400 amps

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Continuous loading would be one factor. Voltage drop, correction factors, and adjustment factors may be other reasons 600kcmil would be used over 500kcmil on a 400A system.
 
Are you talking copper or aluminum? The service calculation should have already taken into account the continuous/non-continuous factor.

If its a dwelling (310.15(B)(6)), you can use 500 KCMIL Al or 350 KCMIL Cu for a service that calculates to 350 amps or less. You use 600 KCMIL Al or 400 KCMIL Cu for a service that calculates to 400 amps or less.

If this is not a dwelling (310.16), you can use 500 KCMIL Cu for services that are 380A or less. Use 600 KCMIL Cu for services that are 420A or less.

If your disconnect has only one breaker, then you need to see 230.90. This basically says that the breaker rating can not be larger than the rating of the service conductor. So if you have a single 400A main, you'd need a 400A rated wire in order to use that main. This means 600 KCMIL Cu for commercial, 400 KCMIL Cu for residential. If you use two parallel 200A panels on a 400A meter, then you can size the wires per the calculated load instead of the breaker size (or sum of breaker sizes).
 
vfrhawk said:
Is the continuous load the deciding factor?
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No. The "calculated load" is the deciding factor. 240.4(B) lets you use an overcurrent device that is rated higher than the ampacity of the conductor (under limited circumstances). But you cannot have a conductor with an ampacity that is lower than the calculated load. So if you calculate a load of 375 amps, you can use 500 MCM copper. But if you calculate a load of 385 amps, you must use at least 600 MCM copper.

Also, if you have a service rated at 400 amps, and if you did not formally calculate the loads (therefore don't know if it is above or below 380), then you must presume the load is 400, and you can't use 500 MCM copper.
 
240.4(B) "Devices Rated 800 Amperes or Less. The next higher standard OCPD (above the ampacity of the conductors being protected) shall be permitted to be used......(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above it's rating (but that shall be permitted to have other trip or rating adjustments)

So when Mark wrote:
"If this is not a dwelling (310.16), you can use 500 KCMIL Cu for services that are 380A or less. Use 600 KCMIL Cu for services that are 420A or less"
Actually 500 KCMIL Cu can be used for 400amp service.
 
GalwayElectric said:
Actually 500 KCMIL Cu can be used for 400amp service.
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Thanks for the backup, Trevor.

Galway: Please note that there is a difference between:
(1) Comparing the cable ampacity to the overcurrent device rating, and
(2) Comparing the cable ampacity to the load.
 
Also for services over 300 A its usually easier to install 2 sets of smaller conductors in parallel. Take a look at table 310.16 and note the ampacity and wire size do not increase proportionally.
I had a 300 amp service with 600 KMCL copper and replaced it with two sets of 3/0 in parallel. Easier to pull and make up.
 
Becareful with the use of the "round up" rule in 240.4(B). In general you can't use that where the code requires a minimum conductor ampacity. That rule does not change the ampacity of the conductor, it only permits the use of an oversized OCPD. In this case the only reason that you can use 240.4(B) is Exception #2 to 230.90(A). The main rule says "Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor." If the exception was not there, you would not be permitted to "round up".
Don
 
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