400 amp 208v Wye Service Size

J

JDye

Member
Location
Missouri
Hello everyone and thank you. I have been a long time forum reader and I'm now currently in the process of Mike Holt's training on the 2023 cycle. I'm sure this type of question has come up before but I had trouble finding it.

That being said, what am I missing?

400 amp 208y/120 three phase main panel service wire size for commercial property. The client moving in is a micro brewery with the only load information being "our family electrician said we maxed out our 200 amp and we need a 400 amp." That's all we have to go with for load calculations. The landlords want us to install the service and they provided a 400 amp main panel. The wire will be aluminum and ran in 2 runs parallel. Each run will be in PVC underground from the utilities disconnect to the panel.

Assuming that most of the loads won't be 3 phase, I'm calculating the neutral as a current carrying conductor. Wire will be modern wire and either of the RHW-2 or THWN-2 multi-rated jacketed wire family. Lugs are 75C.

If I use the 75C column for the aluminum wire, then derate to 80% { 310.15 (c)(1) 4 current carrying conductors} I have to use a 350kcmil wire to get up to exactly 400 amps. The lugs on the meter can and disconnect provided by the city only go up to (2) 250kcmil. I know it's a 320 amp continuous and 400 amp peak meter. Anything above that and the city puts you on demand. If I use the 90C column to derate my wire, I can get by with a 300kcmil but that still doesn't explain why my factory lugs are only good for up to (2) 250kcmil. And by going to the 90C column for my wire wouldn't I violate 110.14 (c)?

If the meter can is only good for 320 amps, is that what I size to? If I do then (2) 250kcmil at 75C gets me to 328 amps which is over 320 amp but a long ways away from 400. At 90C (2) 250kcmil only gets me to 368 amps. I feel like having 400 amp fuses (city's fused disconnect), and a 400 amp main, I should be a lot closer to that 400 amp mark and if the meter fails, that's the city's problem to replace. I know you can round up breaker size to the next size breaker { 240.4 (b) } but I have no way of knowing the trip settings on the used main breaker the client provided.

I had felt confident in my in my 350kcmil calculations until I noticed the factory installed lugs weren't big enough. Then I started to second guess myself and now I feel like I'm missing something drastic and possibly over-sizing my wire but I want to do it right. If I have missed something please quote the reference number so that I may read it myself.

Again thank you.
 
JDye said:
Assuming that most of the loads won't be 3 phase, I'm calculating the neutral as a current carrying conductor.
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Very likely unnecessary. The scenarios in which you have to count the neutral as a current carrying conductor are rare; it's not automatically true just because you have single phase loads. So you probably have only 3 CCCs.

JDye said:
If I use the 75C column for the aluminum wire, then derate to 80% { 310.15 (c)(1) 4 current carrying conductors} I have to use a 350kcmil wire to get up to exactly 400 amps. The lugs on the meter can and disconnect provided by the city only go up to (2) 250kcmil.
Click to expand...
If you had to apply ampacity adjustment for 4 CCCs (you probably don't), you could start with the 90C ampacity just for that calculation.

Also, ampacity adjustment and correction does not apply at terminations. So 2 * 250 kcmil Al will get you to 400A within the meter can. The complexity arises if you were to need a larger size outside the meter can because of the ampacity adjustment or correction, then you'd have to splice to the larger size somewhere.

But based on the information provided, with two parallel conduit runs, you could use 250 kcmil Al for the ungrounded conductors in each conduit run.

Cheers, Wayne
 
wwhitney said:
Very likely unnecessary. The scenarios in which you have to count the neutral as a current carrying conductor are rare; it's not automatically true just because you have single phase loads. So you probably have only 3 CCCs.
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Isn't the neutral counted as a CCC in a Wye service?
 
Little Bill said:
Isn't the neutral counted as a CCC in a Wye service?
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1. If it has 2 hots and 1 neutral then there are 3 CCC.

2. If it has 3 hots and 1 neutral there are 3 CCC because there is presumably no imbalance on the neutral.

3. If it has 3 hots and 1 neutral and there is a lot of imbalance then there will be little current on one of the hots, therefore see #1.
 
Birken Vogt said:
3. If it has 3 hots and 1 neutral and there is a lot of imbalance then there will be little current on one of the hots, therefore see #1.
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It would take a ton of 3n-order harmonics to make the neutral a current-carrying conductor, not just imbalance of the loads between the 3 hots.

I.e. the worst case unbalanced on a 100A wye service from power factor 1 loads would be L1/L2/L3/N currents of (100, 100, 0, 100). That's the same heating as a completely balanced loading of (100, 100, 100, 0), no worse. Mixed cases like (100, 100, 50, 50) would be less heating than either of those. [Heating goes as I2, so just sum the squares of the 4 currents to get a value proportional to heating rate, assuming equal conductor sizes and lengths.]

Cheers, Wayne
 
Birken Vogt said:
2. If it has 3 hots and 1 neutral there are 3 CCC because there is presumably no imbalance on the neutral.
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We are picky about clarity here and this is incorrect. If it has 3 hots and neutral with no harmonic loads, the neutral ONLY carries the unbalanced load between the phases. So what Wayne wrote is not what you said.
 
JDye said:
The client moving in is a micro brewery with the only load information being "our family electrician said we maxed out our 200 amp and we need a 400 amp."
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Sounds like a lot of money spent and work happening with very little upfront info.
We have done a few micro breweries, also working for the 'build to suit' property owner and none of them wanted 208.
I am not sure if its fact or fiction but one had a brewing consultant engineer and they were adamant they needed 240 volts as part of their 'build to suit', there is a larger one that has a 480 service.
 
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