SEC's and PV systems

[nickelec]

Good afternoon all. I'm working with a solar company to provide anfld facilitate the service tie in/switch portion of the project

I had a question maybe some other experience guys here could clarify.

When designing a PV system to tie into an existing structure. Let's say for instance the existing structure has a property line box fed from the utility.

From said property line box there is one set of 500 MCM copper feeding the structure.

If the solar engineer is requiring a 400am service switch and meter for the PV system it is my understanding that you cannot just tie into the existing set of 500s as it would be overloaded.

am I wrong for thinking this way? Basically my question is does the PV system need to go directly to the utility and not tap the existing service entrance conductors?

I would say if the service entrance conductors that are existing from property line box to structure were rated for 800 amps you would be able to tie into it anywhere?

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[kingpb]

A back of the envelope simple one line sketch would be helpful.

 

[wwhitney]

When designing a PV system to tie into an existing structure. Let's say for instance the existing structure has a property line box fed from the utility.

From said property line box there is one set of 500 MCM copper feeding the structure.

If the solar engineer is requiring a 400am service switch and meter for the PV system it is my understanding that you cannot just tie into the existing set of 500s as it would be overloaded.

Not as I understand it.

The conductors from the property line box to the structure are Underground Service Conductors, and not Service Entrance Conductors (SECs), per the Article 100 definitions. 230.42 requires SECs to be sized at 125% of the continuous load (absent 100% rated OCPD), but there is no such requirement for Underground Service Conductors.

So the 500 MCM copper, if limited to the 75C rating, is good for 380A continuous. If your 400A solar OCPD is not 100% rated, your continuous inverter output current will be at most 320A. And (2020) 705.11 just requires that the continuous inverter output current not exceed the service conductor ampacity (no 125% factor). So all is copacetic.

If the 400A solar OCPD is 100% rated, then you'd need to confirm that the continuous inverter output current is no more than 380A, or figure out how to utilize the 500 MCM copper at its 90C ampacity of 430A. The latter seems plausible to me if the property line box has a 90C connection method, and you are able to use a 90C connection method at the structure for the SECs going to your 400A solar OCPD. Those SECs would need to be larger than 500 MCM, as they would be limited to the presumed 75C rating of the 400A solar OCPD.

Cheers, Wayne

 

[nickelec]

Not as I understand it.

The conductors from the property line box to the structure are Underground Service Conductors, and not Service Entrance Conductors (SECs), per the Article 100 definitions. 230.42 requires SECs to be sized at 125% of the continuous load (absent 100% rated OCPD), but there is no such requirement for Underground Service Conductors.

So the 500 MCM copper, if limited to the 75C rating, is good for 380A continuous. If your 400A solar OCPD is not 100% rated, your continuous inverter output current will be at most 320A. And (2020) 705.11 just requires that the continuous inverter output current not exceed the service conductor ampacity (no 125% factor). So all is copacetic.

If the 400A solar OCPD is 100% rated, then you'd need to confirm that the continuous inverter output current is no more than 380A, or figure out how to utilize the 500 MCM copper at its 90C ampacity of 430A. The latter seems plausible to me if the property line box has a 90C connection method, and you are able to use a 90C connection method at the structure for the SECs going to your 400A solar OCPD. Those SECs would need to be larger than 500 MCM, as they would be limited to the presumed 75C rating of the 400A solar OCPD.

Cheers, Wayne

Thanks wayne keep in mind that these under ground service conductors are already feeding a 400 Amp service in an occupied building. Were talking about tapping a PV system to the existing service

 

[wwhitney]

Thanks wayne keep in mind that these under ground service conductors are already feeding a 400 Amp service in an occupied building. Were talking about tapping a PV system to the existing service

Sure, but that doesn't change anything. If you splice into the Underground Service Conductors for your new PV SECs, those Underground Service Conductors upstream (utility side) of the splice will at any given point in time only be carrying current one way or the other. The load current and the PV current can't combine to overload the Underground Service Conductors upstream of the splice. Downstream of the splice, the utility is an effectively unlimited current source already, so adding the available current from the PV won't change anything about how the existing SECs are protected by the existing service OCPD.

Cheers, Wayne

 

[don_resqcapt19]

Will this connection be on the line or load side of the existing meter?

 

[jaggedben]

If you're indeed connecting on the utility side of the existing service disconnect and OCPD then Wayne is correct. The existing service OCPD protects the conductors you are tapping to.

The utility may own conductors or a transformer upstream that they may think could be overloaded. But for conductors covered by the NEC, there is no need to add solar to load when on the line side of the service disconnect and OCPD.

 

[nickelec]

Will this connection be on the line or load side of the existing meter?

So this type of PV set up is a community solar set up . its tied in directly to utility before the Main service OCPD and has its own meter and service switch. The Underground conductors we plan to tap are considered customer cables not utility. see attached picture.

In the picture u can see the Utility(coned) cables and two sets of customer cables in Customer supplied and Installed PLB.


My plan was to Put a wireway at this meter and tie in the PV system . not at the PLB

 

[ggunn]

The utility may own conductors or a transformer upstream that they may think could be overloaded.

This. The rating of the utility transformer will put a limit on how much PV (as determined by inverter maximum power output) you can connect, and the utility may have a further restriction on the percentage of the transformer kVA you can connect.

 

[nickelec]

Here's the picture

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[nickelec]

So this type of PV set up is a community solar set up . its tied in directly to utility before the Main service OCPD and has its own meter and service switch. The Underground conductors we plan to tap are considered customer cables not utility. see attached picture.

In the picture u can see the Utility(coned) cables and two sets of customer cables in Customer supplied and Installed PLB.


My plan was to Put a wireway at this meter and tie in the PV system . not at the PLB

It will be tied in on the load side of service

 

[nickelec]

Will this connection be on the line or load side of the existing meter?

Load side of existing meter with its own SS and Meter

 

[nickelec]

My thinking is it needs to be treated as a separate service.

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[jaggedben]

My thinking is it needs to be treated as a separate service.

Separate service disconnect, not a separate service.
Code cycle matters a bit here. The code has progressively treated it more explicitly like an additional service disconnect over the last few cycles. Before that there were ambiguities about grounding and bonding in particular. But treating it like an additional service disconnect has always been a reasonable approach.

 

[nickelec]

2008 code cycle here. So are we in agreement that I can just tap load side of existing meter ? The one set of 500s?

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[jaggedben]

2008 code cycle here.

Wow that's ancient, especially with respect to solar.

So are we in agreement that I can just tap load side of existing meter ? The one set of 500s?

Yes, see 705.12(A).
The meter is irrelevant to the code. But yes, you can do it.