MOP, Branch Circuit vs. Feeder

[wwhitney]

This discussion about whether the conductors from the panel to fused disconnect are a feeder or branch circuit is purely academic, right? If you trace through Article 430/440 on conductor and OCPD sizing for branch circuits vs feeders, you get the result Dave posted in #4 either way.

Cheers, Wayne

 

[wwhitney]

on occasion I have seen inspectors require the feeder to the disconnect to be sized by the 50 amp panel breaker.

But that is not an NEC requirement.

Cheers, Wayne

 

[kwired]

If you add a fused disconnect, what would the fuses be protecting? The RTU has an MOCP of 50, do the fuses protect the equipment? Do the fuses protect the conductors between the disconnect and the RTU?

So how does one differentiate supplemental OCPD vs a branch circuit OCPD?

Typically must have a local disconnecting means, but can't have supplemental OCPD as a part of the disconnect?

 

[david luchini]

Typically must have a local disconnecting means, but can't have supplemental OCPD as a part of the disconnect?

Are the 50 fuses at the disconnect providing protection for the equipment that has an MOCP of 50? Are they providing SC-GF protection for the conductors between the disconnect and the equipment?

 

[kwired]

Are the 50 fuses at the disconnect providing protection for the equipment that has an MOCP of 50? Are they providing SC-GF protection for the conductors between the disconnect and the equipment?

Does a fuse know when it is a branch device vs a supplemental device? If current reaches the trip curve it responds regardless what you call it.

If there is a device upstream that is set for about the same trip level do they know which one should trip first? I've seen many times where a SC-GF takes out both a branch circuit device and a feeder device. IMO unless you have some sort of supplemental device that is intended to respond faster or limit current to some extent than whatever is upstream in a high current fault there is no telling which will trip first or if they both end up tripping.

If the upstream device is set for the required level for the entire circuit does it matter if the downstream device is a switch or has OCP even if greater than the upstream device? Kind of like a 200 amp main breaker panel supplied by a 100 amp feeder breaker.

 

[david luchini]

Does a fuse know when it is a branch device vs a supplemental device? If current reaches the trip curve it responds regardless what you call it.

If there is a device upstream that is set for about the same trip level do they know which one should trip first? I've seen many times where a SC-GF takes out both a branch circuit device and a feeder device. IMO unless you have some sort of supplemental device that is intended to respond faster or limit current to some extent than whatever is upstream in a high current fault there is no telling which will trip first or if they both end up tripping.

If the upstream device is set for the required level for the entire circuit does it matter if the downstream device is a switch or has OCP even if greater than the upstream device? Kind of like a 200 amp main breaker panel supplied by a 100 amp feeder breaker.

Does the fuse know that there is an upstream device with the same rating? Or....
Does it protect the equipment that has a MOCP of 50?
Does it protect the conductors between the disconnect and the outlet?

 

[kwired]

Does the fuse know that there is an upstream device with the same rating? Or....
Does it protect the equipment that has a MOCP of 50?
Does it protect the conductors between the disconnect and the outlet?

No, but upstream device doesn't know there is a downstream device either.

Yes, but so does the upstream device if they are the same rating.

Yes, but so does the upstream device.

Put a couple more of same devices somewhere in the path and all you gain is some redundancy.

 

[david luchini]

Yes, but so does the upstream device if they are the same rating.

Yes, but so does the upstream device.

So the downstream device is, by definition, a branch circuit OCPD, and not, by definition, a supplementary OCPD.

 

[kwired]

So the downstream device is, by definition, a branch circuit OCPD, and not, by definition, a supplementary OCPD.

I guess so, particularly if they are identical devices. If not identical then other characteristics may come into play.

Seen VFD fuses with much higher "rating" than the upstream device, but also with entirely different trip curve, particularly the response time to high level currents, but that is supposedly for the drive sake (even though if it is called upon to open the drive is toast anyway) and not so much for conductor protection.

 

[wwhitney]

But apparently if you screw the cover shut on your fused disconnect, that makes it a supplementary OCPD instead of the branch circuit OCPD.

Cheers, Wayne