sub-panel feed

[electricguy61]

I could use some help on a dillema I'm facing.

I have an old Zinsco commercial distribution panel that uses QFP breakers. I need to install a small sub-panel (70 amps) approximately 100 ft away. The smallest QFB breaker is 125 amps.

Is it code compliant to install #3 THHN from the 125amp breaker to a 70amp breaker at the sub-panel?

One possible work around would be to install a 125amp sub-panel next to the main and then step-down to 70 amps, but this obviously increases the cost quite a bit.

Please site the applicable code(s)

Thanks for the help

 

[charlie b]

Is it code compliant to install #3 THHN from the 125amp breaker to a 70amp breaker at the sub-panel?

No. Sorry. 240.4 requires that all conductors be protected at their ampacity. There are exceptions, but this is not one of them. You can?t protect a 100 amp rated conductor with a 125 amp breaker.

However, let me suggest two possible, less expensive, alternatives. Which one you use depends on the question of whether the 70 amp sub-panel has a main breaker set at 70 amps or lower. In either case, you need to feed the sub-panel from the 125 amp breaker via a 125 amp rated conductor (i.e., #1 copper). If the sub-panel has a main breaker, then you are done. If it does not, then install an enclosed circuit breaker, rated at 70 amps, just prior to the sub-panel. The #1 conductor comes into the line side of the breaker, but you can use a smaller conductor (i.e., #4 copper) from the breaker to the panel.

 

[cb3]

I don't know the exact code # but the wire size is to small you need 125A wire ampacity for your 125A breaker. #3 is not enough. Maybe someone else can post the code. Increase your wire size to #1 or reduce your breaker size to 100A. And you should still check voltage drop.

 

[electricguy61]

No. Sorry. 240.4 requires that all conductors be protected at their ampacity. There are exceptions, but this is not one of them. You can?t protect a 100 amp rated conductor with a 125 amp breaker.

However, let me suggest two possible, less expensive, alternatives. Which one you use depends on the question of whether the 70 amp sub-panel has a main breaker set at 70 amps or lower. In either case, you need to feed the sub-panel from the 125 amp breaker via a 125 amp rated conductor (i.e., #1 copper). If the sub-panel has a main breaker, then you are done. If it does not, then install an enclosed circuit breaker, rated at 70 amps, just prior to the sub-panel. The #1 conductor comes into the line side of the breaker, but you can use a smaller conductor (i.e., #4 copper) from the breaker to the panel.

Note that the #3 is connected to a 70amp breaker at the other end, thus limiting the current that could flow through the wire. I can't find a code section that specifies on which end the over-current protection must reside.

I know, it's a stretch, but worth asking.

 

[charlie b]

I can't find a code section that specifies on which end the over-current protection must reside.

240.21 begins by saying the protection must be located at the point the conductor receives its power. Then it starts to describe the "Tap Rules" as being exceptions to that rule. Your situation does not match that covered by any of the Tap Rules.

 

[electricguy61]

That's what I needed. Thanks Charlie B

 

[winnie]

The code section that you want is 240.21. The OCPD must be located where the conductors receive their supply, with some exceptions.

What you _want_ to create is known as a 'tap'; this is a conductor with the OCPD at the load end rather than the supply end. Taps are length limited, with different lengths permitted in different conditions. As best I can tell, there is nothing here to help you over the length specified, although you should probably read the section.

-Jon