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flyboy 12-22-2009 12:18 PM

Code requirements for low voltage wiring in conduit with 120V
I would like to install low voltage wiring in a PVC conduit that already has 120 volt hot neutral & ground running thru it. This is for a standby generator outside my house. The existing 120V is for the power. The low voltage (12V) run would be a remote start switch. Location is MA.

My question is this:

What are the code requirements in this situation. Can I run both wires in the same conduit ? Are there special requirements for the type of wire to be used ? Does the low voltage switch inside the house have any special isolation requirements ?

Billy_Bob 12-22-2009 12:49 PM

So far as I know, you can't do that. (National rules are subject to local amendments or not being used at all.)

And you wouldn't want to do that either!

This is a bit complex, but imagine a dog running along a fence and there are horses on the other side of the fence. The horses start running too!

This is the way electricity works when you run one wire next to another. The higher voltage wire gets electricity moving in the lower voltage wire.

And it is possible that the low voltage wire would have enough electricity in it from the other wire, that this would prevent the low voltage circuit from working properly.

Distance is a killer with this type of wiring! All sorts of weird things happen when you have long distances of low voltage wiring running next to higher voltage wires - like in a large warehouse which is several city blocks long.

This is called "inductance"...
(Following is a terribly complex explanation of this that only Einstein would understand! :) )

Best to run the low voltage wire in a separate conduit and add a bit of air or dirt space between conduits.

Same with running wires inside a building. Best if there is maybe a foot of space between low voltage wires and higher voltage wires (as much as possible).

(And there is a possibility of a short from the higher voltage wire to the low voltage wire - a potential safety problem.)

Back to the "inductance" thing, this is a bigger problem when you increase the voltage. Farmers working with metal pipes and fences under high voltage towers have all sorts of problems with this. They could be zapped by just picking up a length of metal irrigation pipe off the ground...

jbfan 12-22-2009 03:09 PM

As long as the insulation on the low voltage wire is rated the same as the high voltage wire, you are ok.
Not he best practice, but allowed.

Anti-wingnut 12-22-2009 04:31 PM


Originally Posted by jbfan (Post 370838)
As long as the insulation on the low voltage wire is rated the same as the high voltage wire, you are ok.

That's the key there. And how much LV wire is rated to 600v?

joed 12-22-2009 04:50 PM


Originally Posted by Anti-wingnut (Post 370869)
That's the key there. And how much LV wire is rated to 600v?

You don't use low voltage wire in this case. You use the same wire you are using for the 120 volts, #14 thwn. Only it has 12 volts on it instead of 120 volts.

micromind 12-22-2009 11:36 PM

Most generator auto-start circuits have circuitry built in to them that'll cancel out the 60 HZ capacitive coupling from the power conductors.

It's completely code compliant to run different voltages in the same conduit provided all conductors are rated for the highest voltage present.

As stated above, run a couple of #14 THWNs, and you'll be fine.


flyboy 12-23-2009 09:39 AM

Thanks for the info.

I am fairly confident that inductive/capacitive coupling will not be an issue in my situation.

The high voltage insulation makes sense. jbfan indicates that this is accepted by code but not the best practice. Is there a specific situation or scenario you are worried about ?

Is there a requirement regarding insulation color for the LV lines ?

micromind 12-23-2009 08:08 PM

The reasons we generally don't put high and low voltage wires in the same conduit are;

1) Possible capacitive coupling. Also known as inductance. This can superimpose the high voltage signal onto the low voltage circuit. Very rarely a problem these days, moreso in years past.

2) If the high voltage wire is big (like over 2/0) is can destroy the smaller low voltage wires when they're being pulled in.

The only color restriction is don't use green, white, or gray. Those are grounds and neutrals. You can use the same color if you wish, it's only a start circuit. Switch closes, generator starts. Switch opens, generator stops.


Michael Thomas 12-24-2009 05:02 AM

As I understand it, the control circuit for a typical residential air conditioner's condenser unit is a power-limited class 2 circuit, and per 725.136 cannot run in the same raceway or conduit as class 1 power and lighting circuits.

qbert 12-24-2009 06:48 PM

You should not run low/line voltages in the same pipe. You may get away with it if your low voltage wire is shielded

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