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Old 04-24-2011, 02:55 PM   #1
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What amp draw ok on common?

Generator is wired for 240. Two legs of 120 each. One black and the other red. The white (may have been yellow originally) only has an amp draw of .3. However, turning on AC unit causes amp draw on common to skyrocket to 18 amps. Electrician tells me AC is wired incorrectly. Is there ever a time common could/should have an amp draw over .5?

Thanks,
Mike
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Old 04-24-2011, 04:44 PM   #2
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Re: What amp draw ok on common?

Quote:
Originally Posted by GameRV
Generator is wired for 240. Two legs of 120 each. One black and the other red. The white (may have been yellow originally) only has an amp draw of .3. However, turning on AC unit causes amp draw on common to skyrocket to 18 amps. Electrician tells me AC is wired incorrectly. Is there ever a time common could/should have an amp draw over .5?

Thanks,
Mike
You'll want to read this: http://www.skoolie.net/forum/viewtop...t=10096#p72743. I think it will make things clearer on how a split phase 240/120V system works.

Hope this helps,
jim
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Old 04-24-2011, 05:34 PM   #3
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Re: What amp draw ok on common?

BTW I have a 21kw isuzu/norpro genset. Ok. Think I got it. Now for the most important piece. I am wired (I believe single phase 240) and therefore my max amps available is 87.5 amps. This is cutting it way too close for my power needs. I am dangerously close to 87.5 and when air conditioner compressor kicks on and amps spike momentarily, the gen dies because it exceeded 87.5. So, I want to rewire the gen for single phase 120 and make my max amps available 175 so I am nowhere near max amps. Only drawback is no support for 240volt devices of which I have none.

Help me confirm I have this right.
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Old 04-24-2011, 06:04 PM   #4
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Re: What amp draw ok on common?

Quote:
Originally Posted by GameRV
BTW I have a 21kw isuzu/norpro genset. Ok. Think I got it. Now for the most important piece. I am wired (I believe single phase 240) and therefore my max amps available is 87.5 amps. This is cutting it way too close for my power needs. I am dangerously close to 87.5 and when air conditioner compressor kicks on and amps spike momentarily, the gen dies because it exceeded 87.5. So, I want to rewire the gen for single phase 120 and make my max amps available 175 so I am nowhere near max amps. Only drawback is no support for 240volt devices of which I have none.

Help me confirm I have this right.

That's a nice genset.

Yes, it sounds like you're set up for 240V split phase.

I'm not sure what the amp load of all your loads totaled is, so I don't know if rewiring would help. However, as you have no need for any 240V equipment, it shouldn't hurt anything either, assuming the generator is safely and efficiently capable of operating that way.

The best benefit you would have from rewiring would be that you would not need to worry about balancing the load across the two Lines, and also, if one Line was close to the max, but the other had plenty of power in reserve, this would keep you from tripping over the limits.

I hope this helps,
jim
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Old 04-24-2011, 06:23 PM   #5
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Re: What amp draw ok on common?

Amp load works out this way, 16 tv's and 16 xbox 360's for a max draw of almost 35 amps. 4 rooftop ac's rated at 10.5 amps each. If I turn on fluorescent lighting, theres another 10 amps or so. Problem is when compressor goes off and the comes back on, the amps spike well above the 10.5 it is rated for. The 240 appeared to be as balanced as it could be for example, all tv's and xbox's up and running the differential or draw on common was .3 amps.

And for the record, very helpful.
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Old 04-24-2011, 08:38 PM   #6
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Re: What amp draw ok on common?

So, now how do I rewire the gen head from 240 to120?
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Old 04-25-2011, 02:20 PM   #7
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Re: What amp draw ok on common?

Quote:
Originally Posted by GameRV
Amp load works out this way, 16 tv's and 16 xbox 360's for a max draw of almost 35 amps. 4 rooftop ac's rated at 10.5 amps each. If I turn on fluorescent lighting, theres another 10 amps or so. Problem is when compressor goes off and the comes back on, the amps spike well above the 10.5 it is rated for. The 240 appeared to be as balanced as it could be for example, all tv's and xbox's up and running the differential or draw on common was .3 amps.

And for the record, very helpful.
Ok... 35A +10A + 42A is only 87A total, which is well within your generator's rating. For an A/C though, it's a good idea to use twice the rating for a guesstimate for the surge when the compressor kicks. So that would make your A/C units total at 84A, which is quite a lot.

Now, when you show the .3A on neutral, is that with all the A/C units turned off? Are the A/C units split so that 2 are on each leg? Does the imbalance happen only for 1 particular A/C unit? It's conceivable that a single A/C unit is bad and drawing way more than it should, throwing everything out of wack.

Assuming everything were in fact equally split, including lights and A/C units, the max a single leg would need to carry would be 74.5A (including compressor surge guesstimate), which is actually very close to your 87.5A limit per leg. Just one A/C unit drawing excessive current would cause that leg to go over. The best way to check the A/C units individually is with a clamp on ammeter on the Line leg of each A/C unit while it runs through a startup and compressor kick cycle. Should take about 10 to 15 minutes per A/C unit to test this. They should all be fairly close to the same readings. An additional benefit to testing this way is that you'll have a better idea of the real actual load, not just the rated load. If one or two of the units are wildly higher than the others, I would suspect them of having a problem needing repair.

OK, so we have 35A for the gaming systems, 10A for lights, and 21A each for A/C units (worst case, compressor kick guesstimate). This brings us to 129A total. Your genset is capable of a total of 175A at 120V (87.5A per leg when split or @240V). This *should* leave you about 46A of wiggle room, which is more than many people's generators produce at peak load. If you can rewire as 120V combined, this *should* eliminate the problem or an A/C unit killing the genset.

Are you positive your genset is rated at 21KW sustained load, and not peak load?

Have you measured actual current draw for the gameing equipment, or are you just using the ratings?

Quote:
Originally Posted by GameRV
So, now how do I rewire the gen head from 240 to120?
I'm afraid that is beyond me. It will likely require the services of a generator service center for the brand you have. Some generators aren't easily rewired to change from 240V to 120V, and some are.

I'm presuming that the problem you're having is that sometimes the A/C causes the generator to cut out/stall/fail, basically killing all the power to everything. If this is the case, I might have a bit of a solution for you. Now, this will cause a slight increase in the energy needs for the generator, but will prevent the "hey! everything just died! I want my money back!" situation.

You could install an appropriately sized battery bank and an inverter for your most important items.

Your gaming gear plus lights is drawing 45A if I remember right. That would be 5400 watts total, or if split into two, 2600 watts per half. What you could do is build 2 500AH battery banks. Then buy 2 3000W Inverter/chargers, one per battery bank. Have each one fed from a different leg in your panel (so they stay balanced), and have them each feed their own sub panels. Plug have your gear/lights into each one. This will pull 2600W from each inverter, and would let you run all your loads (except A/C units) for about 1 hour before your battery bank would be approaching half charge. The inverter/chargers would basically act as UPS gear, and would hopefully be able to help you get through momentary power outages. I chose 500AH because a 3000W inverter draws 250A at 120V at full load, and you don't want to use more than 50% of your battery's capacity as that kills them quicker. So, doubling 250A gives 500A and 500AH is 1 hour at 500A but since you can only use half that, it's 1 hour at 250A.

A less expensive approach would be to put a small UPS on each gaming station (or pair of them). However, this increases the load to the generator a lot more than the other method. Also, some of the cheaper UPS systems don't deal well with power that fluctuates frequency/voltage like generators often do.

I know this is a long post to basically say "take your generator to the shop to get it rewired" and I apologize for that. However, I hope it has given you more food for thought on how to solve the problems you're having.

jim
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Old 04-25-2011, 02:40 PM   #8
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Re: What amp draw ok on common?

Using ratings for amp draw of xbox's and tv's but seems accurate. Current 240 leg (2 legs of 120) is evenly balanced. 21kw is peak and 20kw sustained. It is split phase 120/240 and the common only draws when there is an imbalance (that's what it should do). So no ac's and all xbox's and tv's on, amp draw .3 meaning very well balanced. I think one breaker has 3 systems and all others 2. Turn off all gear and only fire up one ac and common draws almost 10 amps. This is correct because the common is supposed to balance. So one ac drawing 10 amps and common will draw 10 amps. Fire up second ac and if it draws 8 amps, the common will draw 2 amps (the difference between the 2 legs) and maintain balance. At least that's what I read and was eerily familiar to what was happening on the bus. I was confused why one ac gave common draw 10 amps and 2 ac's gave common draw 6 amps. Getting clearer.
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Old 04-25-2011, 05:20 PM   #9
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Re: What amp draw ok on common?

Just got off the phone with Norpro to order a manual. I guess it is worse than I thought. He said it is a 3 phase gen head and not single phase like I thought. That being said, max amp draw is now dropped down to 63 amps at 240. I thought this genset could go single phase or 3 phase. I guess I was wrong and it is 3 phase. That being said, rewiring to 120 will get my max amp draw to 126. He said I could wire it something he called "series delta" 120 and the max amp draw would be 140.

Thoughts?
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Old 04-25-2011, 05:54 PM   #10
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Re: What amp draw ok on common?

Still confused. Attached photo of genset clearly states phase 1.
Attached Images
File Type: jpg GENERATOR 001.jpg (131.0 KB, 1838 views)
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Old 04-25-2011, 05:57 PM   #11
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Re: What amp draw ok on common?

Well, 3 phase power does complicate things a bit.

My first recommendation is to find a good commercial electrician that is comfortable with 3 phase power systems. Preferably one that sells generators to companies. It will be worthwhile to talk with them about your system and get a certified solution.

With all that said, you still have a solvable situation. You still have 19KW to 20KW of normal operational power, with 21KW of surge. BTW, what's the specific model number of your generator? Norpro has some information about their generators online, and the only one I thought sounded like yours didn't list it as being 3 phase. What is the actual voltage between either of the Lines and Neutral?

In any case, total available power hasn't changed. All that has changed is the distribution of the power. In a 3 phase system, you have 3 sources of 120V power. You just need to balance things across 3 legs instead of 2 legs.

This is somewhat more complicated. A commercial electrician with a good reputation for generator installs will have a lot of experience with balancing loads. If you were near the DC/MD/DE area, I could recommend a great electrician to talk with, he is our go-to guy for commercial work where I work. He also installed and balanced our 60KW 3 phase generator.

Generating 3 phase power is generally quite efficient. It's basically how they do it at big power plants. Each leg is 120 out of phase with the next leg (360 in a circle, divided by 3 legs, plus a neutral).

Now, the hard part will be balancing 4 major loads across 3 legs. Since nominal draw for an A/C unit is 10 amps, the lights is 10 amps, and the gaming systems is 35 amps, we just need to find a combination that balances them as closely as possible.

The easiest part is balancing the first 3 A/C units. Obviously, 1 per phase.

Then there's the 4th A/C unit, the lights, and the 35A of gaming gear. If you put the 4th A/C on L1 (10A), the lights on L2, and 10A of the gaming load in L3, then we're sitting at nominal 20A on all 3 legs, for a total of 60A. But this leaves us 25A of gaming gear left to power. If you can then put 8A of gaming load on L1 and L2, and 9A of gaming load on L3, that will get you balanced within approximately 1A, using a total of 85A.

Consider your per-leg limit to be approximately 55A. This puts your total limit at 165, which leaves some overhead for error.

With the above scenario, you should end up with loading like this:
L1 10A A/C, 10A A/C, 8A gaming, total of 28A
L2 10A A/C, 10A Lights, 8A gaming, total of 28A
L3 10A A/C, 19A gaming, total of 29A

This is a total of 85A, which is well below your potential limit of 165A (or actual limit of 175A). Also, the heaviest loaded leg has 26A of wiggle room, and the other two legs have 27A of wiggle room. Compressors kicking shouldn't be an issue.

Now, I mentioned getting a good commercial electrician for a reason. Most residential electricians never have to deal with 3 phase power, and aren't used to working with it. It does require some different skills. Believe it or not, many commercial electricians don't have to deal with it much either, as they're dealing primarily with smaller businesses. Many of the A/C guys (even commercial guys) that have the electrical skills to hook up their A/C gear don't understand 3 phase power either. I've run into many of them in my job.

Whether your generator is producing 240/120, 120 straight, or 3 phase power, you have plenty of energy available to run the loads you have. They just need to be balanced properly, and you need to leave some capacity for A/C startup.

I hope this is helpful,
jim
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Old 04-25-2011, 06:04 PM   #12
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Re: What amp draw ok on common?

Quote:
Originally Posted by GameRV
Still confused. Attached photo of genset clearly states phase 1.
That lines up with the information I found on the website for norpro.
http://www.4norpro.com/pdfs/POWER%20MAX%20I/21PMI.pdf

I'm confused why it says 1 phase 120/240 and also lists 4 wires (which is normal for 3 phase).

Very strange indeed.

I still think you need a good commercial/generator electrician to look over this.

At the main point where the wiring leaves the generator and joins the vehicle, is there a junction box somewhere that you could open and count the number and color of the wires coming from the generator? Also, are there breakers on the generator, and if so how many and what size?

jim
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Old 04-25-2011, 06:08 PM   #13
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Re: What amp draw ok on common?

I will take to the pros. Thanks for help.
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Old 04-25-2011, 11:19 PM   #14
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Re: What amp draw ok on common?

Quote:
Originally Posted by GameRV
BTW I have a 21kw isuzu/norpro genset. Ok. Think I got it. Now for the most important piece. I am wired (I believe single phase 240) and therefore my max amps available is 87.5 amps. This is cutting it way too close for my power needs. I am dangerously close to 87.5 and when air conditioner compressor kicks on and amps spike momentarily, the gen dies because it exceeded 87.5. So, I want to rewire the gen for single phase 120 and make my max amps available 175 so I am nowhere near max amps. Only drawback is no support for 240volt devices of which I have none.

Help me confirm I have this right.
RELAX!
The cut sheet Baadpuppy linked to, and the data plate for your machine, both say you have 120/240-volt single phase, not a three-phase machine. Your machine will put out up to 87.5 amps to 240-volt appliances with no 120-volt loads, or it will put out 175 amps at 120 volts, half on Leg 1 and half on Leg 2. The fourth wire mentioned on the cut sheet has got to be the ground. You already said that you have a black Line 1, a red Line 2, and a white Neutral.

To answer the question of your posts, the Neutral current flow is the difference between the 120-volt loads on Line 1 and Line 2. If they are balanced, it is zero. If one leg is fully loaded and the other has no load, the Neutral will have up to 87.5 amps on it. If you put two 120-volt air conditioners on one line and two on the other for balance, and split up the other loads, you will be fine.

Quote:
Originally Posted by GameRV
So, now how do I rewire the gen head from 240 to120?
This could be easy, or it could be hard, depending on the wiring in the alternator head. For example, Onan "Emerald" 6kW generators are easy to re-wire. They have two 3 kW 120-volt windings coming out to terminals inside the wiring cover. You get 120/240 volts at 3 kW per side if you wire them in series (Line 1 on 1, jumper 2-3 for the Neutral, and Line 2 on 4.) If you wire them in parallel, you get 120 volts at 6kW (Jumper 1-3 for Line 1, jumper 2-4 for Neutral. no Line 2). But the amount of available power does not change! If you had a single 4000-watt appliance, you might need the 120-volt option. Otherwise, either choice works.

If the generator windings do not come out to terminals like this, it may mean un-winding and re-winding the alternator innards to get 120-volt only output. You do not want to do this! If you have 120 volts instead of 120/240 volts coming out of your genset, you must have twice the current flow to get the same amount of power, and the ampacity of the wiring used between the genset and the load center must be at least double. Your losses from cable heating will be greater, and you will have greater fire potential if one of the connections goes bad and starts heating up. Instead of rewiring, just balance your loads as you have been doing, and you will come out ahead.

Also, you have an 1800-RPM generator, not a 3600-RPM generator. This is a good thing. The windings are more complex, as the alternator must put out 2 cycles of AC for each turn of the motor crank instead of one cycle per turn. The motor runs slower and lasts much, much longer. (FYI, 60 cycles per second times 60 seconds per minute equals 3600 revolutions per minute.)

But the generator must turn a constant 1800 RPM to maintain the 60 Hz line frequency. When it hits a sudden load, it may bog down for a second until it can serve up more fuel to generate the extra watts at the fixed RPM.

Think of it like cruise control in your car. If you set the cruise at 60 MPH, you are humming along until you come to a hill. Without cruise control, your eyes see it coming, and without thinking your foot presses harder on the gas. But the cruise control cannot anticipate the hill, it only knows that it was going 60 and is now going 59, 58, 57, so it adds more fuel to get back up to 60. When you get to the top of a hill, you instinctively ease off the pedal, but the cruise control does not ease off the fuel flow until it starts seeing 61, 62, 63. It is the same with a genny trying to hold a certain RPM for 60 Hertz.

Your genset is big enough to power 3-5 RVs. It is at least double your loads with everything on. So the question is, does the genset actually stop when an air conditioner comes on, or does it just stumble while it is turning up the fuel flow? If it stops or stumbles badly, I would look for a defective air conditioner, or a problem with the engine, maybe fuel flow. Is is plumbed into the bus's tanks? Is there a way for air to replace the fuel drawn from the tanks, or are you sucking fuel against a vacuum? If you had a propane genny, I would be asking about enough surface area to boil vapor fast enough (horizontal tank, or multiple vertical tanks in parallel on a manifold).

I seriously doubt that the alternator load and bus "entrance" wiring is the problem. I would look at the air conditioners, the fuel filters, lines, and vents, and if necessary have the engine looked at. If all you are hearing is a stumble and recovery, that may be completely normal. Losing 100 RPM out of 1800 will seem much worse than losing 100 RPM out of a 3600 RPM screamer. You may also want to rig the air conditioner controls for staggered starts, so all four do not try to start in unison.

Good Luck!
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Old 04-25-2011, 11:45 PM   #15
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Re: What amp draw ok on common?

Fuel draws from main tank. Previous post says going from 120/240 and 87 amps to 120 and 175 amps yields no additional power. This is one thing I cannot grasp. 175 sounds better than 87 to me. 50% of load sounds better than 100% but like I said, I can't wrap my head around that concept. If I'm wired 240 and maxing out at 87 I am worse off than Gamerbus with 13kw and 3 ac's. I have 21kw and 4 ac's. My brain says I should be ok but gen sure sounds like it is maxing out at 87. Gamerbus days he needs all 110amps from his 13kw if gen.
Maybe I can shed some light. The other day we ran like 4 xbox's and tv's and 3 ac's for couple hours no problem. I could run all 16 systems for an hour with all 4 ac's in max cool. Thought there was being on max cool would eliminate spike in amps when compressor goes off, then comes back on. I killed the gen (it died) when I threw the a breaker for an electrical heater (double breaker connected to each leg). Thought process was that the extra amp pull from the heaters put it over the max. We had the bus since September and have run the ac's with no problem and lights on. My novice opinion is that I am at the limit of the generator. That's what bugged me, at 21 kw I shouldn't be. Only thing I could think of was 240 vs 120.
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Old 04-25-2011, 11:54 PM   #16
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Re: What amp draw ok on common?

Your machine will put out up to 87.5 amps to 240-volt appliances with no 120-volt loads, or it will put out 175 amps at 120 volts, half on Leg 1 and half on Leg 2.

Does this mean NO re-wiring necessary?
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Old 04-26-2011, 01:25 AM   #17
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Re: What amp draw ok on common?

Quote:
Originally Posted by GameRV
FPrevious post says going from 120/240 and 87 amps to 120 and 175 amps yields no additional power. This is one thing I cannot grasp. 175 sounds better than 87 to me. 50% of load sounds better than 100% but like I said, I can't wrap my head around that concept.
Lets look at watts. 240 volts X 87.5 amps = 21,000 watts. 120 volts x 175 amps = 21,000 watts. It's the same thing.

I say half the load because you were adding 120-volt loads up toward the 87.5 amp limit, which when split evenly between the legs should be about 43.75 amps per side, well under the 21,000 watts the machine can produce when running correctly. The loads you were adding should be compared to the 175-amp capacity of the machine, not the 87.5-amp capacity of each side.

Let's say you had two 2400-watt, 120-volt air conditioners, one wired on Black from L1 to Neutral, and the other wired from Neutral to Red on L2, the two units "share" the 240 volts in series, each taking half. Here's how it works:

*geek alert*
L1 and L2 are wired to opposite sides of the alternator, so when L1 = zero L2 also = zero, using Neutral as a reference. When L1 is rising to +120 volts, L2 is falling to -120 volts. When they hit peak/minimum voltages, L2 starts rising from -120 through zero to +120, and L1 is exactly opposite falling through zero to -120. Neither line is ever has more than 120 volts referenced to neutral, but they are always 180 opposite each other. The 240-volts is only found when measured from Line 1 to Line 2 across the middle.

Think of it like the left and right pedals on a bicycle. 240 volts like is a normal person riding a bicycle with two pedals, one foot going up, one foot going down. 120 volts is like a one-legged person riding a bicycle with only one pedal, working that one leg twice as hard to keep going.

So each air conditioner uses 120 out of the 240 volts, and passes the rest of the current in series to the other unit. If one wants to draw more than the other is using, the difference goes through the middle to/from the Neutral to maintain balanced voltage on each hot leg. In this examples the air conditioners draw 20 amps at 240 volts, each unit using half of the voltage, for 4800 watts combined. .

Now let's rewire for 120 volts. Each unit draws its 20 amps at 120 volts in parallel between Line 1 and Neutral. Both wires now have to supply 20 + 20 amps, or 40 amps. The feed coming to the circuit breaker box now has to handle 40 amps, not 20. The units now draw 40 amps at 120 volts, but still for 4800 watts combined. Each unit is still individually drawing 20 amps at 120 volts to make its own 2400 watts. The power source still has to provide 4800 watts for both. But using the higher voltage and series wiring reduces the need for higher current, reducing heat losses in the wires and devices.

Quote:
Originally Posted by GameRV
Your machine will put out up to 87.5 amps to 240-volt appliances with no 120-volt loads, or it will put out 175 amps at 120 volts, half on Leg 1 and half on Leg 2.

Does this mean NO re-wiring necessary?
That is what I am saying. Split your 120 volts as evenly as possible between Line 1 and Line 2, and if your 120/240 volt wiring was done correctly, it is better not to change it.
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Old 04-26-2011, 08:58 AM   #18
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Re: What amp draw ok on common?

Ok. Leave wiring alone and back to square one as to why gen shuts down. New air filter & fuel filter. I'll look at gas cap for possible vacuum, checking vents as well. Diesel mechanic is of the opinion fuel lines are good.
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Old 04-26-2011, 01:10 PM   #19
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Re: What amp draw ok on common?

Redbear, thanks for jumping in here. Great information!

Quote:
Originally Posted by GameRV
Maybe I can shed some light. The other day we ran like 4 xbox's and tv's and 3 ac's for couple hours no problem. I could run all 16 systems for an hour with all 4 ac's in max cool. Thought there was being on max cool would eliminate spike in amps when compressor goes off, then comes back on. I killed the gen (it died) when I threw the a breaker for an electrical heater (double breaker connected to each leg). Thought process was that the extra amp pull from the heaters put it over the max. We had the bus since September and have run the ac's with no problem and lights on. My novice opinion is that I am at the limit of the generator. That's what bugged me, at 21 kw I shouldn't be. Only thing I could think of was 240 vs 120.
GameRV,

That is a good test. The numbers added up just don't support the genset hitting its limit, unless there is a load that hasn't been accounted for.

I would love to know the results of doing that same test, with a clamp on ammeter on each of the Line legs (L1 and L2), with it set to hold the peak value. Then run all the systems + A/C units on max, and write down the values. Then, flip that breaker to turn on that heater again, and after it dies, write down the new peak values.

As long as both of them are beneath 87.5A, then this would point more to a loading issue with the generator motor rather than an electrical issue with the generator itself, which I think Redbear has alluded to in one of these posts. If either are above 87.5, then this would point more to an electrical load issue, and would also tell you which leg to focus on.

jim
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Old 04-26-2011, 07:51 PM   #20
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Join Date: Dec 2010
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Re: What amp draw ok on common?

It seems the load is balanced perfectly. 2 ac's on each leg. Heaters balanced between legs obviously due to double breaker. Now that I think about it, the electrician showed me the main breaker which I think he said equated to 150 amps. That is located at the top of the panel. Without being in front of it, I can't recall how four breakers added up to 150 though. With that being said, if I was reaching max of 75 for one leg or 150 total, that breaker would have tripped.
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