Upsized EGC

[ggunn]

Say I have two boxes some distance apart, and one of them is next to the service. I upsized the CCC's between the boxes for voltage drop, so I must upsize the EGC commensurately. The CCC's between the box next to the service and the service, however, are sized only for minimum ampacity. Do I need to run the upsized EGC between that box and the service or can I size it to T250.122?

This is not an exam question.

 

[ptonsparky]

FWIW, I only upsize the EG where I have increased the CCCs.

#10s to a terminal block, #4s for 1300’ then down to 12s. 20 amp fuses on both ends.

 

[charlie b]

I agree. Any portion of the run that has conductors that are sized for the minimum required ampacity are not "upsized" in this context, and would not require an upsized EGC.

 

[ggunn]

I agree. Any portion of the run that has conductors that are sized for the minimum required ampacity are not "upsized" in this context, and would not require an upsized EGC.

Yes, but the upsized EGC is to allow enough current to flow to clear a fault in the upsized CCC's, right? That current has to pass through the portion of the ECG between the near box and the service as well, so it occurred to me that it might have to be upsized, too. Maybe I am overthinking this; it certainly wouldn't be the first time.

 

[ptonsparky]

Yes, but the upsized EGC is to allow enough current to flow to clear a fault in the upsized CCC's, right? That current has to pass through the portion of the ECG between the near box and the service as well, so it occurred to me that it might have to be upsized, too. Maybe I am overthinking this; it certainly wouldn't be the first time.

Overthinking? Probably.

 

[augie47]

Overthinking? Probably.

agree. The same logic would mean you would have to have oversized phase conductors the entire length.

 

[infinity]

From an engineering standpoint if you have increased the size of the phase conductors due to more than 3 CCC's in a raceway then I don't see why you would need a larger EGC. If you increased them due to voltage drop compensation then that's a completely different story.

 

[augie47]

From an engineering standpoint if you have increased the size of the phase conductors due to more than 3 CCC's in a raceway then I don't see why you would need a larger EGC. If you increased them due to voltage drop compensation then that's a completely different story.

i would have sworn that one of the Code changes addressed that and removed the increase provision when the ungrounded conductors were increased or ambient or CCCs but I can't find it. maybe its an upcoming change.

Edit: I reviewed a 2020 proposal and it appears there may be a 2020 change to address your concern.

 

[ptonsparky]

From an engineering standpoint if you have increased the size of the phase conductors due to more than 3 CCC's in a raceway then I don't see why you would need a larger EGC. If you increased them due to voltage drop compensation then that's a completely different story.

So I can increase the size of my ___ number CCC by 4 times, run them as far as I want due to over three, and not worry about VD because 10% is suggested not required, thus my #12 EG is still good. As long it is not for VD?

 

[infinity]

So I can increase the size of my ___ number CCC by 4 times, run them as far as I want due to over three, and not worry about VD because 10% is suggested not required, thus my #12 EG is still good. As long it is not for VD?

What I'm saying is that if need to run #6 AWG conductors due to derating on a 20 amp circuit a #12 EGC will still open the OCPD. When the EGC is normally sized according to 250.122 derating has nothing to do with the possibility that the EGC is too small to open the OCPD.

 

[winnie]

If you increase the size of conductor for any reason, you are decreasing voltage drop in that conductor. This lets you have a longer run with an acceptable voltage drop on that circuit.

Say you have to run a mess of 20A circuits. If you run them thermally separated (no derating) you can use 12ga Cu conductors. If for reasons of voltage drop you have to increase the conductor size to 6ga, then clearly you need to increase the size of the EGC.

If, keeping the same length of run you bundle all the conductors together in a single (plastic) conduit, you are forced to derate and use 6ga conductors for ampacity reasons. The 6ga will _also_ reduce voltage drop, and thus as good design you should still increase the size of the EGC.

I am not going to try to argue the code text since I don't have the current NEC on tap...I am simply trying to explain why the reason for increasing the size of the EGC still applies for long enough circuits where larger conductors are forced by derating requirements.

-Jon

 

[ptonsparky]

If you increase the size of conductor for any reason, you are decreasing voltage drop in that conductor. This lets you have a longer run with an acceptable voltage drop on that circuit.

Say you have to run a mess of 20A circuits. If you run them thermally separated (no derating) you can use 12ga Cu conductors. If for reasons of voltage drop you have to increase the conductor size to 6ga, then clearly you need to increase the size of the EGC.

If, keeping the same length of run you bundle all the conductors together in a single (plastic) conduit, you are forced to derate and use 6ga conductors for ampacity reasons. The 6ga will _also_ reduce voltage drop, and thus as good design you should still increase the size of the EGC.

I am not going to try to argue the code text since I don't have the current NEC on tap...I am simply trying to explain why the reason for increasing the size of the EGC still applies for long enough circuits where larger conductors are forced by derating requirements.

-Jon

Thank you.

 

[Adamjamma]

If you increase the size of conductor for any reason, you are decreasing voltage drop in that conductor. This lets you have a longer run with an acceptable voltage drop on that circuit.

Say you have to run a mess of 20A circuits. If you run them thermally separated (no derating) you can use 12ga Cu conductors. If for reasons of voltage drop you have to increase the conductor size to 6ga, then clearly you need to increase the size of the EGC.

If, keeping the same length of run you bundle all the conductors together in a single (plastic) conduit, you are forced to derate and use 6ga conductors for ampacity reasons. The 6ga will _also_ reduce voltage drop, and thus as good design you should still increase the size of the EGC.

I am not going to try to argue the code text since I don't have the current NEC on tap...I am simply trying to explain why the reason for increasing the size of the EGC still applies for long enough circuits where larger conductors are forced by derating requirements.

-Jon

This is the logic I use.. if use number ten wire for a twenty amp circuit I run number ten ground as well... but does that mean I need to? Could I just use numbertwelvewire and trust the neutral?

 

[ptonsparky]

This is the logic I use.. if use number ten wire for a twenty amp circuit I run number ten ground as well... but does that mean I need to? Could I just use numbertwelvewire and trust the neutral?

EG & neutral are different. Would you clarify?

 

[ggunn]

From an engineering standpoint if you have increased the size of the phase conductors due to more than 3 CCC's in a raceway then I don't see why you would need a larger EGC. If you increased them due to voltage drop compensation then that's a completely different story.

Well, yes. If you increase the conductor size for temperature derating (and that's what the number of conductors in a raceway derate is) that's an ampacity requirement and therefore the EGC size doesn't change.

 

[petersonra]

Well, yes. If you increase the conductor size for temperature derating (and that's what the number of conductors in a raceway derate is) that's an ampacity requirement and therefore the EGC size doesn't change.

is this in the code somewhere?

 

[Carultch]

is this in the code somewhere?

Not in 2014 and later. Prior to 2014, it didn't specify, but now it specifies "the minimum size that has sufficient ampacity for the intended installation". That tells me, that you account for bundling and temperature derate factors to determine the what I call the "minimum local size", which is the starting point for this calculation. In otherwords, forgetting about length, what minimum size is required? That's the size that needs no EGC upsizing.

Generally the reason you would upsize is to curtail voltage drop, which is the most common reason you need to proportionally upsize the EGC. The idea is for the EGC to have enough conductance (i.e. not too much resistance) to reliably trip the breaker. Another example where you might upsize the CCC's in a way that would make the EGC upsizing rule apply, is if you use leftover wire from a previous job.

 

[petersonra]

however, the minimum size for the ampacity required certainly includes any adjustments for ambient temperature as the ampacity of the conductors varies with temperature.

 

[jaggedben]

Let's actually look at the language. 2014 NEC quoted here. The blue is revised from the 2011 NEC. Did it change again in 2017? I don't have that handy.

(B) Increased in Size. Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors

Emphasis added.

I think you only have to increase the EGC where you increased the ungrounded conductors. For a different section of the run where you didn't increase them, that is not 'where'. You may or may not even need to run a wire-type EGC in some other section of the run, so clearly the requirement does not apply to the entire run.

Under this language the only reasons I can think of that you have to upsize the EGC are if you oversized the ungrounded conductors because of a) voltage drop, or b) that's what you had on the truck. Really, per physics, it should only be for voltage drop.

 

[petersonra]

Let's actually look at the language. 2014 NEC quoted here. The blue is revised from the 2011 NEC. Did it change again in 2017? I don't have that handy.



Emphasis added.

I think you only have to increase the EGC where you increased the ungrounded conductors. For a different section of the run where you didn't increase them, that is not 'where'. You may or may not even need to run a wire-type EGC in some other section of the run, so clearly the requirement does not apply to the entire run.

Under this language the only reasons I can think of that you have to upsize the EGC are if you oversized the ungrounded conductors because of a) voltage drop, or b) that's what you had on the truck. Really, per physics, it should only be for voltage drop.

"sufficient ampacity" certainly includes ampacity corrections for ambient temperature though.