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Old 04-24-2011, 02:55 PM   #1
Mini-Skoolie
 
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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.
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File Type: jpg GENERATOR 001.jpg (131.0 KB, 1838 views)
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