Two lighting panels on one feeder, what is wrong here ?

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JEFF MILLAR

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Main breaker 800 amps, 480 / 277 volts. Feeding two lighting panels 1 - RUN 4 - 1 / C , 500 THHN TO SPLICE BOX. Then I run, 4 - 1 / C 500 THHN to each lighting panel. Panel each rated same , 3 ph, 4 wire. 480 / 277. Main bus 400 Amps, main breaker 300 amps.
 
Show if I'm understanding your setup:
800 amp OCPD>4-500 Kcmil>splice box with splices and 4-500 kcmil>panel "A" & 4-500 kcmil> panel "B".

Welcome to the Forum. :)
 
I am not entirely sure what you're trying to describe here. Are you describing a single 500 MCM wire per phase going to a splice block and then splitting off to the panel boards as 500 MCM single wire per phase?

It seems to me this is a tap and the 300 amp breaker on the panel boards would make it more or less legal depending on whether it follow the other tap rules if the breaker feeding the circuit was 400 amp or less. But at 800 amps I don't think so.
 
As I understand your post you have (2) 300 amp M/B panels basically connected to one set of 500 kcmil protected by a 800 amp breaker. If so, obviously a violation as you have the possibility of the single set of 500s being loaded to 600 amps on one 380 amp cable protected by an 800 amp OCP device.
(If the original post has typo and that is a 300 amp main feeding the two 300 amp panels that would not be a violation)
 
Thank you for your answer. I could not understand how that could be allowed to happen. Even if the main breaker was set to protect the 500 kcmil. There would still be the two panels being supplied by the one run of 500 kcmil. 300 amp main limited to 80 % load, equals 240 amps with say 80 % diversity factor equals 192 amps x 2 panels equals 384 amps. 1 / C kcmil , 75C EQUALS 380 AMPS. Thank you for your confirmation. Love this forum and reading all the real World problems and experienced shared.
 
JEFF MILLAR said:
Thank you for your answer. I could not understand how that could be allowed to happen. Even if the main breaker was set to protect the 500 kcmil. There would still be the two panels being supplied by the one run of 500 kcmil. 300 amp main limited to 80 % load, equals 240 amps with say 80 % diversity factor equals 192 amps x 2 panels equals 384 amps. 1 / C kcmil , 75C EQUALS 380 AMPS. Thank you for your confirmation. Love this forum and reading all the real World problems and experienced shared.
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Do you mean that if the 800 amp CB were smaller and protected the 500 kcmil feeder? If the calculated load were less than or equal to the OCPD size what would be the issue?
 
Anyway it is tried to be justified is still wrong. Because the 500 kcmil is rated. at. 380 amps. The total loads on the two panels could never exceed that value 380 amps. If the supply breaker 800 amp. frame, was adjusted. lower say 380 amps or 400 amps. Would that meet code ?. The total load on both panels is around 440 amps. Hard to get your head around this one as it is as- built. How should this be designed ? Feeder to the splice box from the supply sized for 600 amps. And from the splice box to each panel sized for 300 amps. Seems like a simple correction. Any comments ?
 
JEFF MILLAR said:
Anyway it is tried to be justified is still wrong. Because the 500 kcmil is rated. at. 380 amps. The total loads on the two panels could never exceed that value 380 amps. If the supply breaker 800 amp. frame, was adjusted. lower say 380 amps or 400 amps. Would that meet code ?. The total load on both panels is around 440 amps. Hard to get your head around this one as it is as- built. How should this be designed ? Feeder to the splice box from the supply sized for 600 amps. And from the splice box to each panel sized for 300 amps. Seems like a simple correction. Any comments ?
Click to expand...
It may be compliant as is utilizing a tap rule. I don't think anyone fully understands the setup. We would need a better description.
 
To clarify 800 amp main breaker, adjustable, load two lighting panels, 300 Amp main breaker in each.Panel 480 / 277 v. Problem from the 800 amp breaker to a splice box is run 4 - 1 / C. 500 kcmil. ( this is not a typo ). And from the splice box to each panel. is run 4 - 1 / C 500 kcmil. The as - build load on each panel. is. around 240 amps.
 
JEFF MILLAR said:
Anyway it is tried to be justified is still wrong. Because the 500 kcmil is rated. at. 380 amps. The total loads on the two panels could never exceed that value 380 amps. If the supply breaker 800 amp. frame, was adjusted. lower say 380 amps or 400 amps. Would that meet code ?. The total load on both panels is around 440 amps.
Click to expand...

Wouldn't meet Code.

JEFF MILLAR said:
Hard to get your head around this one as it is as- built. How should this be designed ? Feeder to the splice box from the supply sized for 600 amps. And from the splice box to each panel sized for 300 amps. Seems like a simple correction. Any comments ?
Click to expand...

Add a second (parallel) set of 500kcmil from the breaker to the splice box.
 
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