Separately Derived Systems and 250.25 & 250.30(C)

[pvgreeze]

Hi all, two questions:

1. Is the working interpretation of 250.25 that the utility side transformer characteristics for a supply-side interconnected PV system must match the characteristics of the existing service?
   • Example - a building has a 2000A 208/120V 3Ph 4W service from the utility. If the PV system on the roof utilizes 480/277V inverters, could a 480/277V - 208V step down transformer be used; that is, the utility side of the transformer is 208V 3Ph 3W delta and does not match the existing service
   • 250.25 does not specifically state that the exact phase and neutral characteristics must match the service, but that they "comply with the requirements of 250.24(A) through (D)." This includes 250.24(C)(3), which includes provisions for a delta-connected service
2. Using the above example, suppose a transformer is installed outside of a building. The POI is inside the building, the PV system is on top of the building. 250.30(C) states that "if the source of the separately derived system is located outside the building or structure supplied, a grounding electrode connection shall be made at the source location to one or more grounding electrodes in compliance with 250.50." Does the equipment grounding conductor (EGC) between the supply-side service disconnect, which is connected to the existing building grounding electrode system (GES) via a properly sized grounding electrode conductor (GEC), satisfy this requirement?
   • This assumes that the primary side (utility side) and secondary side (PV side) grounding pads are electrically connected inside of the transformer by the manufacturer

Be curious to know thoughts. Yes, I know AHJ rules supercede any NEC discussion. Typically I default to utility new service guidelines for any supply side (line side) interconnected PV systems, especially if there is a step down transformer involved.

 

[ggunn]

...a building has a 2000A 208/120V 3Ph 4W service from the utility. If the PV system on the roof utilizes 480/277V inverters, could a 480/277V - 208V step down transformer be used; that is, the utility side of the transformer is 208V 3Ph 3W delta and does not match the existing service

Yes, I have done this many times. The PV transformer should be 208V delta (no neutral) on the utility side and 480/277V wye with a grounded neutral on the inverter side with a kVA rating less than or equal to the maximum kW output of the inverter(s). Such transformers are common off the shelf items for most manufacturers.

 

[wwhitney]

In considering these questions, it is helpful to ignore the fact that the PV system is a source. The transformers involved will all be energized by the utility.

Thus for question (1), 250.25 is only commenting on what you do at the service disconnect and on the utility side of it. Then from the service disconnect, you are free to connect whatever load or interactive source you like. So use a 208V delta : 480Y/277V transformer is fine.

And for question (2), the "source of the SDS" would be the transformer and has nothing to do with the PV location.

Cheers, Wayne

 

[Elect117]

Hi all, two questions:

1. Is the working interpretation of 250.25 that the utility side transformer characteristics for a supply-side interconnected PV system must match the characteristics of the existing service?
   • Example - a building has a 2000A 208/120V 3Ph 4W service from the utility. If the PV system on the roof utilizes 480/277V inverters, could a 480/277V - 208V step down transformer be used; that is, the utility side of the transformer is 208V 3Ph 3W delta and does not match the existing service
   • 250.25 does not specifically state that the exact phase and neutral characteristics must match the service, but that they "comply with the requirements of 250.24(A) through (D)." This includes 250.24(C)(3), which includes provisions for a delta-connected service
2. Using the above example, suppose a transformer is installed outside of a building. The POI is inside the building, the PV system is on top of the building. 250.30(C) states that "if the source of the separately derived system is located outside the building or structure supplied, a grounding electrode connection shall be made at the source location to one or more grounding electrodes in compliance with 250.50." Does the equipment grounding conductor (EGC) between the supply-side service disconnect, which is connected to the existing building grounding electrode system (GES) via a properly sized grounding electrode conductor (GEC), satisfy this requirement?
   • This assumes that the primary side (utility side) and secondary side (PV side) grounding pads are electrically connected inside of the transformer by the manufacturer

Be curious to know thoughts. Yes, I know AHJ rules supercede any NEC discussion. Typically I default to utility new service guidelines for any supply side (line side) interconnected PV systems, especially if there is a step down transformer involved.

For (1), 208 is a wye voltage. 240V is a delta. You might need to confirm the service type. I personally haven't see a utility feed 208V delta. That being said, installing a transformer between them is how your would make the interconnection.

For (2), I will let others respond more thoroughly, but no. You need to ground the transformer effectively. You can probably use a wire sized as a GEC and double the use as both the GEC and EGC but that could be too difficult depending on construction limitations. Like the type of splices or conduit being used. GECs have different rules than EGCs. More than likely you will need to run a separate GEC to the GES to this transformer. And possibly the fused disconnect right before supply side interconnection. You will also need to size your grounding and bonding differently with SSBJs or SBJs rather than just EGCs.

 

[ggunn]

For (1), 208 is a wye voltage. 240V is a delta. You might need to confirm the service type. I personally haven't see a utility feed 208V delta. That being said, installing a transformer between them is how your would make the interconnection.

For a 208/120V wye service he would use a 208V delta primary to 480/277V wye secondary transformer. There should be no connection between the transformer primary and the service neutral.

 

[Elect117]

For a 208/120V wye service he would use a 208V delta primary to 480/277V wye secondary transformer. There should be no connection between the transformer primary and the service neutral.

I thought he was saying the utility was 208V delta. I misunderstood then.

 

[ggunn]

I thought he was saying the utility was 208V delta. I misunderstood then.

He said: a building has a 2000A 208/120V 3Ph 4W service from the utility, i.e. 208/120V wye service. Even if it were 208V delta, it wouldn't change anything with the transformer and inverter connections.

 

[Elect117]

He said: a building has a 2000A 208/120V 3Ph 4W service from the utility, i.e. 208/120V wye service. Even if it were 208V delta, it wouldn't change anything with the transformer and inverter connections.

He also said " This includes 250.24(C)(3), which includes provisions for a delta-connected service". Which is typically used with corner grounded deltas.

I didn't say that it would change anything. Only that it sounded odd to me.

 

[ggunn]

He also said " This includes 250.24(C)(3), which includes provisions for a delta-connected service". Which is typically used with corner grounded deltas.

I didn't say that it would change anything. Only that it sounded odd to me.

Well, the transformer would be delta connected irrespective of the grounding of the service and whether or not there is a neutral; maybe that is what he was referring to.

 

[jim dungar]

Well, the transformer would be delta connected irrespective of the grounding of the service and whether or not there is a neutral; maybe that is what he was referring to.

I agree, he probably meant a delta feeder not a POCO service.

Another case of industry slang getting in the way.