Proportional Increase of EGC for VD

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Jerramundi

Jerramundi

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Chicago
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Licensed Residential Electrician
Hey all, question about some code lingo for you.

My question is in regards to 250.122 (B) Increased Size in which, if we increased the size of ungrounded conductors for the purposes of voltage drop, we are required to increase the size of the EGC proportionately.

I'm working with the 2014 Handbook which has some blue text following the explicit code requirements that gives notes on the code itself and calculation examples.

In this blue text, it says "...where ungrounded conductors are increased in size to compensate for voltage drop... the EGCs must be increased proportionately. In some cases, use of a conductor with a higher insulation temperature rating allows for compliance with ampacity adjustment and correction requirements without having to increase the circular mil area of the conductor.

This underlined section is the portion I'm curious about.

We are demoing an existing sign, extending the run to a new location ~40 ft. away, and installing the feed for a new LED one.
The total length of the run from Panel to New Sign is ~271 ft.
The existing interior conduit run inside the building is 1/2" EMT with an existing 3-wire #12 network handling some other loads.
The existing exterior run is 1/2" Schedule 80 PVC, which will be completely unfilled/empty once the existing feed is removed.
I'm trying to utilize both existing raceways and just extend the existing PVC to the new location to save on cost.

Being mindful of VD and of course project costs, I've calculated that the largest pair of ungrounded conductors I can fit inside the interior 1/2" EMT is 2 x #8 AWG THHN (Hot, Neutral), utilizing the EMT itself as the EGC. I plan to add an EGC conductor for the exterior PVC run to avoid utilizing the earth as a ground-fault return path. Thus, the exterior PVC would contain 3 x #8 AWG THHN wires, which just barely crosses the 40% threshold.

Regarding the above underlined section, how would this apply to me potentially utilizing a slightly smaller EGC such as a #10 AWG, but a different wire type with a higher insulation temperature rating to fulfill conduit fill requirements?

Would I have to match the insulation temperature rating of the required #8 EGC with a different type of wire?
 
Something is off on your area calculations. The internal area of 1/2" Schedule 80 PVC is 0.217 in^2, and the area of #8 THWN is 0.0366 in^2, per Chapter 9. So (3) #8 THWN in 1/2" Schedule 80 would be a 50.6% fill, way over 40%.

Cheers, Wayne
 
First question is are you required to maintain some specified level of voltage drop? How much current does this sign draw, and how tolerant is it of reduced voltage?

I kind of disagree with the underlined portion of the note you mentioned. The purpose of increasing the EGC in a long run is to reduce the resistance of the fault path. If the ungrounded conductor is adding a lot of resistance to the circuit, the EGC will too during a fault. The insulation doesn't matter during a fault, just the impedance of the fault path (the EGC could be bare for all we care). If the run is long, but the ungrounded and EGC should be upsized.

240.4d is going to limit using smaller higher temp conductor exceptions on this circuit, assuming it is a 15 or 20A circuit.
 
wwhitney said:
Something is off on your area calculations. The internal area of 1/2" Schedule 80 PVC is 0.217 in^2, and the area of #8 THWN is 0.0366 in^2, per Chapter 9. So (3) #8 THWN in 1/2" Schedule 80 would be a 50.6% fill, way over 40%.

Cheers, Wayne
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You're correct Wayne. Thank you. I've been bouncing around looking at multiple different solutions and somewhere I mixed up some numbers.

The general questions still stands though regarding utilizing a higher insulation temperature rated conductor for the EGC to avoid having to increase it proportionately.
 
Jerramundi said:
In some cases, use of a conductor with a higher insulation temperature rating allows for compliance with ampacity adjustment and correction requirements without having to increase the circular mil area of the conductor.

This underlined section is the portion I'm curious about.
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I believe I can explain what the Handbook’s blue text (that you underlined) was intended to mean.

Consider a load that calculates out (125% of its continuous load plus 100% of its non-continuous load) right at 30 amps. Thus, you will need a wire that has every bit of 30 amps of ampacity.

Now consider a conduit that holds two single phase, 120 V circuits that will feed these 30 amp loads. That’s 4 current-carrying conductors, so you have to apply an 80% “adjustment factor.” If you had planned to use type THWN wire (75C insulation system), the 75C column’s tabulated ampacity of a #10 is 35 amps. When you apply the 80% factor, the adjusted ampacity becomes 28 amps. That is less than is needed. So you will have to upsize the ungrounded conductors from #10 to #8.

(SIDE NOTE: The question of whether this will also require you to also upsize the EGC depends on which edition of the NEC applies in your area.)

If you instead choose to install THHN wire (90C insulation system), then you get to apply the 80% adjustment factor to the value in the 90C column. Thus, the “adjusted ampacity” becomes 80% of 40 amps, or 32 amps. In that case, you don’t have to upsize your ungrounded conductors. That is what that Handbook note is telling you.
 
Jerramundi said:
The general questions still stands though regarding utilizing a higher insulation temperature rated conductor for the EGC to avoid having to increase it proportionately.
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As I mention in my post #7, the Handbook note is not talking about the insulation system or the size of the EGC.
 
suemarkp said:
First question is are you required to maintain some specified level of voltage drop? How much current does this sign draw, and how tolerant is it of reduced voltage?

I kind of disagree with the underlined portion of the note you mentioned. The purpose of increasing the EGC in a long run is to reduce the resistance of the fault path. If the ungrounded conductor is adding a lot of resistance to the circuit, the EGC will too during a fault. The insulation doesn't matter during a fault, just the impedance of the fault path (the EGC could be bare for all we care). If the run is long, but the ungrounded and EGC should be upsized.

240.4d is going to limit using smaller higher temp conductor exceptions on this circuit, assuming it is a 15 or 20A circuit.
Click to expand...

I'm just going off the general NEC recommendation of 3% for branch circuits and seeing what the largest size wire is that I could fit in the existing conduit to minimize both the cost of the project and the voltage drop as much as possible. I will be pricing out a second option of replacing the existing conduit, but I'm trying to see this one through first. I've read that most equipment can handle up to a 10% VD, but I still have to check with the sign company to see what their equipment can explicitly tolerate.

The #8 wires are the largest I can fit in the interior conduit, minimizing the VD to ~5.5%. However, it's my understanding that I would be required to increase the EGC running through the PVC portion proportionately, which violates conduit fill. I just read this note and was hoping to get some input as to what exactly it was referring to.

The sign itself is only pulling 11.33A continuous, but I'm doing my calculations for a 20A load because... (1) there is a second portion of the sign that has actual lamps for which I was not given a value, (2) they want an outlet there to plug in for general use, and (3) are requesting a couple in ground flood lights around the sign.

No I have not completed the precise calculations YET and understand that is a mortal sin, lol. I'm just spit balling some different potential solutions at this point and was really only curious about that explicit hand book note. Perhaps it's my fault for providing too much information for you guys to nitpick, haha.. but I was just trying to provide some context.
 
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Jerramundi said:
The general questions still stands though regarding utilizing a higher insulation temperature rated conductor for the EGC to avoid having to increase it proportionately.
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The answer is still no. There is no exception to the proportional increase requirement based on insulation temperature rating.

Transform the voltage up at the source of the circuit, and back down at the sign.
 
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charlie b said:
As I mention in my post #7, the Handbook note is not talking about the insulation system or the size of the EGC.
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Are you certain this note doesn't apply to the EGC? It's written directly following 250.122(B) - Increased Size, which is discussing the proportional increase of the EGC when the ungrounded conductors are up-sized for voltage drop purposes...
 
Jerramundi said:
Are you certain this note doesn't apply to the EGC? It's written directly following 250.122(B) - Increased Size, which is discussing the proportional increase of the EGC when conductors are up-sized for voltage drop purposes.
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Yes I am certain. That article tells us to increase the EGC IF we had increased the ungrounded. It is giving us a way to NOT increase the ungrounded.
 
charlie b said:
Yes I am certain. That article tells us to increase the EGC IF we had increased the ungrounded. It is giving us a way to NOT increase the ungrounded.
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I'm not talking about NOT increasing the ungrounded conductors. I'm talking about the proportional increase of EGC. That is, I'm upsizing the ungrounded conductors for a 20A load from #12 to #8 for voltage drop purposes... 250.122(B) requires me to increase the size of the EGC proportionately. Then this note hit me and I was wondering if it was referring to say, for example, the ability to utilize a #10 EGC instead of #8 based as insulation temperature ratings.
 
david luchini said:
The answer is still no. There is no exception to the proportional increase requirement based on insulation temperature rating.
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What if you have a 20A load w/ ungrounded conductors sized according the 75 deg. C column (i.e. #14 AWG THWN - 20A)...

Then, for the purpose of voltage drop, you up size the #14 AWG THWN ungrounded conductors to #12 AWG THWN (i.e. 25A @ 75 deg. C)...

So instead of proportionately increasing the EGC to a #12 AWG THWN...
Could you then pull a 90 deg. C Equipment Grounding Conductor (#14 THHN - 25A)??

Or would this run in direct contradiction to Table 250.122?

Forget the specifics of my particular installation. I just want to talk about this handbook note in particular.
 
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Jerramundi said:
I'm not talking about NOT increasing the ungrounded conductors. I'm talking about the proportional increase of EGC. That is, I'm upsizing the ungrounded conductors for a 20A load from #12 to #8 for voltage drop purposes... 250.122(B) requires me to increase the size of the EGC proportionately. Then this note hit me and I was wondering if it was referring to say, for example, the ability to utilize a #10 EGC instead of #8 based as insulation temperature ratings.
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Not sure why the temperature rating matters but a 20 amp circuit with #12 ungrounded conductor requires a #12 EGC, that's a 1 to 1 ratio. Increasing the ungrounded conductor to a #8 would require the same 1 to 1 ratio so a #8 EGC is required.
 
Jerramundi said:
What if you have a 20A load w/ ungrounded conductors sized according the 75 deg. C column (i.e. #14 AWG - 20A)...
Then, for the purpose of voltage drop, you up size the #14 AWG ungrounded conductors to #12 (25A @ 75 deg. C)...
So instead of proportionately increasing the EGC to a #12...
Could you then pull a 90 deg. C Equipment Grounding Conductor (#14 - 25A)??
Click to expand...

A 20A load would generally require #12 AWG ungrounded conductors to start with. (Ignoring motors, A/C units, etc.)

The proportional increase from #14 to #12 is ungrounded conductors is1.5888.

The proportional increase for a #14 egc is 4110cm * 1.5888 = 6530cm, which just happens to be #12awg.

The insulation rating temperature is irrelevant.
 
david luchini said:
A 20A load would generally require #12 AWG ungrounded conductors to start with. (Ignoring motors, A/C units, etc.)

The proportional increase from #14 to #12 is ungrounded conductors is1.5888.

The proportional increase for a #14 egc is 4110cm * 1.5888 = 6530cm, which just happens to be #12awg.

The insulation rating temperature is irrelevant.
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I understand how to do the math regarding the proportional increase. I'm just trying to make sense of this Handbook Note which explicitly mentions insulation temperature ratings.

I'm in agreement with all of you in regards to increasing the EGC proportionately. That is if I upgrade the #12 ungrounded conductors to #8, the EGC should also be a #8. I'm not refuting this fact. Typically I would do the exact same thing.

I'm just trying to make sense of this Handbook Note which explicitly says, "...where ungrounded conductors are increased in size to compensate for voltage drop... the EGCs must be increased proportionately. In some cases, use of a conductor with a higher insulation temperature rating allows for compliance with ampacity adjustment and correction requirements without having to increase the circular mil area of the conductor.

Given the context of this wording, I can't help but feel like it applies to the EGC, even if only rarely ever in very few circumstances.
 
Jerramundi said:
I'm in agreement with all of you in regards to increasing the EGC proportionately. That is if I upgrade the #12 ungrounded conductors to #8, the EGC should also be a #8. I'm not refuting this fact. Typically I would do the exact same thing.

I'm just trying to make sense of this Handbook Note which explicitly says, "...where ungrounded conductors are increased in size to compensate for voltage drop... the EGCs must be increased proportionately. In some cases, use of a conductor with a higher insulation temperature rating allows for compliance with ampacity adjustment and correction requirements without having to increase the circular mil area of the conductor.

Given the context of this wording, I can't help but feel like it applies to EGC.
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Reread the part in red. Ampacity adjustment and correction requirements have nothing to do with EGCs.
 
@Jerramundi

What good would a higher insulation temperature rating on an EGC accomplish? What if the EGC was bare cu?

Speaking of bare cu, what would the percentage of fill be if you used a bare #8 cu EGC?
 
david luchini said:
Reread the part in red. Ampacity adjustment and correction requirements have nothing to do with EGCs.
Click to expand...
I understand your point and for the most part agree. However the note I'm quoting is contained within 250.122(B) immediately following the requirement for a proportional increase of the EGC. I can't imagine they would put a note in that was completely irrelevant to the parent section.

I can't help but feel like the example I gave above in Post #14 regarding 75 deg. C versus 90 deg. C would be one example that would make sense of this note, even it's the ONLY one.
 
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