NEC 240.4 with multiple disconnects

[mjmike]

Looking at a building operating at 120/240V 3P 4W. Incoming is 3 sets of 600MCM THW AL. all 3 sets connect to the input of metering CT's. On the output of the CT's are 3 sets of 500MCM THW CU but each set goes to an individual 3-phase panel with 400A-3P main. Basically, 3 disconnects. Is this viewed as 1 set of conductors per panel after the CT and parallel before the CT? I am trying to determine ampacity for additional load. Looking at 400.4(B), it looks like the 500 are correct from the output of the CT to each panel (380A conductor and 400A CB is next), but how would it work on the incoming 600MCM AL?

 

[GoldDigger]

The incoming wires are service conductors, so NEC ampacity rules may not apply directly. Depends on where the service point is.
On the NEC side you can decrease the required wire size if the service supplies all the power to a dwelling.

 

[iwire]

Looking at 400.4(B), it looks like the 500 are correct from the output of the CT to each panel (380A conductor and 400A CB is next)

I doubt that is the table you want, that is for flexible cords.

You would want to use a table in Article 310.

but how would it work on the incoming 600MCM AL?

Because the sets of parallel 600s are service conductors exception 3 to 230.90(A) can be used to size the conductors on the total calculated load and not the total of the overcurrent devices.

 

[mjmike]

...Looking at 400.4(B), it looks like the 500 are correct from the output of the CT to each panel (380A conductor and 400A CB is next)?

Typo, I meant 240.4(B).

 

[mjmike]

The incoming wires are service conductors, so NEC ampacity rules may not apply directly. Depends on where the service point is.
On the NEC side you can decrease the required wire size if the service supplies all the power to a dwelling.

Not residential and the service point is at the weatherhead.

 

[iwire]

Typo, I meant 240.4(B).

Now I see and I agree with you.

Those 500 kcmil copper conductors can be protected at 400 amps but the calculated load must remain under 380.

 

[mjmike]

I doubt that is the table you want, that is for flexible cords.

You would want to use a table in Article 310.




Because the sets of parallel 600s are service conductors exception 3 to 230.90(A) can be used to size the conductors on the total calculated load and not the total of the overcurrent devices.

Thanks, that exception is what I was looking for. That works for the 600MCM AL since it will see the load of all 3 panels. The 500MCM, though technically are still service conductors, will only see the load of a single panel.

 

[iwire]

That works for the 600MCM AL since it will see the load of all 3 panels. The 500MCM, though technically are still service conductors, will only see the load of a single panel.

Exactly, that.

Now to add load to this service technically you need to do load calculations to make sure you do not overload the 3 sets of 600 aluminum.

 

[mjmike]

for the 230.90(A) exception #3, since this is an existing facility I believe 220.87 could be used basing it on 1 years max demand then 220.87(2) for existing load and new load.

 

[iwire]

for the 230.90(A) exception #3, since this is an existing facility I believe 220.87 could be used basing it on 1 years max demand then 220.87(2) for existing load and new load.

I agree with you 100%.