208V 3Phase with 115V Single Phase

[fifty60]

I have equipment that is powered from a 208V 3 phase source. Inside the equipment I have a separate piece of equipment that will have its own 115V single phase power source. The 3 Phases conductors enter the equipment and are inserted into a distribution block, and from there feed the rest of the equipment through terminal blocks. The non plug end of the power chord (hot, neutral ground) for the 115V equipment enters the equipment and is inserted into terminal blocks. The single piece of electronics equipment is then powered from these 115V terminal blocks.

My concern is with grounding the equipment. I have my main equipment grounding conductor which is bonded to the accessible parts of the equipment and back to the 208V 3 phase source. Do I only connect the 115V piece of equipment and the 115V power chord ground into a single terminal block, and not bond it to any other accessible parts on the equipment?

Summation of problem: 1 piece of equipment, two seperate power sources, how are the two sources grounded?

 

[GoldDigger]

I have equipment that is powered from a 208V 3 phase source. Inside the equipment I have a separate piece of equipment that will have its own 115V single phase power source. The 3 Phases conductors enter the equipment and are inserted into a distribution block, and from there feed the rest of the equipment through terminal blocks. The non plug end of the power chord (hot, neutral ground) for the 115V equipment enters the equipment and is inserted into terminal blocks. The single piece of electronics equipment is then powered from these 115V terminal blocks.

My concern is with grounding the equipment. I have my main equipment grounding conductor which is bonded to the accessible parts of the equipment and back to the 208V 3 phase source. Do I only connect the 115V piece of equipment and the 115V power chord ground into a single terminal block, and not bond it to any other accessible parts on the equipment?

Summation of problem: 1 piece of equipment, two seperate power sources, how are the two sources grounded?

Basic answer:
Do not tie the neutrals together. As for the EGC, I see no harm in connecting them together, it happens with raceways all the time. Code may not be fine with that, but it is not a current carrying conductor so parallel rules do not apply.

If the two sources can be separately disconnected (i.e. plug-in) then you need to make sure that even if only one of the sources is plugged in you still have a solid equipment grounding connection.

 

[texie]

The EGC must be run with the circuit conductors. If all circuit conductors are in the same raceway or cable, you could use 1 sized for the largest circuit in the raceway.
Since one circuit in your install is a cord, that is separate from the other circuit, it will have to have its own EGC.

 

[kwired]

If this separately derived system is within a specific piece of equipment and is a part of the premises wiring you may not even have to ground it.

Read 250.20(B) carefully:

250.20 Alternating-Current Systems to Be Grounded.

(A) not applicable to this thread


(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to less than 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:

(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts
(2) Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor
(3) Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor

Does not mean you can't ground it, but as I see it, this section doesn't apply if it is not premises wiring.

 

[Smart $]

Most likely both power sources utilize the same grounding electrode system. The circuit EGCs are bonded to the respective loads. If it just so happens that both loads have common non-current-carrying-metal parts there is no violation.