Hi,
This is more of a poll than a question specifically because I have my opinion on it already but its been an area of contention between other engineers, electricians, and AHJ's before:
This is a common scenario, a photo-voltaic system that exceeds the 120% rule and can't be back-fed into a panel. The service is instrument transformer rated (i.e. more than 400 amps) and the utility requires there CT/PT cabinet to be cold sequence (i.e. Main switch before the cabinet) - this is where the problem comes in.
So the service looks like this: |3000A Main|---|CT Cabinet|---|Pull Section|---|Loads Section| (This is all bussed, no conductor or lugs; the buss is rated for 3000A)
The loads section doesn't have over current protection other than the service main and you can't tie the PV ahead of that main because its a cold sequence service so you'd be ahead of the meter.
Let's say the PV is 1MW AC @ 480 and needs a 1600A tie in point.
Let's assume we have manufacturer permission to do so and there is no UL issue...keep it simple...can you add 3000A OCPD for the loads section and tie in on a 3000A buss in the area of the pull section? In other words technically you have 3000A of utility feeding into the buss and 1600A of PV on the buss (4600A of source on a 3000A buss) but you can never exceed 3000A because 100% of loads are behind the new breaker. As far as I can tell the NEC doesn't really have anything specific that addresses this...unless I am missing something. Some electricians swear by this some AHJs in our area swear against it so I want to see what the community has to say. Basically the argument for is that the NEC doesn't say you can't and logically it makes sense that can (draw out the vectors you will never exceed 3000A on the buss), what AHJs say is that this is a supply side interconnect and the NEC only allows a supply side interconnect when metering is HOT sequenced, meaning you are tapping on the supply side of the service main. The usually continue the argument stating what prevents someone from adding another loads panel in that same pull section, and the electrician responds saying they will placard the panel to have no new loads.
I would also add that in this case that pull section would likey become very obviously unavailable for anything new to be added because of space constraints but maybe that is a more plausible argument for smaller equipment (say this was a 600A service and we were trying to feed in 600A of solar using enclosed breakers, you may still have a position on the buss where new loads could be tied ahead of the loads panel). Personally, I would also think that any new loads section to a bussed piece of gear would likely have a permit (I would think large enough work you don't have a fly by night guy adding bugs to a conductor and adding a new loads panel without telling anyone).
One related actual question that recently came up because this scenario lends itself well to ask while I have your attention: Again say you can modify the panel and manufacturer gives its blessing, and also assuming that you are in the camp that you can in fact just add OCPD to the loads section and everything is good (or say we aren't exceeding the buss if you dont agree with that), in this scenario could you use 3000A fuses in that pull section to protect the loads section and lug the PV to the line side of the fuses . In other words it is not a fused disconnect, just fuse holders added to the buss and fuses placed there. The only thing I could think of is that it is serviceable equipment and the fuses themselves need a disconnecting means to be replaced, the main breaker is less than 6 feet away, would a placard identifying that as the disconnect to service the fuses be adequate? Does it actually need a placard (I know there is a difference between can and should, I am just wondering the code says here)? Is there anything I am missing?
Thanks,
Steve
This is more of a poll than a question specifically because I have my opinion on it already but its been an area of contention between other engineers, electricians, and AHJ's before:
This is a common scenario, a photo-voltaic system that exceeds the 120% rule and can't be back-fed into a panel. The service is instrument transformer rated (i.e. more than 400 amps) and the utility requires there CT/PT cabinet to be cold sequence (i.e. Main switch before the cabinet) - this is where the problem comes in.
So the service looks like this: |3000A Main|---|CT Cabinet|---|Pull Section|---|Loads Section| (This is all bussed, no conductor or lugs; the buss is rated for 3000A)
The loads section doesn't have over current protection other than the service main and you can't tie the PV ahead of that main because its a cold sequence service so you'd be ahead of the meter.
Let's say the PV is 1MW AC @ 480 and needs a 1600A tie in point.
Let's assume we have manufacturer permission to do so and there is no UL issue...keep it simple...can you add 3000A OCPD for the loads section and tie in on a 3000A buss in the area of the pull section? In other words technically you have 3000A of utility feeding into the buss and 1600A of PV on the buss (4600A of source on a 3000A buss) but you can never exceed 3000A because 100% of loads are behind the new breaker. As far as I can tell the NEC doesn't really have anything specific that addresses this...unless I am missing something. Some electricians swear by this some AHJs in our area swear against it so I want to see what the community has to say. Basically the argument for is that the NEC doesn't say you can't and logically it makes sense that can (draw out the vectors you will never exceed 3000A on the buss), what AHJs say is that this is a supply side interconnect and the NEC only allows a supply side interconnect when metering is HOT sequenced, meaning you are tapping on the supply side of the service main. The usually continue the argument stating what prevents someone from adding another loads panel in that same pull section, and the electrician responds saying they will placard the panel to have no new loads.
I would also add that in this case that pull section would likey become very obviously unavailable for anything new to be added because of space constraints but maybe that is a more plausible argument for smaller equipment (say this was a 600A service and we were trying to feed in 600A of solar using enclosed breakers, you may still have a position on the buss where new loads could be tied ahead of the loads panel). Personally, I would also think that any new loads section to a bussed piece of gear would likely have a permit (I would think large enough work you don't have a fly by night guy adding bugs to a conductor and adding a new loads panel without telling anyone).
One related actual question that recently came up because this scenario lends itself well to ask while I have your attention: Again say you can modify the panel and manufacturer gives its blessing, and also assuming that you are in the camp that you can in fact just add OCPD to the loads section and everything is good (or say we aren't exceeding the buss if you dont agree with that), in this scenario could you use 3000A fuses in that pull section to protect the loads section and lug the PV to the line side of the fuses . In other words it is not a fused disconnect, just fuse holders added to the buss and fuses placed there. The only thing I could think of is that it is serviceable equipment and the fuses themselves need a disconnecting means to be replaced, the main breaker is less than 6 feet away, would a placard identifying that as the disconnect to service the fuses be adequate? Does it actually need a placard (I know there is a difference between can and should, I am just wondering the code says here)? Is there anything I am missing?
Thanks,
Steve
~RJ~