30 amp disconnect

[wireday]

I have a 30amp breaker feeding a 30amp disconnect,Can I install 20amp fuses into disconnect and use for 20amp load?

 

[Dennis Alwon]

I have a 30amp breaker feeding a 30amp disconnect,Can I install 20amp fuses into disconnect and use for 20amp load?

Absolutely assuming the wire is #10 that feeds the disconnect

 

[kwired]

Absolutely assuming the wire is #10 that feeds the disconnect

If supplying a motor load that is not an absolute requirement.

One scenario is a 11 amp motor could have up to 30 amp breaker but up to 20 amp fuse. 7.5 hp @ 460 volts or 3 hp @ 200 volts motors (both three phase) can work out that way, but both those motors can also be on 14 AWG conductor, for the entire circuit.

But yes most general applications the first segment of what OP mentioned needs to be 10 AWG conductor

 

[Dennis Alwon]

If supplying a motor load that is not an absolute requirement.

One scenario is a 11 amp motor could have up to 30 amp breaker but up to 20 amp fuse. 7.5 hp @ 460 volts or 3 hp @ 200 volts motors (both three phase) can work out that way, but both those motors can also be on 14 AWG conductor, for the entire circuit.

But yes most general applications the first segment of what OP mentioned needs to be 10 AWG conductor

If there are fuses in the disconnect doesn't that make the conductors feeding the disconnect a feeder and not a branch circuit? How could you then have 30 amp breaker and a 12 or 10 gauge wire feeding the disconnect.

 

[infinity]

Absolutely assuming the wire is #10 that feeds the disconnect

Also assuming that tap rule doesn't apply.

 

[kwired]

If there are fuses in the disconnect doesn't that make the conductors feeding the disconnect a feeder and not a branch circuit? How could you then have 30 amp breaker and a 12 or 10 gauge wire feeding the disconnect.

It would still comply with requirements in 430 parts IV and V.

A feeder supplying motor loads can potentially have higher OCPD than what otherwise is the general rule. We may not see it all that often though.

We might commonly see feeder supplying a panelboard, but 408.36 will limit the OCPD to the bus rating. In an arrangement with no panelboard you could have feeder conductors sized per art 430 part II and the overcurrent protection per part V

 

[Dennis Alwon]

It would still comply with requirements in 430 parts IV and V.

A feeder supplying motor loads can potentially have higher OCPD than what otherwise is the general rule. We may not see it all that often though.

We might commonly see feeder supplying a panelboard, but 408.36 will limit the OCPD to the bus rating. In an arrangement with no panelboard you could have feeder conductors sized per art 430 part II and the overcurrent protection per part V

A feeder can have a higher overcurrent protective device but it must have the proper conductor size as a feeder. No?

 

[kwired]

A feeder can have a higher overcurrent protective device but it must have the proper conductor size as a feeder. No?

Generally, yes. but:


430.62(B):


Those "other installations" are more commonly seen, people run say a 200 amp feeder with 200 amp OCPD when they possibly could have had say a 175 amp breaker and conductor that is less than 175A. If supplying a panelboard with the feeder you can't ordinarily exceed the panelboard rating with the OCPD, but where you may be more likely to take advantage of this sort of thing is with a feeder to a gutter or tap box and then supplying multiple combination starters.

 

[Dennis Alwon]

Generally, yes. but:

View attachment 2560901
430.62(B):

View attachment 2560900
Those "other installations" are more commonly seen, people run say a 200 amp feeder with 200 amp OCPD when they possibly could have had say a 175 amp breaker and conductor that is less than 175A. If supplying a panelboard with the feeder you can't ordinarily exceed the panelboard rating with the OCPD, but where you may be more likely to take advantage of this sort of thing is with a feeder to a gutter or tap box and then supplying multiple combination starters.

I don't see where that would allow a #12, for instance, to feed a fused disconnect where the feeder is protected at 30 amps. I understand the branch circuit part can be #12 but not the feeder. In fact it is basically a branch circuit but by definition the first half is a feeder. I tried writing a change for that but the cmp would not accept it.

 

[jap]

I don't see where that would allow a #12, for instance, to feed a fused disconnect where the feeder is protected at 30 amps. I understand the branch circuit part can be #12 but not the feeder. In fact it is basically a branch circuit but by definition the first half is a feeder. I tried writing a change for that but the cmp would not accept it.

I agree completely.

In addition, the load that is there now may not be the load that is there in the future.

That that little extra may be needed.

Don't be choking the feeders to those disconnects.

JAP>

 

[kwired]

A common application I have that is essentially what OP asked about is with 25 HP motor @ 480 volts, start with a 70 amp breaker (because that is what my Square D motor slide calculator suggests, NEC would allow up to 90 amp breaker here. Run 8 AWG NEC minimum conductor size. Land that at a pump panel at the motor location. I use pump panel because I get better price at my supplier for those than for a combination starter, thing is the ones they stock and have the better pricing on have fuseholders. End up using 50 amp fuses because that is what slide calculator suggests, NEC max could be 60 amp fuse. I run into similar situation with other motor sizes as well, this just happens to be one that is a little common on farms for me.

If the provisions in 430.62 do not allow this then explain what it does allow? This is simple because it only supplies one motor but there is provisions in 430.62 for motors and other loads on the feeder as well.