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#88548 - 06/24/04 06:45 AM 12VDC  
PEdoubleNIZZLE  Offline
Member
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
I am an electrical engineering student working on a renewable energy (solar, wind, etc) project for homes. I have a few general code questions.
1) Can you use 12VDC on normal hose wiring components (light switches, boxes, etc.) I know to follow current guidelines and keep it seperate from 120/240V. Or do I have to use switches specially made for 12V?
2) Can I use Romex/THHN/etc. with the same current that the NEC states for 120VAC use, or is it different for 12VDC loads? (I have a book that breifly touches electronics and low voltage stuff, and it says 9amps for #12 wire, but I think they are just being "too safe" in case anyone decides to try the experiments at home)
3)Can I use romex that I mark "12VDC" or do I have to use another system such as conduit and THHN? I know lower voltage wires don't really need protected, like door bells, but I'm dealing with wires that are protected at 20A per branch
4) are there color codes for 12VDC? I've seen Red positive Black Negative, but is that set in stone, or is it a well adopted practice?
5) Does the NEC have anything on battery boxes or battery rooms for lead acid batteries (vetalation, heating, etc)
6) Can anyone point me to a web site that sells 12VDC outlets that fit in a normal electrical box?

Thanks for any help that anyone can give me. I know a little about code..... enough to rewire a house and torque the lugs on stuff, but past that I am still learning code, as it may help me get farther as an EE (ya know, like inventing an outlet that tells you how many amps you have left before the breaker blows... or the outlet with the grounds up... and down... LOL)


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#88549 - 06/24/04 09:23 AM Re: 12VDC  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
1- No! The devices must be rated for DC and many of the manufacturers have dropped DC ratings. Cooper offers #77V-BX single pole switch and a #783V-BOX 3-pole that do not appear in their catalog. Device boxes and other wiring methods are ok.

Special care should be taken when using any device as they are marked with a maximum voltage that they may be used with, where as solar based systems use a nominal value as their rating. A typical relay rated 12 VDC has a typical voltage tolerance of 12 VDC +10% to -25%. This will not work on a 12 VDC solar system directly which may be rated at 20+ VDC open circuit +25% for ambient temperature which is well out of range. Also if your load is such that it is always on a battery, the battery output would typically be 11.5 VDC at low vltage disconect to 14.5 VDC at charge again outside the range of the device.

Also a 48 VDC rated circuit breaker can at max be used on a 24 VDC solar system in order to stay within its voltage rating. See NEC 690.7 for how to calculate the voltage of a solar system. (Note: I use solar versus photovoltaic to speed up my typing).

2- Yes, but realize you are dealing with 12 VDC not 120 VAC. To get the same wattage delivered to a load with the same voltage drop you are dealing with a factor of 100 (your resistance must be 100 times less due to I squared*R losses) so be aware of your voltage drop to each of your loads and size your wires accordingly. Rule of thumb - If you are going to use 12 gage wire at its 20 amp rating, unless it is a very short distance, you will have to upsize.

When protecting at 20 amp, take care that the voltage drop will allow the operation of the overcurrent device during a fault.

For long runs in wet locations THWN has leakage concerns that may interfere with proper operation of some devices that may rely upon insulation integrity. XHHW or RHW would be better choices. This is not a typical situation that you may run into.

3- If you have multiple voltage within a building then you need to identify them.

4- A black grounded conductor is not allowed. Typically the negative is grounded, but which ever one is, either positive or neative, it must be white. The color scheme you are refering to is a hold over from electronics work and you will see many systems wired with that scheme which is not allowed unless the system is ungrounded per NEC 690.41. See NEC 200.6.

5- Yes but not all that good, see article 480.

6- I do not know of any. For outlets when needed I convert to 120 VAC using an inverter. The devices that operate on 12 VDC are all hardwired, and portable 12 VDC devices are not found in the applications that I have worked with.

Shane


#88550 - 06/24/04 01:03 PM Re: 12VDC  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Some suggestions follow that are intended for qualified, trained and experienced electrical workers. (1999) NEC Article 720 — Circuits and Equipment Operating at Less than 50 Volts. may apply.

An example of DC-rated wiring devices is Hubbell 328DC-series.

Because voltage drop is critical, consider something like roughly 4-AWG aluminum conductors, with roughly 10-AWG copper pigtailed with a tap connector like T&B 63105 installed with T&B-specified tooling. Increased box volumes should not be overlooked.


#88551 - 06/24/04 08:08 PM Re: 12VDC  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
This is maybe the 20th time I've heard of people wanting to bring low voltage in as a supply. What is the thinking on this? I can't think of much benifit.

Sabrown those were some nice answers! Sounds like you work with DC a lot?

Oh, "inventing an outlet that tells you how many amps you have left before the breaker blows" Please don't, somebody will buy them thinking maxing out is ok!

How-ever, if you invented a breaker with a simple amp reading, and a RED zone at 75% -100% of it's rating, you got a winner!

[This message has been edited by e57 (edited 06-24-2004).]


Mark Heller
"Well - I oughta....." -Jackie Gleason

#88552 - 06/24/04 11:45 PM Re: 12VDC  
PEdoubleNIZZLE  Offline
Member
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
Thanks everyone for your help. This project is mainly to teach us about alternative energy systems. I'm trying to score extra points by going by code. (as well as determine feasability). all the wiring I am talking about happens after regulation, so I'm not worried about the open voltage from the panels (the wind gennys get an even higher voltage.) The project isn't permanent wiring, it's just mock-up of two walls in a room. I hate these projects because they don't incorpoate codes or cost, only the electrical theory. As for the outlet, i was just kidding. But the breaker idea seems cool. Maybe a joint venture? :ccol:
Oh, as for the load, I know I need 10X the amps for 1/10 the volts to get the same power. We're using compact fluorescents since they are more efficient and use less current for the same lighting.
As for the Idea in general, I think it's okay for remote homes (see http://www.otherpower.com ). Oh well, it's a grade.
Thanks again,
Josh


#88553 - 06/25/04 03:03 AM Re: 12VDC  
PEdoubleNIZZLE  Offline
Member
Joined: Jun 2004
Posts: 176
McKeesport, PA, USA
I forgot to mention, The project is just for compact fluorescent lights and computerized controls (house of the future crap.... personally i'd go with x-10 instead). Anyways, each light is a 3 light fixture using 3 14W CPFs, which would be 3.5amps, protected with a 7.5A fuse (or something similar, still only in the design phase), with 12 gauge wire. The total load is 25A or less, since the reserve capacity on the battery is 7hrs@ 25A@ 70deg. F. I'm using a deep cycle marine house battery for the design. The Mock-up (from last post) will be a smaller scale prototype that will have 1 light and a cigarette lighter socket to plug in a walkman CD player or something, all hooked up to a small 12V battery, like a NiCad or something.
Thanks for all the Help,
Josh


#88554 - 06/28/04 09:16 AM Re: 12VDC  
sabrown  Offline
Member
Joined: Dec 2002
Posts: 302
Ogden, Utah, USA
E57,

DC projects are becoming more common, I think I reviewed (designed by someone else under my supervision) 4 of them this month and I am currently designing a system where the power company is pulling out of a site, which will be some straight solar, and some hybrid (solar and generator) with confusing (for me at least to keep it all straight in my head) controls. I am just glad that I won't have to design the load shedding load bank on top of it.

These are just a few of the months projects, but I am bragging which doen't belong here, just that sometimes I enjoy it. It helps me to feel good about myself.

Shane



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