Upsizing EGC for voltage drop

[JP23]

A plan shows a 100 amp 480V breaker feeding two conductors that are upsized from #3 to 350MCM
for voltage drop. The plans call for a #4 grounding conductor with the 350's. I'm calculating something different.

#350 MCM 350,000CM Upsized conductor
#3 52,560CM 100 amp conductor
#8 16,510CM ground

350,000/52,560*16,510 = 109.941CM

#1/0 105,600CM
#2/0 133,100CM

Do I need to use a 2/0 for this run? Seems huge so I'm wondering if I'm doing the calculation incorrectly.

 

[ramsy]

Put your values in me spreadsheet based on NEC tables.

Says #8 must also be up-sized for voltage drop, to #1 AWG, or 83690 CM.
Puts 350cir.mil 480vac feeder 2500 feet 1-way ~ 3% VD, if Pf=1 in PVC conduit.
Puts both 350's & #1 THxN in 2" PVC-A, or PVC-EB. All other raceways are 2-1/2"
End of feeder Point-To-Point fault current @ Pf=0.85 is 1800A on 150kva Xfmr, 2220A on 2500kva.
Puts 100A feeder/terminal temperature rise at 5°C above Ambient when using 350 cir.mils.

This speadsheet uses 2011 NEC tables, references some IEEE Std's & Recommended Practices, but forgot how I did it.

 

[Smart $]

...
Do I need to use a 2/0 for this run? ...

Yes.

 

[ramsy]

Yes.

If engineering supervision can supersede NEC tables, then #4 EGC can be used from the plans.

Apparently my engineered spreadsheet value of #1 EGC, only works for my house, if not inspected.

 

[ramsy]

Apparently my engineered spreadsheet value of #1 EGC, begins with #2, rather than, (#3 with excessive Temp. Rise @ 75.4c)

350,000/66,360*16,510 = 87,078 cir.mils

#1 = 83,690 cir.mils

 

[JP23]

Thanks for the responses! I asked to engineer to revisit his calcs to see if he
is still good with the #4 EGC.

 

[ramsy]

Engineer should except an 85% power factor, for more reasonable diversity of loads, and therefore #1 EGC from NEC table calculation shown above.

 

[petersonra]

Do I need to use a 2/0 for this run? Seems huge so I'm wondering if I'm doing the calculation incorrectly.

It appears this is the right size to me if your CM sizes are correct.

It does seem a little bizarre.

why not skip the wire type EGC and run it in emt or rigid?

 

[JP23]

It appears this is the right size to me if your CM sizes are correct.

It does seem a little bizarre.

why not skip the wire type EGC and run it in emt or rigid?

It's all underground PVC but I see your point. BTW, the engineer revised it to 1/0
Thanks!

 

[Smart $]

...#350 MCM 350,000CM Upsized conductor
#3 52,560CM 100 amp conductor
#8 16,510CM ground

350,000/52,560*16,510 = 109.941CM
...
Do I need to use a 2/0 for this run? ...

Yes.

It's all underground PVC but I see your point. BTW, the engineer revised it to 1/0

1/0 is still a violation. 250.122(B) is quite specific... and nothing in Code permits deviation under engineering supervision.

 

[ramsy]

..nothing in Code permits deviation under engineering supervision.

Engineering supervision has superseded NEC tables for years.
2011 NFPA-70 310.60(C)(4)(D)

 

[ramsy]

I don't understand why the tables can't be used for #1 equipment grounding conductor (EGC), if size before voltage drop begins with #2 for proper terminal/ termination temperature below 75C

 

[david luchini]

Engineering supervision has superseded NEC tables for years.
2011 NFPA-70 310.60(C)(4)(D)

I think you mean 310.15(C). 310.60(C)(4)(D) is for conductors rated 2001 to 35,000V.

But the engineering supervision is for cable ampacity, not equipment ground conductor sizing.

 

[Smart $]

...
But the engineering supervision is for cable ampacity, not equipment ground conductor sizing.

:thumbsup:

 

[ramsy]

... and nothing in Code permits deviation under engineering supervision.

Now that we all agree - Code does permit deviation under engineering supervision - for more than 1 ampacity table:

Do we also agree, Code allows a size #1 EGC, if size before voltage drop can be #2, and 350 cir.mils after voltage drop?

350,000/66,360*16,510 = 87,078 cir.mils

#1 = 83,690 cir.mils
#1/0 = 105,600 cir.mils

 

[Smart $]

Now that we all agree - Code does permit deviation under engineering supervision - for more than 1 ampacity table:

Do we also agree, Code allows a size #1 EGC, if size before voltage drop can be #2, and 350 cir.mils after voltage drop?

350,000/66,360*16,510 = 87,078 cir.mils

#1 = 83,690 cir.mils
#1/0 = 105,600 cir.mils

No, I do not agree.

The size used must equal or exceed the result of the calculation, even if you round to four significant figures.

 

[JP23]

1/0 is still a violation. 250.122(B) is quite specific... and nothing in Code permits deviation under engineering supervision.

Thanks for that input.

 

[ramsy]

The size used must equal or exceed the result of the calculation, even if you round to four significant figures.

Thank you for that.

Nothing less than 1/0 allowed, therefore #4 on the plans was an error with the values given.

 

[ramsy]

Does 250.122(B) apply to any "increase in size" from ambients, continuous loads, CCC's, motor-power factors, and voltage drop?

Does 250.122(B) apply to Neutral feeders, per 215.2(A)(2), except for "continuous loads" in 215.2(A)(1) Exception #2?

 

[Smart $]

Does 250.122(B) apply to any "increase in size" from ambients, continuous loads, CCC's, motor-power factors, and voltage drop?

Any increase to larger than the minimum required by Code. Ambient temperature correction, adjustment for number of current-carrying conductors in raceway/cable, factoring for continuous/motor loads are all Code-required for sizing. Voltage drop compensation, "only had larger size on the truck", and similar are size increases not required by Code.

Does 250.122(B) apply to Neutral feeders, per 215.2(A)(2), except for "continuous loads" in 215.2(A)(1) Exception #2?

Applies to any circuit conductor. However, I do not consider permitted size reduction(s), where section text contains the phrase shall be permitted, as the required minimum size. But there are those that may. Perhaps your friendly neighborhood AHJ. Too few have voiced their opinion the matter here to provide any impression on general consensus.