Dedicated GFCI circuits - Mulitwire Branch or not?

[Rufnek100]

I'm looking to set up my 120G and will be running two dedicated circuits in order to be able to provide for critical equipment redundancy.

Right now my plan is run two 20A GFCI receptacles in a double gang box. I figure the easiest way to make this happen is run a single 12/3 NM wire, creating a multiwire branch circuit (shared neutral). After some research, it looks like a I need to run a 2 pole breaker in order to be within code (or two single pole breakers connected out of phase with a disconnect tie bar). The GFCI receptacles will be wired for single location protection only with nothing on the load side and pigtailed neutrals.

With this setup, in the event one circuit trips for whatever reason, I effectively shut down both circuits. This seems to me like I've given back some of the original goal, which is redundancy.

Should I stay with the mulitwire branch configuration or does it make more sense to run two independent circuits with their own dedicated 12/2 and single pole breakers?

I'm not an electrician, but am comfortable doing this myself - I welcome opinions and perspectives, especially from any electrician reefers that may have faced the same issue. Thanks!

 

[sleepydoc]

?
Not sure why you think one circuit tripping would shut down both circuits? The GFCIs interrupt the circuit at the outlet, so they're not a concern, and circuit breakers only interrupt the hot wire, so that shouldn't affect things either.

 

[mmn]

I think sharing a neutral at the box for 2 20a ckts would be not a good idea. You have the potential (npi) to dump 40 amps on that wire. If you are pulling wire anyway, why not pull two neutrals? The bus bar at the box is designed to handle the load, but not one wire.

Don't know about code, but I wouldn't do that.

 

[dkeller_nc]

Might be another issue. GFCI monitors the power between the hot and the neutral. In the event that they don't match, the GFCI outlet/breaker trips, since the assumption is there's a current leak to ground.

I'm thinking a shared neutral between two circuits is an automatic mismatch between hot and neutral, and an automatic trip. I've never tried this - I always run 3 or 4 wire (for arc-fault) dedicated to a specific circuit.

 

[Rufnek100]

Sleepydoc - both circuits trip because by code a 2 pole breaker or two single pole breakers connected by a tie bar is required (if the breaker side trips). But you're correct if they trip at the GFCI receptacle then the other circuit is not affected.

mmn - if the circuits are connected out of phase (different buss bars) then the currents cancel each other out on the neutral side. But you are right in that the potential exists to dump 40A on a 12 ga wire and overload it in the event the two circuits are connected on the same phase.

dkeller - it's probably simpler to just pull two wires and be done with it.

At least this is what my research has shown me thus far; then again I'm not an electrician (an engineer, which is probably part of the problem lol). Sometimes I overthink these things...

 

[Windy2]

Pull 3 wires while you are at it, you will probably need it later.

 

[sleepydoc]

You're correct about them needing to be on opposite poles, and your explanation to MMN is correct as well. I'll have to check my code book - but I don't believe a tie bar is required for a two pole breaker. Our old house had a few circuits wired like this and they didn't put a tie bar on them when we had a new service panel installed ~2005, and off hand I can't think of a reason it wold be necessary.

If you have GFCI outlets installed, it wouldn't be an issue. I'd have to check on GFCI breakers; not sure about them. Either way, if you haven't pulled the wire, I'd 2nd just pulling 2 sets of wire. It's not that much more work or money and then you know it's not an issue and you don't have to worry about the breakers getting connected properly.

 

[KStatefan]

I would just run two separate circuits. The code does say that each circuit be disconnected simultaneously. I do not have a code book in my office but will look at one this afternoon if someone does not quote the actual article before that.

 

[max6297]

You cannot share a neutral on a two GFI circuits. The GFI's will stay tripped.

 

[Rufnek100]

Thanks for all the feedback - I believe the simultaneous disconnect requirement came about in the 2008 NEC revision, but prior to that it was okay to not have them. Something about protecting idiot non-electrician DIY homeowners like myself from shocking themselves... Ha ha.

KStatefan brings up another question I haven't thought about - even if I run two independent circuits, do I still have to have simultaneous disconnects because they're in the same box? Or does that just apply to multiwire branch circuits?

But I'm definitely thinking just pull two wires at this point.

Thanks so far to everyone for their input!

 

[KStatefan]

You cannot share a neutral on a two GFI circuits. The GFI's will stay tripped.

You can share the neutral as long as it is not downstream of the GFCI device.

 

[KStatefan]

KStatefan brings up another question I haven't thought about - even if I run two independent circuits, do I still have to have simultaneous disconnects because they're in the same box? Or does that just apply to multiwire branch circuits?

I am not aware of a code requiring that. I will look when I go out to the shop.

 

[SGT_York]

The circuits would need to be on separate phases, per NEC, which is why they require the simultaneous disconnects. The separate phases would negate the electrical load on the neutral wire so you could never exceed 20 amps on the joint neutral wire. Also bear in mind this will create a potential for 240 volts in the box.

A much better way would be to run two 12-2 lines on a double breaker (two breakers in one slot) so you are only pulling 120v but you'd need two neutral lines. A bit more money and effort but much safer in the end.

Don't run one neutral off two breakers on the same phase.

Also you could not use GFCI breakers as mentioned before, GFCI outlets as you described would work just fine.

 

[Windy2]

Mistaken post.

 

[dkeller_nc]

Keep in mind that with 2 120V circuits on different phases, the voltage/current won't completely cancel out. In power distribution system based on three-phase 460V, the three lines are 120 degrees out-of-phase with one another, so no two conductors are 180 degrees out-of-phase with one another (which would be required for complete voltage/amperage cancellation).

BTW - if you do pull 2 12 AWG romex cables, I wouldn't recommend hooking them to a ganged breaker. Since a nuisance GFCI trip can be catastrophic for a reef tank, you'd like different circulation equipment on different GFCI breakers, so that your tank's not left without oxygenation in case of an inadvertent GFCI trip when you're not at home.

 

[KStatefan]

I did not get from the OP that he was working with a 3 phase system.

 

[KStatefan]

.

KStatefan brings up another question I haven't thought about - even if I run two independent circuits, do I still have to have simultaneous disconnects because they're in the same box? Or does that just apply to multiwire branch circuits?

The way I read the NEC is powering two separate GFCIs you do not need the breaker with tie bars. If you were supply a single yoke with two circuits you would.

 

[SGT_York]

The interpretation will depend on your city's inspector if you go that route.
Two 12-2 wires on the same phase is what I did and would recommend.

 

[Rufnek100]

Thank you for the insight everyone, it's been very informative and I've learned something and hopefully avoided some pitfalls.

I'll be pulling two 12/2 wires on independent single pole 20A breakers and going to 2 GFCI outlets wired for single location protection. I've got plenty of room left in the box for two regular sized breakers.

I'm running an Apex controller with two energy bars, each of which will be plugged into a separate circuit. That way I can split my heaters, pumps, powerheads, and lights across the two circuits and have some redundancy in the event I trip one half of the system.

 

[Shawn O]

Right now my plan is run two 20A GFCI receptacles in a double gang box.

I'm just curious about this. Why is it you only want two receptacles? Also, why would you want them in the same box? Seems to me that down the road, a different owner living in the home will likely kill the power to the box to work on it and get killed by the other line in the box that's not shut off. Also, do you only expect to ever need to plug in four things for the tank? One would think two separate double gang boxes would be more useful for the tank and for afterward at some point when there is no tank there.

Like I said, just curious.