Feeders Vs Branch Circuits

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Yes he can make a difference as a feeder calculation is different than a branch circuit. A while ago we had a discussion and the example given showed that a larger wire was needed for the feeder-- I believe...
Do you have an example? I cant think of a case where this would be true.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
An internal fuse usually does not change anything. Supplemental protective devices do not impact the definition of a feeder.
I agree, but why would an external OCPD at the load that only feeds that load be any different?
 

kwired

Electron manager
Location
NE Nebraska
Feeders are the aggregate circuit that serves a distribution device (panelboard, load center) containing multiple branch circuits. Feeders connect subpanels from the main panel, or the main panel from the service equipment.

Branch circuits connect the branch overcurrent device (e.g. branch breaker) to each load. Or to a daisy-chained group of loads, as is the case with receptacle and lighting circuits. The way to tell that it is a branch circuit, is that it will not have multiple paralleled overcurrent devices on the load side of the circuit.

In both cases, you can de-energize the feeders and branch circuits with customer-owned disconnects. Service conductors come ahead of feeders, ahead of the main service disconnect, with the essential difference being that you need the utility to shut down your service, to de-energize them.
But you can have a feeder supplying a panel that in turn only has additional feeders leaving it.

Still goes back to definitions having the best clarification - a branch circuit is between the final overcurrent device and outlet(s). If a load connects to the portion in question it has to be a branch circuit.

Feeder can get just a little more tricky. Say I put a receptacle for an RV or any other kind of mobile unit that has a panelboard as the first thing supplied by the power supply conductors. By definition that circuit to the receptacle on the "main structure" is probably a branch circuit. But by definitions the supply conductors to the panel in the mobile unit is a feeder from the perspective of individual loads within that mobile unit.
 

kwired

Electron manager
Location
NE Nebraska
The origin of the circuit is immaterial, as is its size. All that matters is the location of the final overcurrent protective device. effectively, Feeder circuits terminate in branch overcurrent protective devices while Branch circuits terminate in loads.
Only when you are past the service disconnect. Ahead of service disconnect there are no branch circuit or feeders.

Separately derived or on site power production can be feeder or branch circuit - art 100 definitions will determine which it is.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Do you have an example? I cant think of a case where this would be true.
As I understand the rules for overcurrent protective device for a feeder and a branch circuit for motors or a/c units the feeder cannot use the round up to the next size breaker where the branch circuit can take advantage of this. Not sure this would make a big difference but that is one case where it may be affected.

I don't believe that was the example we talked about years ago. I remember it had something to do with an a/c unit on the roof being fed by a feeder rather than a branch circuit that caused the issue. It may have been an existing situation where the unit got changed but the feeder was no longer large enough but if it were defined as a BC then it would be fine...I can't seem to come up with an example.
 

kwired

Electron manager
Location
NE Nebraska
As I understand the rules for overcurrent protective device for a feeder and a branch circuit for motors or a/c units the feeder cannot use the round up to the next size breaker where the branch circuit can take advantage of this. Not sure this would make a big difference but that is one case where it may be affected.

I don't believe that was the example we talked about years ago. I remember it had something to do with an a/c unit on the roof being fed by a feeder rather than a branch circuit that caused the issue. It may have been an existing situation where the unit got changed but the feeder was no longer large enough but if it were defined as a BC then it would be fine...I can't seem to come up with an example.
I disagree. Both 430.62(A) and 430.63 indicate shall have a rating not less than, slightly differently worded in each but is essentially what both are saying. Not less than means you must round up if not at a standard size.

Where it may be interpreted that you can not increase overcurrent device is in 430.62(B), though I don't necessarily think it says that there either.
 

kwired

Electron manager
Location
NE Nebraska
You might want to read 430.62(A) again.

Roger
Ok I was taking wrong approach at this, but we need to clarify what Dennis is saying as I think it can be misunderstood fairly easily.

Rounding up to the next size is part of what is allowed for the largest motor, and same value you would use for a single motor. Then you add the other motors or other loads involved at 100%

A 10 amp motor and two other 5 amp motors or other loads gives you 10 x 2.50 (thermal mag breaker) = 25 + 5 + 5 = 35 amp breaker is allowed.

That happens to be a standard size, but if final result were 36 amps - I see what you pointed out as meaning it still must be protected at 35 amps, if it is a conductor with less than 35 ampacity, which this example seems it would allow as little as 22.5 amp conductors.

So my example we could have a 35 amp breaker on 12 AWG 75C copper. If we ran 10 AWG conductor we still would need 35 amp breaker. If we ran 8 AWG we could increase overcurrent protection to as much as 50 amps.
 

kwired

Electron manager
Location
NE Nebraska
Yes you can round up with motors on a branch circuit but where does it say you can round up with feeders.
But you have already done the rounding up on the largest motor and then added your additional loads, usually will not give you issues with holding during starting, it is just the final result that can't be rounded up.

In my example the 10 amp motor is allowed to use up to 25 amps. Make it the only load and it probably never trips a 20 amp breaker and even some cases never trips a 15, 25 is just the max that NEC allows as a general rule.
 

kwired

Electron manager
Location
NE Nebraska
But you have already done the rounding up on the largest motor and then added your additional loads, usually will not give you issues with holding during starting, it is just the final result that can't be rounded up.

In my example the 10 amp motor is allowed to use up to 25 amps. Make it the only load and it probably never trips a 20 amp breaker and even some cases never trips a 15, 25 is just the max that NEC allows as a general rule.
My 10 amp motor example maybe a bad example as it comes out right on a standard device so no next size up not happening anyway.

How about a 13 amp largest motor and two 5 amp motors. 13 x 2.5 = 26.5 >>> that can have a 30 amp breaker protecting that largest motor, then add two 5 amp motors and you get 40 amp breaker able to protect the feeder for all three. Min conductor size is 13 x 1.25 = 16.25 + 5 + 5 + 26.25 amps. If you want more than 40 amp feeder OCPD on this then you must increase conductor size to match up with the feeder conductor ampacity - could still use the next size rule in that situation though. 95 amp conductor (probably some ampacity adjustments involved to come up with 95) could still be on 100 amp breaker if load is not over 95 amps.

The next size up on overcurrent protection is still allowed to factor into the largest motor just not the end result.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I don't believe that was the example we talked about years ago. I remember it had something to do with an a/c unit on the roof being fed by a feeder rather than a branch circuit that caused the issue. It may have been an existing situation where the unit got changed but the feeder was no longer large enough but if it were defined as a BC then it would be fine...I can't seem to come up with an example.
Thanks Dennis.

I think for a motor only branch circuit, the feeder OCPD would be the same size. But for a branch circuit for motor and "other loads" the feeder OCPD may be a different size. I'll have to look closer when I have the Code in front of me.
 

kwired

Electron manager
Location
NE Nebraska
Thanks Dennis.

I think for a motor only branch circuit, the feeder OCPD would be the same size. But for a branch circuit for motor and "other loads" the feeder OCPD may be a different size. I'll have to look closer when I have the Code in front of me.
Though there could be some variances allowed in some cases, I think generally the rule is minimum conductor ampacity is 125% largest motor, plus other load(s).

Max OCPD on conductors selected by this method is the max OCPD allowed for the largest motor plus other load(s).

Should you decide to run larger than minimum conductor you likely will be protecting that conductor per general purpose feeder rules.
 
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