GFCI Line Protection

[mityeltu]

I have been asked a question I cannot answer.

The scenario is this: old construction home. Breaker panel is grounded, but the branch feeds are 2-wire only - no ground. Wire is #12awg. Branch feeder breaker is 20A. The question I was asked is this: if a GFCI is installed in the first receptacle location of the branch circuit, will this device protect downstream outlets? This seems possible, but I don't understand exactly how a GFCI works.

An additional question is, can a 30A breaker be swapped for the 20A and not lose protection of the cable? From Ugly's book, it does not look good, and I told him as much, but I don't do alot with NEC any more and would like your input.

Thanks.

 

[JDBrown]

... if a GFCI is installed in the first receptacle location of the branch circuit, will this device protect downstream outlets?

Yes. A GFCI works by comparing the current flowing through the hot and neutral wires, and disconnecting power if the difference between the two is more than about 6 mA. It does not need an equipment ground in order to work, and it will protect the downstream outlets as long as they are connected to the load side of the GFCI device. The relevant Code section for this is NEC 406.3(D)(3)(b) and 406.3(D)(3)(c).

Another issue you will want to be aware of is that MWBCs don't play nicely on the load side of a GFCI. This is because if the neutral wire is shared by two circuits, it will carry current from both circuits, but the GFCI will be comparing it to the current on only one of the "hot" wires. This will cause the GFCI to trip, since the hot and neutral currents will only be the same if there is no load at all on the other circuit that is sharing the neutral. Because of this, any shared neutrals will need to be separated before the GFCI.

Along those same lines, the GFCI will trip if there is a neutral-ground connection at some point on its load side, or if the neutral conductors of different circuits are tied together on the load side of the GFCI. Neither of these is allowed, but they seem to crop up all over the place in the real world anyway. Just a couple of things to look for if you get the GFCI installed it and immediately trips.

... can a 30A breaker be swapped for the 20A and not lose protection of the cable?

This is not allowed. NEC 240.4(D)(5) says that # 12AWG conductors cannot be protected by anything more than a 20 Amp OCPD. I think there are some exceptions for motors, but it sounds like you're talking about a circuit that feeds receptacles. So no, you can't go higher than a 20 Amp receptacle with #12 AWG wire.