Multi-family Apartment Building

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Grouch

Grouch

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New York, NY
If you design / install a solar PV system for an apartment building with multiple apartments, say for example 25 apartments, and there's base building areas such as lobbies, corridors, cellar with all the back-of-house areas... I have the following questions (each with my answer in red, if you can please correct where needed):

1. Can the output of the inverter be connected to the line side of the service switchboard (which is allowed by the 2008 section 690.64(A)), thereby providing power to the whole building (apartments, base building areas, etc.)? My answer is yes.

2. Can the output of the inverter be connected to the line side of the house distribution panel (at the feeder coming into the panel), thereby providing power to only the base building areas? My answer is no, because it has to be connected via a dedicated circuit breaker or fusible disconnecting means once you are on the load side of the service disconnecting means, based on 690.64(B).

3. Can the output of the inverter be connected to a dedicated circuit breaker or fusible disconnecting means on the house distribution panel, thereby providing power to only the base building areas? My answer is yes.

4. Are you free to choose either 1 or 3? My answer is yes, dependent on the size / KW rating of the array.
 
The question you have to ask yourself is how will the PV production be allocated to the meters on the site? If you interconnect on the supply side of the utility metering then you need some kind of PV production meter and a deal with the utility to allocate the PV production to specified meters in the billing system. Not all utilities do this. If you have to back feed a specific meter then you have to put the POI on the load side of that meter. None of your questions indicate how you are handling the metering.
But strictly looking at NEC issues you can do any of the three options you have listed. Option two would tap into the circuit between the meter and the panel and go to a PV disconnect with an OCPD, most likely a fused safety switch.
 
Presumably whomever is footing the bill wants to get the lion's share of the benefit. That would seem to me to point toward the tap being downstream of the house panel. it seems to me that option 1 ends up with no one getting any credit on their meter.
 
pv_n00b said:
But strictly looking at NEC issues you can do any of the three options you have listed. Option two would tap into the circuit between the meter and the panel and go to a PV disconnect with an OCPD, most likely a fused safety switch.
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The way i read it though, and this is 2008, if you follow 690.64(B), it doesn't let me do option #2... you can't connect to the line side of a panel or fusible board... it has to be on its load side, either through a breaker of fusible switch... since you have to follow (B)(1) through (B)(7). Or am I wrong?
 
Grouch1980 said:
If you design / install a solar PV system for an apartment building with multiple apartments, say for example 25 apartments, and there's base building areas such as lobbies, corridors, cellar with all the back-of-house areas... I have the following questions (each with my answer in red, if you can please correct where needed):

1. Can the output of the inverter be connected to the line side of the service switchboard (which is allowed by the 2008 section 690.64(A)), thereby providing power to the whole building (apartments, base building areas, etc.)? My answer is yes.

2. Can the output of the inverter be connected to the line side of the house distribution panel (at the feeder coming into the panel), thereby providing power to only the base building areas? My answer is no, because it has to be connected via a dedicated circuit breaker or fusible disconnecting means once you are on the load side of the service disconnecting means, based on 690.64(B).

3. Can the output of the inverter be connected to a dedicated circuit breaker or fusible disconnecting means on the house distribution panel, thereby providing power to only the base building areas? My answer is yes.

4. Are you free to choose either 1 or 3? My answer is yes, dependent on the size / KW rating of the array.
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1. Yes.

Bear in mind that if you do this, if you are on the line side of all service meters, you would treat this as its own service, dedicated to the PV. It would use the same overall electrical infrastructure (transformer, service lateral, etc) as the rest of the services on the site. However, for bookkeeping, it would have its own service meter and its own account. It would continuously accumulate in the negative kW-hrs, unlike the other meters that accumulate positive kW-hrs. You would need an agreement with the utility to allow for assigning the credit to the house meter and various tenants. In my region, the utilities call this agreement a Schedule Z.

Also, no matter where you interconnect, you can't really control where any specific kilowatt-hour of energy will go, so it will have the possibility of going to any of the tenants no matter what. The metering keeps track of net power that passes through any given point, and accumulates it to get net energy accordingly (depending on its specific configuration, e.g. net meter, secure-forward, etc).

2. Yes. That would make it a feeder tap governed by 240.21(B). It would need to occur at a dedicated OCPD/disconnect that is immediately adjacent to the tap point, within one of the lengths permitted by 240.21(B). To do a feeder tap in this example, the feeder would need an OCPD on both sides of it, to prevent the house panel from drawing more than its rating.

As an example, consider a 1200A aggregate service to feed to the whole building, with a 400A house loads panel, that is fed from a 400A meter/main, and then eventually feeds a 400A MLO house loads panel. Suppose you want to tie-in a 200A system on this feeder. If you did this, you would potentially overload that house panel with 560A worth of total current, assuming its breakers total a lot more than 400A (which load breakers in general usually do). You therefore need a 400A breaker on the load side of your tap point, to be the device that trips before the house panel gets overloaded.

In NEC2014, it was clarified in Article 705, that you can do this. There is a section for load side of the service disconnecting means, and a subsection for interconnecting on feeders.

3. Yes.

4. Yes, depending on the amp rating of the array.

What would impact this decision, is whether you can satisfy one of the busbar interconnection rules, at each busbar from the POI to the service point. Generally, if you can satisfy the 120% rule at a low-ranked subpanel, it becomes more likely that you can satisfy this rule the higher up the panelboard ranks you go. Assuming each subpanel is comes from a breaker at the opposite end of the bus in the upstream panel.

5. Whether 1 or 3, you're always stuck to the 600V DC limit correct?

This would count as a commercial application. The 600Vdc limit from 690.7 is for single family and duplex dwellings, and applications where you'd have a 120/240V split phase service. Anything large enough to have a 3-phase service where the NEC applies, is most likely going to count as commercial and allow 1000Vdc systems, provided that all 1000V wiring and equipment is limited to only qualified personnel.
 
Understood on all fronts, thanks!

I see how it's allowed now for #2. and if in your example that was a 400 amp MCB panel, we would not need that 400 amp breaker on the load side of the tap. So if it was clarified in the 2014 NEC, looks like this was allowed as well in the 2008... it's just that it wasn't clear.

For #5... I should've read that read more closely. it says it right there in the text one and two family dwellings no more than 600 volts.

Great, thank you all.
 
Grouch1980 said:
I see how it's allowed now for #2. and if in your example that was a 400 amp MCB panel, we would not need that 400 amp breaker on the load side of the tap. So if it was clarified in the 2014 NEC, looks like this was allowed as well in the 2008... it's just that it wasn't clear.
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I wouldn't say it that way, as I see those words as a little misleading, but that is the correct conclusion. You still would need the 400A OCPD on the load side of that tap, it's just that it is already there, due to it being the MCB of the panel.
 
Grouch1980 said:
5. Whether 1 or 3, you're always stuck to the 600V DC limit correct? My answer is yes.
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I know this was answered, but one more comment: You didnt say what the service voltage and system is, but if it is NOT 277/480, you will be quite limited on 1000VDC string inverters. I think fronius is the only one that will do 1KV at 240 or 208. I have frequently seen transformers used on a 120/208 service so that large 480 inverters can be used.
 
Carultch said:
I wouldn't say it that way, as I see those words as a little misleading, but that is the correct conclusion. You still would need the 400A OCPD on the load side of that tap, it's just that it is already there, due to it being the MCB of the panel.
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Agreed. In the 2020 NEC See 705.12(B)(1).
 
electrofelon said:
I know this was answered, but one more comment: You didnt say what the service voltage and system is, but if it is NOT 277/480, you will be quite limited on 1000VDC string inverters. I think fronius is the only one that will do 1KV at 240 or 208. I have frequently seen transformers used on a 120/208 service so that large 480 inverters can be used.
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and THAT answers a question that was on my mind as well. I came across a design done by another firm that had two 36kw inverters (1000VDC input and 277/480 on their outputs) connecting to a combiner panel, and then the combined feeder going to a 75kva step-down transformer bringing it down to 120/208 volts. I was wondering why they didn't just use 1KV at 120/208 volts to begin with... so it seems that these are not readily available on larger inverters.
 
Grouch1980 said:
and THAT answers a question that was on my mind as well. I came across a design done by another firm that had two 36kw inverters (1000VDC input and 277/480 on their outputs) connecting to a combiner panel, and then the combined feeder going to a 75kva step-down transformer bringing it down to 120/208 volts. I was wondering why they didn't just use 1KV at 120/208 volts to begin with... so it seems that these are not readily available on larger inverters.
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Yeah I think the best you can do is a fronius primo which does 1KV strings at 208 but the biggest one is only 15KW. I would really try and avoid the transformers but there is going to be a point where you need so many 15KW inverters and the current at 208 will be up there such that the transformers may be worth it. So yeah that is pretty common. I have only personally worked on one where that was the case. They actually used autos no isos (I didnt design). I think it was this series:


But you could use a regular isolation transformer too. Going all the way from 208-480 I dont think there is much gain in using an auto.
 
electrofelon said:
Yeah I think the best you can do is a fronius primo which does 1KV strings at 208 but the biggest one is only 15KW. I would really try and avoid the transformers but there is going to be a point where you need so many 15KW inverters and the current at 208 will be up there such that the transformers may be worth it. So yeah that is pretty common. I have only personally worked on one where that was the case. They actually used autos no isos (I didnt design). I think it was this series:


But you could use a regular isolation transformer too. Going all the way from 208-480 I dont think there is much gain in using an auto.
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Yeah, interconnecting a largeish PV system with a 208/120V service is problematic. There aren't any big 1000V DC 208/120V native inverters, so you are stuck with either a large number of small inverters and all that goes with that or reasonably sized 480/277V inverter(s) and a 480V-208V transformer.
 
On the one-line diagram, they showed two (2) inverters (transformerless, ungrounded types) going to a combiner panel (277/480 volts), then the combiner panel goes to a 75kva step-down transformer, bringing the voltage down to 120/208 volts. So it looks like its a Y-Y transformer. On the transformer secondary they showed a fused disconnect switch, which then connected to the utility. 2 questions came to mind:

1. they showed a connection to ground (cold water main and ground rod) on the fused disconnect switch on the transformer secondary. but isn't a grounding connection instead needed on the primary of the transformer (after the combiner panel)? The transformer is forcing the array / inverters / combiner panel to be a separately derived system (however with current flowing the other way to the service), which has no grounding connection. why ground on the switch instead? The switch is connected to utility which already has a service switch that's connected to grounding electrodes anyway.

2. This switch on the transformer secondary is labeled as service entrance rated. does this need to be SE rated?
 
First, to avoid confusion, let's use the terms "480 side" and "208 side"

The correct transformer is a 208 delta to 277/480 wye. Think of it as a "regular " SDS with the inverters as a load and the grounding electrode on the 480 side. Now I have heard several times on this forum of Y-Y Transformer being used. I am not sure what the purpose is. Those bring in confusion about to do with the 120/208 neutral and whether th XO and HO should be connected together (which would make it a non SDS.)
 
I just looked at it again. they definitely showed a Y-Y transformer, with a neutral on each side. and also they showed the GEC going to the electrodes on the 208 volt side of the transformer (via the 208 volt switch). so you're saying the GEC should connect to the 480 volt side of the transformer? that would make more sense. as well as using a delta-wye transformer.
 
Grouch1980 said:
I just looked at it again. they definitely showed a Y-Y transformer, with a neutral on each side. and also they showed the GEC going to the electrodes on the 208 volt side of the transformer (via the 208 volt switch). so you're saying the GEC should connect to the 480 volt side of the transformer? that would make more sense. as well as using a delta-wye transformer.
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Yes, on the 480 side. For NEC transformer secondary conductor and transformer protection purposes the 480 side is the secondary (well there is 705.30(B))
Someone doesn't know what they are doing. Show them this thread and I'm sure they immediately believe that the random guy on an internet forum knows what he is talking about 🤣
 
electrofelon said:
Yes, on the 480 side. For NEC transformer secondary conductor and transformer protection purposes the 480 side is the secondary (well there is 705.30(B))
Someone doesn't know what they are doing. Show them this thread and I'm sure they immediately believe that the random guy on an internet forum knows what he is talking about 🤣
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Maybe we can tell him to believe 2 random guys instead of 1. 😀
 
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