NEC 225.30(C) - 2000A per voltage or 2000A total for all voltages?

[hmspe]

This came up in a discussion with another engineer today. Consider a building supplied by two outside feeders, both from the same remote 277/480V service. One supplies a 500 KVA transformer that feeds a 1600A, 120/208V distribution section. The calculated load is 800A. The other feeder supplies a 1000A, 277/480V distribution section with a calculated load of 500A.

NEC 225.30(C): Additional feeders or branch circuits shall be permitted where the capacity requirements are in excess of 2000 amperes at a supply voltage of 600 volts or less.

For NEC 225.30(C) do we handle each voltage separately? Do we add calculated loads without regard to voltage (800A + 500A = 1300A)? Do we use the the ratings of the distribution sections without regard to voltage (1600A + 1000A = 2600A)? Do we use the transformer secondary rated amps instead of the distribution section rating (1388A + 1000A = 2388A)? Do we use the transformer primary rated amps so both feeders are calculated at the same voltage (602A + 1000A = 1602A)? Or maybe something else? Is there a clarification from the CMP on what NEC 225.30(C) means?

For that matter, what does "capacity requirements" mean? Is that calculated load? Equipment rated amps? Calculated load plus 20% for expansion? Something else?

Thanks in advance for any comments.

 

[texie]

This came up in a discussion with another engineer today. Consider a building supplied by two outside feeders, both from the same remote 277/480V service. One supplies a 500 KVA transformer that feeds a 1600A, 120/208V distribution section. The calculated load is 800A. The other feeder supplies a 1000A, 277/480V distribution section with a calculated load of 500A.

NEC 225.30(C): Additional feeders or branch circuits shall be permitted where the capacity requirements are in excess of 2000 amperes at a supply voltage of 600 volts or less.

For NEC 225.30(C) do we handle each voltage separately? Do we add calculated loads without regard to voltage (800A + 500A = 1300A)? Do we use the the ratings of the distribution sections without regard to voltage (1600A + 1000A = 2600A)? Do we use the transformer secondary rated amps instead of the distribution section rating (1388A + 1000A = 2388A)? Do we use the transformer primary rated amps so both feeders are calculated at the same voltage (602A + 1000A = 1602A)? Or maybe something else? Is there a clarification from the CMP on what NEC 225.30(C) means?

For that matter, what does "capacity requirements" mean? Is that calculated load? Equipment rated amps? Calculated load plus 20% for expansion? Something else?

Thanks in advance for any comments.

You raise some interesting points. This is one of those areas of the NEC that leaves a lot of room for interpretation. I think many AHJ's would go along with your installation but this is one of those things you need hash out for sure and written in blood with the AHJ first.

 

[hmspe]

My design approach to 225.30(C) and 230.2(C)(1) is to apply the 2000A limit per voltage configuration, but I'm not sure that that is what the actual text says. My associate took the "total amps irrespective of voltage" approach. I'm not sure that that is what the actual text says, either. The actual building we were discussing has had many "upgrades" over the years and now has two 480V feeders to bus duct plus two 480V feeders to outdoor step-down transformers. All the feeders come from a service in another building. Having the transformers outside the building complicates this configuration. If the transformers were inside the building it would be logical to calculate all the feeders at 480V. With exterior transformers it is not quite so clear whether to treat this as a dual voltage building or a single voltage building.

The panel response to Log #1692 to the 2011 NEC ROP establishes that 230.2(C)(1) is based on calculated load, not mains ratings, but it does not address how to handle multiple service voltages. I went back as far as the 1999 cycle on the ROPs and did not find anything that addressed how to handle a building with multiple supply voltage configurations with respect to 225.30(C) and 230.2(C)(1).

 

[kevin]

I am in the "total amps irrespective of voltage" camp. Use "calculated amps" only. You have 800 amps at 208V plus 500 amps at 480V for a total of 1300 amps.
225.30(C) is of no help, but 225.30(D) Different Characteristics should allow your design to be acceptable.

 

[Smart $]

My design approach to 225.30(C) and 230.2(C)(1) is to apply the 2000A limit per voltage configuration, but I'm not sure that that is what the actual text says. My associate took the "total amps irrespective of voltage" approach. I'm not sure that that is what the actual text says, either. The actual building we were discussing has had many "upgrades" over the years and now has two 480V feeders to bus duct plus two 480V feeders to outdoor step-down transformers. All the feeders come from a service in another building. Having the transformers outside the building complicates this configuration. If the transformers were inside the building it would be logical to calculate all the feeders at 480V. With exterior transformers it is not quite so clear whether to treat this as a dual voltage building or a single voltage building.

The panel response to Log #1692 to the 2011 NEC ROP establishes that 230.2(C)(1) is based on calculated load, not mains ratings, but it does not address how to handle multiple service voltages. I went back as far as the 1999 cycle on the ROPs and did not find anything that addressed how to handle a building with multiple supply voltage configurations with respect to 225.30(C) and 230.2(C)(1).

Well first, you are mixing terms (supplied by service and supplied by feeder)... which makes it more difficult to hone in on what is and isn't permitted.

Second, IMO, you cannot supply a building by feeder under 225 if it is already supplied by a service under 230... which it sounds like you are saying this other building already has a service supply (which goes back to my first comment). Yet, I would agree this would be negated if the capcity requirements exceed 2000A... you could then supply by service and feeder from a remote location.

I think the rule for exceeding 2000A is per voltage system, regardless of it being service or feeder supplied. That said, I was thinking prior to reading your second post about whether involved transformers are inside or outside. If outside, the secondary is treated as the supply, not the primary. If inside, the primary is treated as the supply.

 

[Smart $]

...
225.30(C) is of no help, but 225.30(D) Different Characteristics should allow your design to be acceptable.

As noted in my first reply to this thread, that would depend on whether the transformer is inside or outside.

 

[hmspe]

Well first, you are mixing terms ...

Not intentionally. The service is in building "A". The feeders supply building "B". I proofread several times in an effort to be sure I used the correct terms, but maybe not. My first post was a hypothetical because I thought it would be a clearer example than the actual field job. The field job is too complicated to make a good example.

225.30(C) and 230.2(C)(1) are parallel sections, and from my research 225.30(C) was derived from 230.2(C)(1). If there is no clarification to be found for 225.30(C) it seems logical to look for comments on 230.2(C)(1).