So I have been going through the 2020 NEC. I have something that seems a bit tricky to me. I wanted to get some other opinions on the subject before I handed over my verdict.
We have multiple MCC's that have parallel 120VAC powered 24VDC output switching power supplies in parallel. It's the control voltage for our systems in various incarnations so that we don't need to use our gloves, hoods and shields when troubleshooting a significant majority of our equipment. This power is distributed by powering a 480V primary to 120V secondary transformer, to a 20A 120V branch circuit, which is then followed by separate breakers for each individual PSU. The particular MCC that this 24VDC supply is in is MCC9B.
For most of these systems, it's fine. But we have one backup generator at the plant that keeps some critical equipment going in the event of a power outage. The ATS for this generator and the emergency lighting and load panel is named MCC9E. The Emergency panel is fed by a 480V-208V 3 phase step-down transformer, which in turn is fed from the ATS.
Finally, we have a control panel with a PLC, some 24V distribution and a bit of other equipment in it that is all powered from the supply in MCC9B.
Currently, the 24VDC supply in MCC9B is fed by a bucket with a circuit breaker which feeds a transformer in the bottom of the MCC section with the control power. I hope explained all of this relatively clearly. Basically, I have a PSU in one MCC section, an ATS feeding an emergency MCC, a transformer, which feeds a lighting and load panel and then a PLC cabinet fed from the PSU.
As I'm reworking a lot of this DC power, what I'm considering doing is disconnecting the transformer in the bottom of MCC9B and feeding it from the emergency lighting and load panel so that the PLC and associated equipment stays powered up during an outage. There is a 24VDC UPS in place. I'm thinking that as long as we install appropriate signage, this really shouldn't be an issue. The only other thought I have is the grounding and bonding, but the transformer is already bonded to the building metal, and so is the substation (medium voltage switchboard) feeding this set of equipment, so I don't think that running an EGC with the circuit from the emergency panel would cause issues, and would also still provide an effective fault current path.
Thanks for reading. I just wanted to see if I was potentially missing anything. I don't really like the idea of doing what I'm thinking all that much at first glance, but I also can't see any major issues with it from a code standpoint.
We have multiple MCC's that have parallel 120VAC powered 24VDC output switching power supplies in parallel. It's the control voltage for our systems in various incarnations so that we don't need to use our gloves, hoods and shields when troubleshooting a significant majority of our equipment. This power is distributed by powering a 480V primary to 120V secondary transformer, to a 20A 120V branch circuit, which is then followed by separate breakers for each individual PSU. The particular MCC that this 24VDC supply is in is MCC9B.
For most of these systems, it's fine. But we have one backup generator at the plant that keeps some critical equipment going in the event of a power outage. The ATS for this generator and the emergency lighting and load panel is named MCC9E. The Emergency panel is fed by a 480V-208V 3 phase step-down transformer, which in turn is fed from the ATS.
Finally, we have a control panel with a PLC, some 24V distribution and a bit of other equipment in it that is all powered from the supply in MCC9B.
Currently, the 24VDC supply in MCC9B is fed by a bucket with a circuit breaker which feeds a transformer in the bottom of the MCC section with the control power. I hope explained all of this relatively clearly. Basically, I have a PSU in one MCC section, an ATS feeding an emergency MCC, a transformer, which feeds a lighting and load panel and then a PLC cabinet fed from the PSU.
As I'm reworking a lot of this DC power, what I'm considering doing is disconnecting the transformer in the bottom of MCC9B and feeding it from the emergency lighting and load panel so that the PLC and associated equipment stays powered up during an outage. There is a 24VDC UPS in place. I'm thinking that as long as we install appropriate signage, this really shouldn't be an issue. The only other thought I have is the grounding and bonding, but the transformer is already bonded to the building metal, and so is the substation (medium voltage switchboard) feeding this set of equipment, so I don't think that running an EGC with the circuit from the emergency panel would cause issues, and would also still provide an effective fault current path.
Thanks for reading. I just wanted to see if I was potentially missing anything. I don't really like the idea of doing what I'm thinking all that much at first glance, but I also can't see any major issues with it from a code standpoint.
