Multi-circuit receptacle strip

[thomrk]

I have a client that wants me to connect some existing three phase, 120V, receptacle strips. The strips are only three-phase in the sense that every other outlet on the strip can be wired to a different circuit or phase. He would like me to connect each strip to three, single phase, 20 amp circuit breakers so if there is a short or overload in one of the outlets, it doesn't knock off the entire strip. The strips are being used to power critical telecommunication racks that can't go down for any reason.

Is it legal to connect one electrical outlet strip to three separate circuit breakers or does it have to be connected to a 3-pole circuit breaker?

 

[Jljohnson]

If the strip shares a neutral among the 3 circuits, the breakers must be handle tied. They do not need to be common trip, however.

 

[thomrk]

Thanks. Do you happen to know exactly what the NEC number that this requirement is stated?

 

[GoldDigger]

The strips are being used to power critical telecommunication racks that can't go down for any reason.

Take a close look at the wiring of the strips. To me telecommunications racks screams non-linear loads, and that means that the neutral current in a shared neutral can easily be as much as three times the phase line current. That is a larger problem than just common trip if the strips will be loaded to more than 1/3 of each phase ampacity.

Ideally there would be a dedicated neutral for each phase carried all the way back to the panel.

See 210.4 for disconnect requirements for MWBCs.

If the strips will be part of a redundancy plan where each piece of equipment will have two power supplies deliberately connected to different supply circuits, then you will never have a full load on both circuits from that piece of equipment at the same time (except for inrush.)

 

[thomrk]

Yes, you are correct that each telecommunication rack will contain two receptacles strips and each strip will be fed from a separate UPS. However, I don't see how a receptacle strip is classified the same as a duplex outlet as referenced in NEC 210.4. NEC 210.4 refers to a single device (Duplex outlet) that is split to make two separate outlets. Isn't a receptacle strip closer to a raceway that has individual receptacles installed in it? Using that logic, if you fed multiple wall receptacles on different circuits but a common neutral, you would have to use multiple poled breaker and I have never seen that done before.

 

[Jljohnson]

210.4 in the 2011 NEC does not reference a single receptacle or strap. It references multi-wire branch circuits

 

[GoldDigger]

Yes, you are correct that each telecommunication rack will contain two receptacles strips and each strip will be fed from a separate UPS. However, I don't see how a receptacle strip is classified the same as a duplex outlet as referenced in NEC 210.4. NEC 210.4 refers to a single device (Duplex outlet) that is split to make two separate outlets. Isn't a receptacle strip closer to a raceway that has individual receptacles installed in it? Using that logic, if you fed multiple wall receptacles on different circuits but a common neutral, you would have to use multiple poled breaker and I have never seen that done before.

That is exactly what would be required. Except that identified handle ties (with no common trip) would be allowed instead of a multipole breaker. This requirement is relatively new to the NEC, so that may explain why you have not seen it in old wiring. Some feel that the world is moving in the direction of not using shared neutrals for branch circuits except for actual three phase loads.
The critical point is that if you open only one breaker for maintenance or rewiring, you will still have current flowing through the common neutral and this will create a hazardous condition if you deliberately or inadvertently open the neutral.

The point was driven home to me while working on a Edison circuit in our house while in high school! At the time I thought that I was safe having turned off the breaker to the circuit that I was planning to tap onto. When I removed the wire nut to add a new neutral wire, sparks flew.

 

[thomrk]

Okay, I think I understand the issue, but this would also apply to multiple circuits used for feeding lighting. I see a lot more conductors and separate raceways in the future for electrical designs.

 

[eHunter]

I have a client that wants me to connect some existing three phase, 120V, receptacle strips. The strips are only three-phase in the sense that every other outlet on the strip can be wired to a different circuit or phase. He would like me to connect each strip to three, single phase, 20 amp circuit breakers so if there is a short or overload in one of the outlets, it doesn't knock off the entire strip. The strips are being used to power critical telecommunication racks that can't go down for any reason.

Is it legal to connect one electrical outlet strip to three separate circuit breakers or does it have to be connected to a 3-pole circuit breaker?

What brand of PDU(power strip) are you working with?
How many individual white neutral conductors does it have?
Most of the PDUs that I have worked with have multiple(2-3) whips.