GFI Breaker for Feed for Sub Panel

jonny1982

Member
Location
CA
If a sub panel is Fed from a GFI breaker, is everything that that panel feeds considered GFI protected? For instance if the sub panel feeds outlets outside, do they have to be GFI's, or are they considered GFI protected because of the breaker feeding the panel?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
It would be GFCI protected but there are some circuits that may require GFCI protection in the branch circuit not the panel feeder.
 

bwat

EE
Location
Western PA
Occupation
EE
It would be GFCI protected but there are some circuits that may require GFCI protection in the branch circuit not the panel feeder.
I wasn’t aware of this. Do you know the reasoning behind this? I can’t think of why gfci protection in the branch is ever safer than the feeder if they are the same leakage current levels being used.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I don't have the book in front of me but if I remember correctly certain pool equipment requires the protection in the branch circuit.
 

bwat

EE
Location
Western PA
Occupation
EE
I don't have the book in front of me but if I remember correctly certain pool equipment requires the protection in the branch circuit.
But do you happen to know the reason for that requirement? Or have a guess? I can't think of why it would be that way.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
It would be GFCI protected but there are some circuits that may require GFCI protection in the branch circuit not the panel feeder.
I wasn’t aware of this. Do you know the reasoning behind this? I can’t think of why gfci protection in the branch is ever safer than the feeder if they are the same leakage current levels being used.
If the GFCI breaker in the feeder panel is a 2-pole with a load neutral output, it's possible that some leakage current from a 120V branch circuit on one phase could be effectively cancelled at the GFCI sense coil by leakage current in a branch circuit on the other phase . For example, if there's a ground fault on each phase with identical impedances, then there would be no net common-mode current on the L1, L2, N conductors that pass through the toroidal transformer of the GFCI and so it wouldn't trip. That's because the L1 and L2 voltages are 180 degrees apart and therefore the leakage currents from the two ground fault impedances will be 180 degrees apart, and so their magnetic fields will cancel at the toroid.

Obviously, a MWBC fed by a 2-pole breaker could have this same issue. But I think it's unlikely this issue would be significant in real life circumstances.
 

bwat

EE
Location
Western PA
Occupation
EE
If the GFCI breaker in the feeder panel is a 2-pole with a load neutral output, it's possible that some leakage current from a 120V branch circuit on one phase could be effectively cancelled at the GFCI sense coil by leakage current in a branch circuit on the other phase . For example, if there's a ground fault on each phase with identical impedances, then there would be no net common-mode current on the L1, L2, N conductors that pass through the toroidal transformer of the GFCI and so it wouldn't trip. That's because the L1 and L2 voltages are 180 degrees apart and therefore the leakage currents from the two ground fault impedances will be 180 degrees apart, and so their magnetic fields will cancel at the toroid.

Obviously, a MWBC fed by a 2-pole breaker could have this same issue. But I think it's unlikely this issue would be significant in real life circumstances.
I'd buy that vowel. Possibly a stretch as you mentioned, but it's at least reasonable enough for me for a possible reason for why. Thank you.
 

qcroanoke

Sometimes I don't know if I'm the boxer or the bag
Location
Roanoke, VA.
Occupation
Engineering
The closer I could get to what requires GFI protection before I installed it the better I'd like it.
 

brantmacga

Senior Member
Location
Georgia
Occupation
Electrical contractor
But do you happen to know the reason for that requirement? Or have a guess? I can't think of why it would be that way.
Pool lighting requires GFCI within the branch circuit.

I’m pretty sure most GFCI’s will say the circuit length should be 100’ or less. I don’t know the substantiation but I’ve always assumed the 100’ limitation is the reason for requiring it within the branch circuit, to ensure you don’t have too far of a distance between the lighting and GFCI device.


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