AHJ discussion with 250.122(B) and 4CCC adjustment.

[mjmike]

We got a plan review back from the AHJ where they noted to upsize the ground conductor where the ungrounded conductors are increased per 250.122(B). However, the phase conductors are larger due to ampacity adjustment and not due to voltage drop. (They were increased in size due to the circuit being 3-phase 4W 208 and considering the neutral as a CCC. Therefore we have 4CCC and derated by 80% based on the 90 degree column as opposed to the 75 degree column per 110.14(C).)

We noted that per the 2011 NEC handbook in the text below 250.122(B), it indicates the ground does not need to increase in size due to ampacity adjustment. "Increases in ungrounded circuit conductor size for the purposes of ampacity adjustment, correction, or both are not required to be considered in applying the provisions of 250.122(B)." However, the AHJ is using the 2008 NEC. Does similar language exist in the 2008 NEC? We were not able to locate it.

We are hoping we can simply indicate the conductors are larger due to ampacity adjustment which does not require upsizing the ground.

We then explained to the AHJ how we derated the conductors using the 90 degree column and referenced section 110.14(C). The AHJ noted you can't do that unless all the terminals etc are rated for 90 degree C. We then explained to the AHJ that if after applying the derating that the size we show is the first acceptable size conductor then we really didn't upsize.

Even if we wanted to upsize the ground proportionally to accommodate the AHJ, and we are starting with the first available conductor size based on 80% of 90 degree column (even though we are using the 75 degree terminations), what conductor would we then use as a starting point?

 

[wwhitney]

FWIW the wording of 250.122(B) is the same in 2008 and 2011:

(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors.
​

Cheers, Wayne

 

[Carultch]

We got a plan review back from the AHJ where they noted t upsize the ground conductor where the ungrounded conductors are increased per 250.122(B). However, the phase conductors are larger due to ampacity adjustment and not due to voltage drop. (They were increased in sizoe due to the circuit being 3-phase 4W 208 and considering the neutral as a CCC. Therefore we have 4CCC and derated by 80% based on the 90 degree column as opposed to the 75 degree column per 110.14(C).)

We noted that per the 2011 NEC handbook in the text below 250.122(B), it indicates the ground does not need to increase in size due to ampacity adjustment. "Increases in ungrounded circuit conductor size for the purposes of ampacity adjustment, correction, or both are not required to be considered in applying the provisions of 250.122(B)." However, the AHJ is using the 2008 NEC. Does similar language exist in the 2008 NEC? We were not able to locate it.

We are hoping we can simply indicate the conductors are larger due to ampacity adjustment which does not require upsizing the ground.

We then explained to the AHJ how we derated the conductors using the 90 degree column and referenced section 110.14(C). The AHJ noted you can't do that unless all the terminals etc are rated for 90 degree C. We then explained to the AHJ that if after applying the derating that the size we show is the first acceptable size conductor then we really didn't upsize.

Even if we wanted to upsize the ground proportionally to accommodate the AHJ, and we are starting with the first available conductor size based on 80% of 90 degree column (even though we are using the 75 degree terminations), what conductor would we then use as a starting point?

NEC2014 added language that states "from the minimum size that has sufficient ampacity for the intended installation".

This means that if only ampacity, OCPD, terminals, wire ampacity, and derates govern the conductor sizing, that you did not "increase in size" and do not need to increase the EGC. If by contrast, you increased in size due to leftover wire from a previous job, or to accomodate voltage drop, then that is when you have to proportionally upsize the EGC.

110.14(C) states "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, temperature corrections, or both", which is precisely the opposite of what the AHJ is telling you. This is the primary reason why there is a value in having 90C wire, even if terminals are only rated for 75C.

 

[charlie b]

Your situation will be governed by the NEC code cycle in effect in your area. If it is 2014, then carultch has your answer. If it is earlier, then I suggest discussing the 2014 version with the AHJ.

 

[mjmike]

Your situation will be governed by the NEC code cycle in effect in your area. If it is 2014, then carultch has your answer. If it is earlier, then I suggest discussing the 2014 version with the AHJ.

When we discussed with the AHJ, we referenced the 2011 NEC for not increasing the ground for ampacity and they indicated we had to use the 2008 NEC. They do not appear open to using a newer code version. Therefore, what code references would we be able to use from the 2008 NEC for not increasing the ground due to ampacity adjustment?

If there is not verbiage and we do the calculation to upsize the ground, we are having difficulty determining what our starting conductor size would be. For example, for a 225A ckt thhn/thwn-2 CU, the derated conductor would be a #250 MCM (good for 232A at 80% of 90 deg column). Would we use #4/0 with an amperage of 230A from the 75 degree column as if 3 CCC?

 

[GoldDigger]

I think that if you can look at the adoption process for the 2014 change you will see statements by the CMP that the language simply clarified what was intended by the earlier wording and was not a change in policy.

 

[mjmike]

I think that if you can look at the adoption process for the 2014 change you will see statements by the CMP that the language simply clarified what was intended by the earlier wording and was not a change in policy.

I agree with you. However, the AHJ is not open to looking at any other code addition then the currently 2008 adopted code we must somehow to prove the ground does not change when applying ampacity adjustment.

 

[electrofelon]

As an aside, the neutral of a three phase system is rarely a CCC. Granted the NEC is very vague on what constitutes a "nonlinear" load, but IMO something being "electronic" doesn't make it "nonlinear".

 

[charlie b]

Here is the essence of the situation:

We then explained to the AHJ that if after applying the derating that the size we show is the first acceptable size conductor then we really didn't upsize.

The problem with the earlier versions of this article, including the 2008 and 2011 versions, is that they speak of "increased in size." But they don't answer the question, "Increased from what?" What is the starting point, the wire size that could have been used, whereas you choose to use a larger size?

I had been trying to get this clarified for several cycles, and it finally got clarified. GoldDigger said it right: they didn't change the code, they only clarified the intent. Let's take a specific example:

• Install (5) 20 amp, single phase circuits in the same conduit.
• That's 10 current-carrying conductors, and requires a derating factor of 50%.
• Select THHN wire. You can therefore start the derating from the 90C column.
• If you try to use #12, you will derate the 30 amp value to 15 amps.
• Since you can't protect a wire that has a 15 amp ampacity with a 20 amp breaker, you cannot use #12 wire.
• Similar logic tells us that a #10 wire will be acceptable with 20 amp breakers (derated ampacity is 20 amps).
• Therefore, the use of #10 instead of #12 is not an increase in size. It is merely the selection of the smallest wire size that has sufficient ampacity.
• I conclude that using #10 wire for the hot and neutral and a #12 wire for the EGC is compliant, even with the 2008 edition of the NEC.

 

[Carultch]

But they don't answer the question, "Increased from what?"

Bring in parallel sets, and the 2014+ language still has ambiguity in the question "increased in size from what?". As an example, consider a 400A circuit, built with two parallel sets of 400 kcmil Cu. How do you know whether the starting point for this calculation has to be two parallel sets of #3/0 Cu, or could be a single set of 600 kcmil Cu (which is in your favor)?

 

[charlie b]

That's a good point, and I agree there is room for interpretation. My take would be that since you chose to use two parallel sets, then the smallest wire that would have had sufficient ampacity is the 3/0. So you would be increasing from 167.8 KCMIL to 400 KCMIL, and the ground wire would go from a #3 to a 2/0.

 

[ggunn]

We got a plan review back from the AHJ where they noted to upsize the ground conductor where the ungrounded conductors are increased per 250.122(B). However, the phase conductors are larger due to ampacity adjustment and not due to voltage drop. (They were increased in size due to the circuit being 3-phase 4W 208 and considering the neutral as a CCC. Therefore we have 4CCC and derated by 80% based on the 90 degree column as opposed to the 75 degree column per 110.14(C).)

Why did you do that?