Feeder Sizing

[mstrlucky74]

Have some feeders where only the switch size( in amps) is shown and the fuse size of the switch. These feeders are feeding other panels. When sizing these feeders do you go off the switch size, fuse size or panel size? I think the breaker or switch should be sized to the panel being fed, correct? Thanks,

 

[infinity]

The designer should tell you the switch size, fuse size, feeder size and panel size. Without that info you're just guessing.

 

[mstrlucky74]

The designer should tell you the switch size, fuse size, feeder size and panel size. Without that info you're just guessing.

He does give switch, fuse and panel size. Left out feeder sizes. It's a budget and I'd like size based on what's given.

 

[tw1156]

He does give switch, fuse and panel size. Left out feeder sizes. It's a budget and I'd like size based on what's given.

Size based upon the OCPD for the Feeder. For example, if it's a 600A rated panel, but it's fused for 450A, size the feeder to 450A. I think this is what you're asking right?

 

[mstrlucky74]

Size based upon the OCPD for the Feeder. For example, if it's a 600A rated panel, but it's fused for 450A, size the feeder to 450A. I think this is what you're asking right?

yes it is thanks. So your saying to size based on the switches fuse and not the switch size? Why is that? Just curious, thanks.

 

[tw1156]

yes it is thanks. So your saying to size based on the switches fuse and not the switch size? Why is that? Just curious, thanks.

The switch size only indicates the max size of the OCPD (fuse for example) that can be placed in it. Example Disconnect Frame sizes are 30, 60, 100, 200, 400, 600, but you're not required to use those same size fuses in them. You can have a 600A Frame Disconnect with 450A fuses if your load only needs to be sized for this. Just so I'm understanding your terminology, when you refer to switch, do you mean Disconnect?

 

[mstrlucky74]

The switch size only indicates the max size of the OCPD (fuse for example) that can be placed in it. Example Disconnect Frame sizes are 30, 60, 100, 200, 400, 600, but you're not required to use those same size fuses in them. You can have a 600A Frame Disconnect with 450A fuses if your load only needs to be sized for this. Just so I'm understanding your terminology, when you refer to switch, do you mean Disconnect?

Thanks, got it. I mean fused switch in a switchboard.

 

[Gshell82]

I basically asked this same question and got the same answer. Here is the thread:

http://forums.mikeholt.com/showthread.php?t=191768&highlight=

 

[infinity]

Thanks, got it. I mean fused switch in a switchboard.

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.

 

[petersonra]

I agree. Use whatever the OCPD rating is.

 

[mstrlucky74]

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...:huh:)

 

[mstrlucky74]

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.

 

[infinity]

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...:huh:)

If you're given the switch size, the fuse size and the panel size there is no assuming. Size the conductors to fuse size.

 

[mstrlucky74]

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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[petersonra]

Thanks! Which code section is this in?

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240 part I

 

[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.

 

[mstrlucky74]

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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[GoldDigger]

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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