Downsizing the ground wire for line loss?

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dduffee260

dduffee260

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I believe it says that if you install #10 current carring conductors that you have to also install a #10 ground wire, you cannot go down to a #12. Like when a 6/2 type NM cable has 2- #6's but then has the #10 gound wire. But when it is 10/2 type NM it has a #10 ground wire.
My question is this. If you increase the wire size for voltage loss like in a parking lot lighting type project. Let's say you install 2- #10 wires to a light pole for a distance of 100 ft or so. This would be on a 20 amp breaker.
Can you install a #12 ground wire or do you have to increase the ground wire size since the other wires are #10?
 
Short answer: Use #10.

Longer Answer: First, you figure out the minimum size of the circuit conductors, based solely on ampacity. Next, you figure out the minimum size of the EGC, based on the overcurrent device rating (Table 250.122). Then, if you choose to use a larger size circuit conductor for any reason (voltage drop being one possible reason), then you must increase the size of the EGC in proportion to the increase in size of the circuit conductors. Reference: 250.122(B).
 
This gets weird but here it is.

I will use parking lot lighting as an example.

Say you used 6 AWG to overcome voltage drop.

Now you use a 15, 20 or 30 amp breaker or fuse at the beginning of the circuit.

In this case the EGC would have to be 6 AWG as 15, 20 and 30 amp circuits require an EGC with a 1 to 1 ratio of the circuit conductors.

Now swap out the 15, 20 or 30 amp breaker for a 60 and the EGC could be 10 AWG.

This is all based on the rules found in 250.122(B).
 
I disagree Bob (I don't get to say that very often). ;)

Let's say you started with a 16 amp lighting load. It's continuous, so you plan to use a 20 amp breaker and #12 wire. Now you look at distances, and figure out that you need to upsize to #6 for voltage drop considerations. In other words, 16 amps of continuous load flowing through #6 conductors gives satisfactory voltage at the end of the run. As you pointed out, this requires you to upsize the EGC to #6.

You are suggesting that you can use a 60 amp breaker, thus converting this to a 60 amp branch circuit, and then saying you only need a #10 EGC. Not so, I says. You would be designing a system that could pass 60 amps through wires that would not give an adequate voltage for anything more than 16 amps.

I'll admit that I can't think of the article I could cite as being violated by this design concept. But it's just plain un-good.
 
charlie b said:
it's just plain un-good.
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I don't think we really disagree here.:cool:

I gave my opinion of what the NEC requires or allows. :)

I do not think intentionally installing a 60 amp breaker in this circuit (and providing OCP at each pole) simply to get away with a 10 AWG EGC is good.

A case like this would be a good time for 250.4(A)(5) to give more guidance about what exactly is an effective ground fault path.

How effective is effective?

Bob
 
I could just be stirring a pot here but.... In Bob's example, arguement could be made that the 60 amp breaker was used to allow for future additions to the circuit. Wires were not upsized for voltage drop since they are breakered at 60 amps. This means a # 10 E.G. is required. If I'm thinking properly, it also means you must put invidual fuse protection in for each and every light fixture (or should I say "luminaire") on this circuit. 250.122 says that if branch circuit wiring is upsized, equipment ground wire must be upsized in direct proportion to the circular mil area of ungrounded conductors so on smaller conductors, a 1 to 1 upsize is required but if we start talking larger conductors this is not so. for example if you increase a 400 MCM conductor to 500 MCM for whatever reason (voltage drop is most likely one), that equates to a 20% increase so you must increase your EGC conductor by 20%. I would never actually do what I pointed out in my pot-stirring example but sometimes can't help myself from splitting a hair.
 
Bob, your post about the larger breaker letting you use smaller EGC is something that came up on our job a few months back when we really got focused on 250.122, in our case it was for voltage drop. The weird thing in our case was that we were running everything in pipe that complied with 250.118. So if our calcs told us we had to upsize the EGC from #12 to #8 or #6 we could either upsize it or RUN NO EGC AT ALL. We ran one every time but there were a lot of times where it would have been easier with a smaller EGC (needed larger pipe for fill, harder to pull etc.). Seemed like 250.122 should read "where required" instead of "where installed". Or am I missing something?

By the way Bob, thanks for the welcome awhile back.
 
Interesting, Keep talking im learning more and more.

Ive always thought that the EGC was sized acordingly to the Over current Device ONLY. But apparently it is sized according to the ungrounded conductors as well.

Please help me a little more to understand, What they mean in 250.122 (B).

...Shall be increased in size proportionaely according to circular mil area of ungrounded conductors

So if I have a 20 amp OCP, number 12 wire, but then i find out that is a long run or maybe it is DERATED for ampacity because of the number of wires in the conduit and i need to increase the size of ungrounded conductors to #10, Even though the OCP is still 20 amp, Does this mean i increase the ground to #10???

What do they mean by proportionately, What ratio do they use for bigger wires. Where can i find that in the code?? Thanks,
 
brother said:
So if I have a 20 amp OCP, number 12 wire, but then i find out that is a long run or maybe it is DERATED for ampacity because of the number of wires in the conduit and i need to increase the size of ungrounded conductors to #10, Even though the OCP is still 20 amp, Does this mean i increase the ground to #10???
Click to expand...

That's what it means. If the run is long enough that voltage drop (which is a product of load current and circuit resistance) is an issue, the EGC needs to have that same voltage drop taken into account to assure operation of the OCP device.
 
LarryFine said:
That's what it means. If the run is long enough that voltage drop (which is a product of load current and circuit resistance) is an issue, the EGC needs to have that same voltage drop taken into account to assure operation of the OCP device.
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What about Derating?? Theres no voltage drop, but had to derate for the number of conductors in the conduit. :)
 
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