How to size egc

hhsting said:
In the attached pic I have equipment grounding conductor note #8 between combiner and the AC disconnect. How do I size the egc?
Click to expand...

Sorry forgot to attach picture. See below


0f17bfee2d24826192d8de5a859e6ba5.jpg



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Table 250... look there for solar installs (that appears to be what you're doing) points to 690.41 and following. 690.42 says go back to use the sizing in Table 250.122. Basically size the EGC based on the breaker size.
 
They show a 45A breaker on the right side, seemingly in its own breaker enclosure (for some reason I don't understand). So by Table 250.122 the EGC between there and the combiner can be 10awg cu, ordinarily.
 
jaggedben said:
They show a 45A breaker on the right side, seemingly in its own breaker enclosure (for some reason I don't understand). So by Table 250.122 the EGC between there and the combiner can be 10awg cu, ordinarily.
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They upsized the phase and neutral conductors to #4 awg copper but they provided #8 equipment grounding conductor between combiner and ac solar disconnect. Would #8 awg be undersized since they upsize phase/neutral conductors?
 
hhsting said:
They upsized the phase and neutral conductors to #4 awg copper but they provided #8 equipment grounding conductor between combiner and ac solar disconnect. Would #8 awg be undersized since they upsize phase/neutral conductors?
Click to expand...
Well, nothing in the NEC says that you have to take advantage of 75C rated insulation if you have it. So the minimum size conductor for 45A is #6, based on the 60C ampacity column of Table 310.16. Meaning the ungrounded conductors are upsized by 2 AWG (from #6 minimum to #4 provided). As the minimum size EGC for 21-60A is #10, upsizing the EGC by 2 AWG as well to #8 complies with 250.122(B).

[AWG is a geometric sizing system, so the ratio of areas for any conductors whose AWG size differs by a given number is the same.]

Cheers, Wayne
 
hhsting said:
Anyone?


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Keep in mind that the EGC is sized according to the OCPD (breaker) - not the other conductor sizes - in Table 250.122... assuming that sizing the EGC is still the question.

To help out - and I'm guessing about what you're thinking... if you're sizing the current-carrying conductors, you'd use table 310.16, a number 6 conductor @75°C is rated for 65A. But that's not the question you're asking... or were asking. The EGC is sized differently. The assumption is that in normal systems/designs, the EGC never carries current, so it should not be similarly sized.
 
hhsting said:
In the attached pic I have equipment grounding conductor note #8 between combiner and the AC disconnect. How do I size the egc?
Click to expand...
If required, use the ratio between selected and required ungrounded conductor.
CM
#4: 41740
#6: 26240
41740/26240= 1.5907, round down to 1.59

Table 250.122; 45 amp OCPD, #10
CM
#10: 10380
10380*1.59=16504.2
CM
#8: 16510
 
Tulsa Electrician said:
If required, use the ratio between selected and required ungrounded conductor.
Click to expand...
Unnecessary to look up the cmils for AWG 14-4/0, each increase in AWG by 1 unit is a fixed area ratio increase (approximately sqrt(1.59) per your calculation). So if the ungrounded is bumped up 2 sizes, then the EGC gets bumped up 2 sizes.

Once you get to 250 kcmil ungrounded or larger, though, your method is necessary.

Cheers, Wayne
 
I agree as long as you use 60 c and a 6 to 4 for a ratio of 1.59 as your base.

45 amps, minimum conductor size.
#6@ 60c = 55
#8@ 75c = 50

Most will use 75c if applicable. ( No NM cable) and thus #8 egc based on a ratio of 8 to 4 (2.53) would be to small. It would require #4 EGC.

If an inspector saw a THHN in lieu of NM. He call them on it.
If he seen NM it would have a #8 which would apply. Then if they ran 2 #4 THHN and a #8 EGC in raceway it would be a violation. Metal raceway no big deal.
Non metallic raceway more concern as the wire type egc is the sole fault carrying path.

#4 @60c is 70 amp. Therefore the 75c equivalent must be a 70 amp conductor or in this case # 4 @75c. If the load was stated then the 75c conductor may be a different size.

If the calculation came up with a 65 amp conductor required then a #4 @ 60c and #6 @ 75 c. Then the EGC ratio calculation would be a #8 for both.

The key to the answer for me is the calculated ampacity for the conductor change.
Then it's up to the inspector to evaluate the installation.

Pretty sure he in on the 2017 NEC.

I do like the the tip on awg sizing and will pass along to the guys. Never looked at that way.

Thanks.
 
Tulsa Electrician said:
Most will use 75c if applicable. ( No NM cable) and thus #8 egc based on a ratio of 8 to 4 (2.53) would be to small. It would require #4 EGC.
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I will disagree. 110.14(C)(1)(a)(2) lets us treat the 75C/90C insulated conductor as 60C rated if we so choose, as we have a circuit of 100A or less.

It obviously makes zero sense to say that if the circuit is wired with THWN-2 it requires a larger EGC than if it were wired in NM cable (for a fixed size of OCPD and ungrounded conductor).

Cheers, Wayne
 
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