3x500 for 1200A panel

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greenspark1

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Why do I keep seeing 1200A panels fed by 3 sets of 500 kcmil? 500 is good for 380A @ 75C so per 240.4 can't be used for a 1200A panel. Is there a history here that I don't know of?
 
greenspark1 said:
Why do I keep seeing 1200A panels fed by 3 sets of 500 kcmil? 500 is good for 380A @ 75C so per 240.4 can't be used for a 1200A panel. Is there a history here that I don't know of?
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It could be a 1200A panel that doesn't fully utilize all 1200A. Such as a 1100A fused disconnect upstream, or 6 breakers inside that only add up to 1100A.
 
greenspark1 said:
Why do I keep seeing 1200A panels fed by 3 sets of 500 kcmil? 500 is good for 380A @ 75C so per 240.4 can't be used for a 1200A panel. Is there a history here that I don't know of?
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It could be the supervised industrial exemption in 240 VIII
 
JFletcher said:
Curious as to why the mfgs havent pushed to get 500MCM to 400A, or make like 525MCM.
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Currently no wire and cable company wants to dig into the weeds and buck the 310.15(B)(16) complacency.Even if we did want to increase the ampacity of a conductor we would have mountains of submittals, code panels to convince, rival companies to dispute....among the obvious things. (not so obvious...MONEY)

As for DLO.....fine but you wont find it acceptable for building wire in 310.10 and 310.104 so....at the end of the day for normal building wire the values of 310.15(B)(16) will continue to rule the day....and probably wont change in my lifetime.
 
It is permissible to use the next higher standard fuse or breaker size....generally per exception 310.15 A 2 and 310.15 C


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rudiseldb said:
It is permissible to use the next higher standard fuse or breaker size....generally per exception 310.15 A 2 and 310.15 C


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You can often use the next standard OCPD size, but you cannot apply the same argument to allow the wiring itself to carry more than the calculated load.
 
MasterTheNEC said:
....
As for DLO.....fine but you wont find it acceptable for building wire in 310.10 and 310.104 so....at the end of the day for normal building wire the values of 310.15(B)(16) will continue to rule the day....and probably wont change in my lifetime.
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If it is only marked DLO, that is correct, but it is easy to find DLO/RHH or RHW and that is code compliant. The only issue is what ampacity it can be used at, as there is no provision for the ampacity of non-standard sized conductors in the NEC.
 
Then that is the problem. As I said. If it's DLO then you are not going to get to use it at that ampacity for its other ratings. I believe we all know that....it's the DLO and it's increased ampacity value that is being discussed.

Comments based on the 2017 National Electrical Code.
 
jim dungar said:
This used to be an acceptable practice. I think it became explicitly prohibited in the late 90's.
Habits learned early are hard to break.
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I am not sure it was ever permitted. Even in the 1978 code you could only round up to the next standard sized OCPD where the OCPD was 800 amps or less, just like in the current code.

It was commonly done for transformer secondary conductors because many code users did not understand the difference between conductor ampacity and the permission in 240 to use an oversized OCPD. Even though it was commonly done, it was a violation. This was addressed in the 2005 NEC with a provision in 240.21 that said you could not use the "round-up" rule for tap conductors or transformer secondary conductors.
 
winnie said:
What is the ambient temperature :) :) :)

-Jon
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Can't play that card. :happyno::p

Maximum permitted circuit ampacity is based on 110.14(C) coordination, which uses the 75°C column of 310.15(B)(16). There is no temperature correction for this determination. So 380A is the maximum permitted circuit ampacity for 500kcmil copper conductors. Refer to the second paragraph of 310.15(B) opening statement for confirmation.
 
MasterTheNEC said:
Then that is the problem. As I said. If it's DLO then you are not going to get to use it at that ampacity for its other ratings. I believe we all know that....it's the DLO and it's increased ampacity value that is being discussed.

Comments based on the 2017 National Electrical Code.
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The published DLO ampacities are most often based on 90°C and free air and are consistent with ampacities in Table 310.15(B)(1)...of course there are very few applications where you are permitted to use those ampacities.

However, just because they added RHH to the DLO does not change the physical size of the conductor and since it is larger than the sizes shown in the ampaciity tables, it will have a greater ampacity. This increased ampacity can be used under the provisions of 310.15(C).
 
don_resqcapt19 said:
The published DLO ampacities are most often based on 90°C and free air and are consistent with ampacities in Table 310.15(B)(1)...of course there are very few applications where you are permitted to use those ampacities.

However, just because they added RHH to the DLO does not change the physical size of the conductor and since it is larger than the sizes shown in the ampaciity tables, it will have a greater ampacity. This increased ampacity can be used under the provisions of 310.15(C).
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But not under the context of the conversation. We are talking building wire.

Comments based on the 2017 National Electrical Code.
 
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