75*C vs 90*C

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Vlietracer64

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I'm working off a spec for new service provided by companies in house electrician.
New 480V 1200-Amp service to gear with GFCI main breaker, now spec called out 3 runs of 500-MCM wire to feed from pole mounted transformer bank.

This works at 90*C but the spec provided just states breaker (1200A Adj, 3P [NGS 1200A Frame], Trip 1200 A, 310+, (4) 4/0-500 kcmil),
Dose not call out temp rating of connections and always been little confusing to me so i's always been taught to always size off 75*C unless it was a service where we crimped lugs on both ends of wire and bolted to bussing but then had to verify buss was good for 90*C so it was always standard to use 75*C

Since largest conductor on this breaker is 500 mcm i would need 4 runs of 350 MCM in 4 conduits to make this proper.

Ive had a few other post about transformers today from there spec and everything has been sized for 90*C not sure if person who wrote spec had a clue or im missing something between 75 and 90*C uses?

Thank you
 
The person writing the spec is not acquainted with 240.4(C)
The only way it would be permissible would be if the terminated on a 90o breaker which is highly unlikely at 480v gear.
 
As Augie stated forget using the 90° C ampacity and calculate your conductors based on the 75° C ampacity. Since you're over 800 amps (no next standard size up rule) you'll need to meet or exceed the 1200 amp OCPD size with your conductors.
 
Looks like ill be submitting a corrected bid,
Got a call back and they stated im making it more complicated, their head of maintenance used to be a commercial electrician and knows what he's doing and another contractor already gave them pricing and didn't mention anything was wrong :happysad:
 
Vlietracer64 said:
...their head of maintenance used to be a commercial electrician and "knows what he's doing" ...
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Big deal. I used to be a commercial electrician, too, and I'm reading this thread so I can learn something about something outside of my expertise.

The typical commercial electrician couldn't design anything. They already have the plans and don't know enough to question them.
 
I see (often older) installations all the time that seem to use the "next size up" rule above 800 amps. I use to always think that they must have thought they could always use next size up, but that is a good point - maybe they were (incorrectly) using the 90 degree column for conductor sizing - never thought of that.
 
Same as you guys, i did commercial / industrial work for many years but its easy when your making wire pull and transformer connection off a designed, engineered stamped drawing
 
oldsparky52 said:
If this is a bid, I suggest you bid it the way it's shown and make sure your bid says per plans. Then, before signing a contract, hash it out.
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Not so much of a bid as a in house design and build project they want pricing on, im going to re-work it and price it correctly and have a meeting with them this friday to discuss as it will not pass if built as per there current design
 
Vlietracer64 said:
I'm working off a spec for new service provided by companies in house electrician.
New 480V 1200-Amp service to gear with GFCI main breaker, now spec called out 3 runs of 500-MCM wire to feed from pole mounted transformer bank.

This works at 90*C but the spec provided just states breaker (1200A Adj, 3P [NGS 1200A Frame], Trip 1200 A, 310+, (4) 4/0-500 kcmil),
Dose not call out temp rating of connections and always been little confusing to me so i's always been taught to always size off 75*C unless it was a service where we crimped lugs on both ends of wire and bolted to bussing but then had to verify buss was good for 90*C so it was always standard to use 75*C

Since largest conductor on this breaker is 500 mcm i would need 4 runs of 350 MCM in 4 conduits to make this proper.

Ive had a few other post about transformers today from there spec and everything has been sized for 90*C not sure if person who wrote spec had a clue or im missing something between 75 and 90*C uses?

Thank you
Click to expand...

500 kCMIL AL or CU? How is it to be run? Buried in conduit underground? In a building ?
 
Vlietracer64 said:
They Specked 500 MCM copper in conduit underground from utility pole to 1200-Amp Main breaker in outdoor 3R gear

3 - 4" comduits
4 500 mcm CU per conduit
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Ampacity of 500kCMIL CU @ 75C 310.15(B)(16) = 380 Amps
Maximum Ambient temp of earth 30" deep (in my region) = 18.89 C
T310.15(B)(2)(a) adjustment @ 75C = 1.11
380 * 1.11 = 421.8 A
Adjust for 4 current conductors (Y system neutral counts) 310.15(B)(5) 421.8 * .8 = 337.44A
337.44 * 3 = 1012.31
Well its close but no go I'd say 1000A OCPD
If you did not have to count that neutral in the Y system you would be good.
 
tortuga said:
Ampacity of 500kCMIL CU @ 75C 310.15(B)(16) = 380 Amps
Maximum Ambient temp of earth 30" deep (in my region) = 18.89 C
T310.15(B)(2)(a) adjustment @ 75C = 1.11
380 * 1.11 = 421.8 A
Adjust for 4 current conductors (Y system neutral counts) 310.15(B)(5) 421.8 * .8 = 337.44A
337.44 * 3 = 1012.31
Well its close but no go I'd say 1000A OCPD
If you did not have to count that neutral in the Y system you would be good.
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110.14(C)(1) does not permit you to do that. It specifies the maximum conductor ampacity is the ampacity shown in Table 310.15(B)(16).
 
don_resqcapt19 said:
110.14(C)(1) does not permit you to do that. It specifies the maximum conductor ampacity is the ampacity shown in Table 310.15(B)(16).
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Thanks Don good point I have always looked at 310.15(A)(2) Exception. Its interesting that 110.14(C)(1) ignores all the derating that might need to happen as well.
We always terminate a service lateral on a CT mounting base not a breaker like these:
CT mounting bases
Ill have to check the temp rating of the lugs on there.
Cheers
 
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