Reply
 
Thread Tools Display Modes
 
Old 01-06-2012, 09:06 PM   #31
wtd
Bus Nut
 
wtd's Avatar
 
Join Date: Jun 2005
Location: California City, CA
Posts: 267
Year: 1982
Coachwork: Thomas TransitLiner
Re: 12 volt

Quote:
Originally Posted by bansil
... I liked your calculations they where "visually" correct(I don't know electrical like an electrical engineer" ...
Me neither , but if you can make heads or tails outta the Machinist's Handbook, this is a snap.
Here's a link to table B.310.1 (it's a pdf)-> http://search.yahoo.com/r/_ylt=A0oGd...k/73497_15.pdf Wow, that's a HUGE link! There's other stuff in the file, but the table you want is on page 15.17.

To calc the current capacity of a conductor at a specific temperature all you need is the conductor size and the maximum temperature where you want to install it and the insulation type of your conductor.
Romex consists of 2 THHN (Thermoplastic, high-heat, non-metallic), usually with a max. temperature of 90C. Just for an example of how to use the chart we'll use this.
Get the chart up, follow the top row over to find the insulation type (THHN) at the max. temp. 90C, then just follow that column down 'til you get to your wire size - we'll use 12 ga., so you find 27 at the column row intersection. THis is the ampacity of the conductor at 30C, in open air.

If the anitcipated ambient temperature of the installation will be more than 30c we follow the THHN column down to the bottom of the chart to the 'Correction Factors' section. Find the maximum temperature on the left, say 60C (140F). At the column/row intersection we find 0.71. 'Derate' the ampacity of the conductor by multiplying the ampacity by the derating factor. Gives you a temperature corrected ampacity of 19.6 amps - in open air.

If you have several cables bundled together, you have to further derate the ampacity of each cable depending on how many cables are in the bundle. For this we use Table NEC table b.310.15(B)(2)(a), found here http://www.houwire.com/products/tech...cle310_15.html.
Say we have 3 cables bundled together we have 6 individual conductors so we find the derating factor for 6 conductors to be 0.80.
So just multiply the temperature adjusted ampacity by the 'bundling' factor 19.6 x 0.80 = 15.7 amps. So that's the totally corrected current carrying capacity of your 12 ga. cable in a 140F environment.

Now just use a breaker just the next size smaller than the ampacity of the conductor, so the breaker will trip before the wire overheats and fails.

Pretty simple, just a few steps.

Tom
__________________
The Rolling Motel Room
Where DIY meets WTF
wtd is offline   Reply With Quote
Old 01-07-2012, 04:55 PM   #32
Bus Crazy
 
Join Date: Jan 2008
Location: Upstate NY (Mohawk Valley)
Posts: 1,096
Re: 12 volt

From the original post:
Quote:
Originally Posted by bcschro
. . . Do I have to take out the 12v wire I ran and replace it with 12 RV two strand wire? And where can I buy 12v RV 2 strand wire at. . .
I was over-thinking the original question, as I had never heard of "RV 2 strand wire." I was picturing two solid conductors twisted together inside the insulation, which did not make sense. The wire would be as inflexible as one solid conductor, and would be very expensive to manufacture.

Others seemed to interpret this reference as "2 conductor" wire, with separate insulated hot and ground wires. The conductors would either be together in another layer of insulation, or it could be "zip" cord, with the insulation of the two wires joined side-by side.

Hours after making my first post, it dawned on me that someone probably told bcschro that "Number 2" stranded wire was needed for a particular high-current application. Duh!

********
So it seems we have had quite a discussion since then. Remember, we all have different purposes for our buses. There are three levels of concern, and all of us operate at one or more of these levels, sometimes alternating between them:

1. Will it work?
2. Is it safe?
3. Is it compliant with the National Electrical Code?

1. There are a lot of things that "work," whether they are safe or not. The example cited of the washing machine running on lamp cord working for a while (until it didn't) is one of these. People who temporarily cut corners because they know what they are doing, and know and accept the risks, may be fine for them, but they should not teach their practices to other people, nor leave their jury rigs anyplace where other people might come into contact with them.

I don't care about stories of what you "got away with." (Except for entertainment value )

For a personal example, once I was deep in the woods with two other people, driving a truck and a van. When we wanted to leave, the van would not start because we had been using the accessories for an hour or more. We didn't have jumper cables, but we did have a 20-foot length of #6 wire. We cut it in half, bared the ends, and left the truck running while two of us held the bare ends with our thumbs onto the battery posts of the two vehicles. The third person started the van. YOW! I and the other person burned our thumbs on the hot wire. This "hillbilly" emergency measure "worked," but I would never teach someone to do it, and would always instead teach: "Always carry jumper cables."

2. There are a lot of things that are basically "safe," but not to code. A heavy-duty extension cord with no splices in it, run direct between the source and the device while being drilled through wooden partitions would "work" and be "safe," as long as there was nothing that could mechanically cut into it. But I think it would certainly not be code, as I think cord must not be used in permanent installations (with possible exceptions of links to stationary machinery). Running the cord through a sharp hole in a metal partition without a protective bushing would be different.

The question of "safe" should not only include how you intend to use it, but also protection against uninformed strangers. If the toddler from the next campsite comes over and touches your bus while standing barefoot in wet grass, that cannot be a fatal mistake.

3. Wiring to code is a different animal. And to add to the confusion, the code is constantly changing. When I was a kid, there was no such thing as a ground-fault breaker. We had fuses that screwed into 'medium Edison' holders in a fuse box. This was the kind where people were known to replace a blown fuse with a penny to get unlimited current and no more blown fuses. Remodeling in the early 1960's, the house got circuit breakers, but still no ground fault interrupters. Today, you cannot wire a bathroom, kitchen, or outdoor outlet without one. Should all the old houses be torn down just because the safety pointer has moved? Are owners of older houses criminally liable because new technology has been developed, but they did not implement it?

Another example: In my current job, I inherited two 5.5 kW solar systems built in the early 1990s. Each system has two arrays of 9 strings each. Each was installed with two circuit breakers, one for each array. Today, the solar section added to the code says that each system would need 18 over-current protection devices (fuses or circuit breakers), one for each individual string. Are the systems that have "worked" and were formerly "safe" for 20 years now "unsafe" because a section of code has been added?

I won't advise anyone to do any work that is not to code, but neither will I insist that anyone comply with the RV sections that were written by sticks-n-staples makers for sticks-n-staples makers to protect their business. Commercially made RVs never catch fire, do they? Still, many "best practices" can be gleaned from knowing the sections, even if some of them seem to be overkill (or over not-kill?)

********
When you ask about electrical issues, a layman and an electrician will give you different answers.
Ask about wire size, and a layman might answer something like:

#16 - 10 amps
#14 - 15 amps
#12 - 20 amps
#10 - 30 amps
# 8 - 40 amps
# 6 - 50 amps
# 4 - 75 amps
# 2 - 80 amps

There are probably tens of thousands of homes wired by homeowners using rules of thumb like these on short wire runs, after reading "Popular Mechanics" and using listed devices obtained from the hardware stores. The work could have been done without a licensed electrician, either secretly or where allowed by building codes. I'll bet there are thousands of vehicles similarly wired, too.

If you ask an electrician what wire to use, the answer MUST be "it depends." His license and all his future paychecks depend on it.

All wire has some resistance, and resistance generates heat when current flows through it. Therefore, all wires and devices generate some amount of heat when in use. A licensed electrician must consider the accumulation of heat in de-rating the capacity of wire and devices.

The electrician must consider: "How long is the wire run?" "What does the wire run through?" "Is it exposed or enclosed?" "How many other wires are bundled with it?" "What is the environment?" There are various tables, charts, or calculations used to arrive at the values that will pass the code.

If Lorna wires her own bus like the picture she posted, and knows that the bundle powers a night light, a table lamp, a microwave and a laptop, though not to code it would be completely "safe" for her purposes, as long as protection against metal framing cutting into the wire insulation was employed. An electrician could not do the same thing for other people, because the wiring would have to be able to disburse the heat while the owner perhaps used two space heaters, a waffle iron, toaster, microwave, and hair dryer, all at the same time. The wiring would have to be over-sized, spaced apart, or de-rated to allow for that possibility.

********
Something no one else has covered is rodents. Have you ever taken apart a piece of non-working equipment, only to find the remains of a mouse with its teeth still sunk into the AC wires, memorializing its last act on earth? Have you ever seen a building where a porcupine has eaten a hole through a wood floor, had a rubber propane hose for lunch, and ate all the telephone wire insulation it could reach for dessert? I personally like to use metal conduit or armored cable when I can, knowing how much rodents love the taste of plastic.

********
Disclaimer:
I am a "radio man," not an electrician. I do not "play [an electrician] on TV." And I did not ever sleep at a Holiday Inn Express.
I do not have copies of the electrical code, except for the excerpts quoted on the internet. But I do have to know enough about the code to be able to to sign off on electrical sub-contractors' proposals and completed work. For example, I had to devise a plan to properly add a 70 kW emergency generator to part of a commercial building having 216 kW electrical service. Another project I signed for was a half-megawatt battery backup system. And I have worked on dozens of skoolies, a few transits, plus a couple of coaches.
__________________
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.
Redbear is offline   Reply With Quote
Old 01-07-2012, 06:31 PM   #33
wtd
Bus Nut
 
wtd's Avatar
 
Join Date: Jun 2005
Location: California City, CA
Posts: 267
Year: 1982
Coachwork: Thomas TransitLiner
Re: 12 volt

Redbear -

My question, way back when, regarding the picture of the installation was -
How do you provide overcurrent protection for any one of those circuits?

Once you've derated it properly the conductors are only rated at 9.6 amps.
Do you just go ahead and throw a 15 amp breaker in there and hope for the best? Try to find a five amp breaker somewhere?
Nomally breaker size is determined FROM the ultimate installed conductor capacity - not the other way around. You don't stick in a twenty amp breaker and then look at an extension cord capacity chart, pick 12 gauge for the wiring and go town.


In all that discourse that was essentially my only question, and so far has not been addressed.
Connecting a conductor rated at less than 10 amps up to a 15 amp breaker and relying on users to limit the current used in the circuit, doesn't exactly seem like 'overcurrent protection' to me. Why put in a breaker at all? Just wire is solid and tell everyone not to plug in more than a table lamp and a TV at once.

A second question that I had, which is of no real importance other than curiousity is - why would you use conductors that are good for 27 amps in free air and then install them in such a way as to derate them to about a third of their capacity?
My guess is that they weren't about to derate them and they were simply seen as 12 gauge conductors with their full capacity. Heck the installer might have even thought he was being 'conservative' by putting in 15 amp breakers.

The question of heat on conductors isn't just limited to the electrician because he has a license to lose. The reason for derating cables is simply safety - do you really think that heat doesn't affect your installation simply because you have no license to lose?

Tom
__________________
The Rolling Motel Room
Where DIY meets WTF
wtd is offline   Reply With Quote
Old 01-07-2012, 06:58 PM   #34
Bus Geek
 
lornaschinske's Avatar
 
Join Date: Jun 2004
Location: Roswell, NM
Posts: 3,587
Year: 1986
Coachwork: BlueBird
Chassis: 40 ft All American FE
Engine: 8.2LTA Fuel Pincher DD V8
Rated Cap: 89
Re: 12 volt

Wrote to Fred, heard from Fred. End of discussion as far as I am concerned. 35 yr experience electrical. Years (decades) of high end coach conversions. 16 years and over a 100,000 miles on his own personal coach. VS a no name individual who has converted one (?) school bus. No contest.
__________________
This post is my opinion. It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.
Fulltime since 2006
The goal of life is living in agreement with nature. Zeno (335BC-264BC)
http://lorndavi.wordpress.com/blog/
http://i570.photobucket.com/albums/s...ps0340a6ff.jpg
lornaschinske is offline   Reply With Quote
Old 01-07-2012, 07:58 PM   #35
Bus Crazy
 
Join Date: Jan 2008
Location: Upstate NY (Mohawk Valley)
Posts: 1,096
Re: 12 volt

wtd, I understand. And we will never know the answer to your questions, because all we see are the wire runs, not the entire system overview.

But we all live with 'unsafe' wiring. If I plug a lamp with an 18-gauge cord into a 20-amp outlet with 20-amp breaker, the cord is essentially unprotected (except for the Brits with fuses in the plugs). And though our vehicles may have fuses for dome lights, tail lights, heater, and cigar lighter, most of them have self-resetting circuit breakers for critical systems like headlights or windshield wipers. A short in the wiring for one of those circuits results in the breaker periodically resetting over and over again until something gives . . .

Lorna,
I have been reading your posts about the plans for your skoolie, and you have my blessing for your wiring, though I know you are not looking for it, nor do you care whether I give it or not. You are an 'informed consumer' of your own work, and I am positive that you will do it in a safe manner. By the way, I am jealous of anyone with the ability to do cabinetry, as I cannot cut a straight line in wood even with a table saw . . .

As to Fred's work, it looks five times better than the crap that came out of the Fleetwood factory in our "RVIA Certified" pop-up. And I have no complaints about this wiring for his own coach, as he also is an 'informed consumer' of his own work. But wtd does have a point. If doing that work to sell conversions to other 'uninformed consumers,' it's a fact of physics that bundling the wires decreases each wire's safe rating. Fred may be taking that into consideration, or he may just be trusting that he can "get away with it." Either way, if someone copies his methods without doing the rating calculations, there's a possibility of overloading.

By the way, I have "40 year electrical" experience, including audio/video, radio frequency, telephone, stage lighting, residential, commercial, and vehicles. I have worked with systems from 5 volts to 2500+ volts. I have seen failures, fires, and lightning strikes. But that does not make me an electrician. And I have zero bus conversions to date. Based on his posts, I will presume that wtd is an electrician, until I learn otherwise.
__________________
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.
Redbear is offline   Reply With Quote
Old 01-07-2012, 11:23 PM   #36
wtd
Bus Nut
 
wtd's Avatar
 
Join Date: Jun 2005
Location: California City, CA
Posts: 267
Year: 1982
Coachwork: Thomas TransitLiner
Re: 12 volt

Quote:
wtd, I understand. And we will never know the answer to your questions, because all we see are the wire runs, not the entire system overview.
Wow, #1 on the 'List of Dodges' - if you don't want to answer, ask for more information. Used it many a time myself.
Imagine the conductors at the bottom going to a power source and the other end of the conductors at the top going to devices.
Nothing overly complicated, nothing that you could put in there that would change anything I've noted about this installation. Is there?
Makes about as much sense as looking at a flat tire on your car and not knowing whether you should change it or not because you need to see the other 3 tires.
Couldn't understand why this would be a question, but then read the rest of your post and understood the ultimate purpose of the post.

Quote:
But we all live with 'unsafe' wiring. If I plug a lamp with an 18-gauge cord into a 20-amp outlet with 20-amp breaker, the cord is essentially unprotected (except for the Brits with fuses in the plugs).
You actually think that this is similar to using 10 amp conductors and a 15 amp breaker?
This is like saying that as pedestrians we are at risk of being struck by a car, so we might as well ignore crosswalk signals.

As to how long who's done what and for how long - doesn't matter to me a whit if you the world's oldest electrician, a one eyed paraplegic pickpocket or the King of Siam, if you can make a point and back it up with some references and data that's verifiable, that's fine with me.
References to me aren't 'Fred Hobie done it that way' or 'I've been getting away with doing it this way for umpteen years'.
Gosh if I went into my boss' office with a design and he asked how I came up with this and I said - 'Some other guy did it this way', I could probably hear the laughter all the way to the bus stop while I waited for a ride to the unemployment office.
There is a large body of information out there that's available to everyone and isn't hard to understand. Physics are physics, there are no dispensations based on who you are.

Tom
__________________
The Rolling Motel Room
Where DIY meets WTF
wtd is offline   Reply With Quote
Old 01-10-2012, 06:42 PM   #37
Bus Nut
 
bus-bro's Avatar
 
Join Date: May 2007
Location: Whidbey Island, WA.
Posts: 679
Year: 1984
Coachwork: Blue Bird
Chassis: All American
Engine: 3208 na boat anchor
Rated Cap: 2
Re: 12 volt

Quote:
Originally Posted by bus-bro
Geesh Lorna. On some bus sites, recommending the use of extension cords for fixed wiring has caused many a battle.
Told you so!
bus-bro is offline   Reply With Quote
Old 01-10-2012, 07:54 PM   #38
Bus Geek
 
lornaschinske's Avatar
 
Join Date: Jun 2004
Location: Roswell, NM
Posts: 3,587
Year: 1986
Coachwork: BlueBird
Chassis: 40 ft All American FE
Engine: 8.2LTA Fuel Pincher DD V8
Rated Cap: 89
Re: 12 volt

You know, if you have that much of a problem with what I post, then why don't you do as I have done? Put me on your "foe" list. I have done the same to you.
__________________
This post is my opinion. It is not intended to influence anyone's judgment nor do I advocate anyone do what I propose.
Fulltime since 2006
The goal of life is living in agreement with nature. Zeno (335BC-264BC)
http://lorndavi.wordpress.com/blog/
http://i570.photobucket.com/albums/s...ps0340a6ff.jpg
lornaschinske is offline   Reply With Quote
Old 01-10-2012, 09:01 PM   #39
Bus Nut
 
bus-bro's Avatar
 
Join Date: May 2007
Location: Whidbey Island, WA.
Posts: 679
Year: 1984
Coachwork: Blue Bird
Chassis: All American
Engine: 3208 na boat anchor
Rated Cap: 2
Re: 12 volt

Jeesh, all that I was saying was that the use of extensions cords as fixed wires always causes a fuss, whenever, wherever posted. It's kind of like the chemicals and black tanks posts on the RV forums, always a fight.

(alas, Lorna will never see this post. )
bus-bro is offline   Reply With Quote
Old 01-10-2012, 11:06 PM   #40
Bus Nut
 
bus-bro's Avatar
 
Join Date: May 2007
Location: Whidbey Island, WA.
Posts: 679
Year: 1984
Coachwork: Blue Bird
Chassis: All American
Engine: 3208 na boat anchor
Rated Cap: 2
Re: 12 volt

Too late for that, bcschro.

I2 volt isn't a NEC thing, as far as I know. As Lorna posted, the boater's ABYC regs are a good guide to use. The RV industry has a 12 volt "code", but it's not much. I found this in the solar stuff, it has some really good info concerning DC wiring, if you can wade through it: http://www.altestore.com/store/media/pd ... es2005.pdf
bus-bro is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
48 volt subsystem? aaronsb Alternative Fuels | Electric, Propane, Wood Gasification, etc. 3 09-26-2014 10:26 AM
12 volt mr 16 light bulbs chev49 Conversion General Discussions 1 04-30-2012 12:29 PM
6 Volt Batteries... danielm Electrical, Charging and Solar 3 07-14-2010 04:10 PM
Hot? Need a 12 volt fan? trx Conversion General Discussions 3 10-30-2007 11:18 AM
12 Volt appliances fmtaylor Heating, Cooling and Appliances 9 02-13-2006 05:04 PM

» Virginia Campgrounds

Reviews provided by

Powered by vBadvanced CMPS v3.2.3

All times are GMT -5. The time now is 09:52 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.