1. Senior Member
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Originally Posted by infinity
Then use the "weakest link" for the feeder, if the switch is 200 amps but the panel or fuse size is 125 amps then the feeder needs to only be 125 amps.
Thank. I thought there would be hard and fast rules(NEC) for sizing the conductors. Calculated load, fuse, breaker etc. Seem like you are assuming( although I know probably right...)

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Originally Posted by petersonra
I agree. Use whatever the OCPD rating is.
I understand but technically is you have a switch at a certain amperage and a fuse in the switch which OCPD are you referring to the switch size or fuse. I understand others have said fuse size...trying to be clear on what your saying. Thanks.

3. Originally Posted by mstrlucky74
Thank. I thought there would be hard and fast rules(NEC) for sizing the conductors. Calculated load, fuse, breaker etc. Seem like you are assuming( although I know probably right...)
If you're given the switch size, the fuse size and the panel size there is no assuming. Size the conductors to fuse size.

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Originally Posted by infinity
If you're given the switch size, the fuse size and the panel size there is no assuming. Size the conductors to fuse size.
Thanks! Which code section is this in?

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Originally Posted by mstrlucky74
Thanks! Which code section is this in?

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240 part I

6. The feeder conductors are sized based on the load. The fuses (OCPD) are sized to protect the conductors. The switch just holds the fuses, i.e. you can't physically put 450A fuses in a 400A switch, so you go to the next size up, a 600A switch.

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Originally Posted by Jraef
The feeder conductors are sized based on the load. The fuses (OCPD) are sized to protect the conductors. The switch just holds the fuses, i.e. you can't physically put 450A fuses in a 400A switch, so you go to the next size up, a 600A switch.
Got it and know those fuses won't fit. I'm following all the feedback but now you contradict what other have said by saying to size the conductors based on the load and not the fuses.

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Originally Posted by mstrlucky74
Got it and know those fuses won't fit. I'm following all the feedback but now you contradict what other have said by saying to size the conductors based on the load and not the fuses.

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Same difference? You size the conductors to carry the load (or more), then you size the fuse to protect the conductors (or smaller).
You have the option to make both conductors and fuses larger, provided there is no code requirement to protect the load too (as with motor(s)).

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