#3/0 rating

Status
Not open for further replies.
M

Mason Schuler

New member
We are refeeding a 225 amp panel from the existing switch gear. We used #4/0 due to the amperage of the panel. The electrical engineer has told us that #3/0 is acceptable for the panel. Is #3/0 rated to 225 amps?
 
I won't say, "yes and no." Instead, I'll say, "yes, but no." :wink:

If you look in the 90C column of Table 310.16, you will see that 3/0 copper THHN is rated for 225 amps. That is the "yes" part, and I suspect that is the only part that your engineer noticed. The "but no" part comes from 110.14(C). You don't get to use the 90C ampacity unless all termination points are rated for 90C. Odds are that they aren't. So a 3/0 is really limited to the 200 amps shown in the 75C column.
 
Mason Schuler said:
We are refeeding a 225 amp panel from the existing switch gear. We used #4/0 due to the amperage of the panel. The electrical engineer has told us that #3/0 is acceptable for the panel. Is #3/0 rated to 225 amps?
Click to expand...

I assume this is commercial work so ---NO--- 3/0 is good for 200 amps unless you have terminals and insulation that is rated 90C. I don't think you'll find too many terminals rated 90C
 
From the table 310.16, with 90 C insulation, yes, 3/0 is rated at 225A however, you must consider derating if necessary (more than 3 CCC's) and also the termination rating of the device you are landing you wires to. Most terminations are only rated at 75 C so therefore you'd have an allowable ampacity of 200. I say you did the right thing by running 4/0.
 
Does the rating of the panel have anything to do with it?
Wouldn't the calculated load determine what size wire you use?

If the load is under 200 amps 3/0 would be okay as long as the OCPD was 200A, wouldn't it?
 
wirebender said:
Does the rating of the panel have anything to do with it?
Wouldn't the calculated load determine what size wire you use?

If the load is under 200 amps 3/0 would be okay as long as the OCPD was 200A, wouldn't it?
Click to expand...
I believe you have to design the conductor feeding a panel/subpanel with the maximum current in mind.
 
hardworkingstiff said:
I agree. Since the main was 225, the OP did the correct thing by pulling the 4/0.
Click to expand...

The OP didn't say anything about a main. Or about the OCPD.

He just said an existing 225A panel. The engineer said 3/0 was good. How do we know the engineer didn't do a load calc?

Is there somewhere in the NEC that says to size conductors by the panel rating?

If so, when I hook up an A/C unit with a 60A A/C disconnect do I need to pull #6 regardless of the load?
 
wirebender said:
The OP didn't say anything about a main. Or about the OCPD.

He just said an existing 225A panel. The engineer said 3/0 was good. How do we know the engineer didn't do a load calc?

Is there somewhere in the NEC that says to size conductors by the panel rating?

If so, when I hook up an A/C unit with a 60A A/C disconnect do I need to pull #6 regardless of the load?
Click to expand...

You are absolutely correct. If the panel is protected by 200 amp breaker or less than the 3/0 is fine.

The panel amp is not important, thus a 225 amp panel may be protected by a 100 amp breaker with the proper size conductors as long as the load does not exceed the 100 amps.
 
Status
Not open for further replies.
Top