Proper way to tie PV system

M

Mr.Y

Member
Location
Iowa
Occupation
Electrical engineer
I have a question on a project with a single-phase electrical service.

There’s a shed with a 60A 1 phase MLO panelboard at the corner of the property that’s fed from the main garage with a 1PH 100A panelboard, and the client is planning to install a 12kW PV grid tie system on the shed roof.

What's the correct way to tie the PV system into the existing setup?
Due to the bus size it can't backfeed the shed panel via breaker, would it be acceptable to tap PV disconnect directly to the shed feeder?
Or due to the main panelboard size will need to run a new separate conductors to the meter (which is several hundred feet away)?

Thanks!

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Is the 100A panelboard the service panel, i.e. the service size is 100A? If not, you need to specify the full one-line diagram between the shed and the meter.

As for the wires running to the shed, if they have a 60A ampacity, then can support both a 60A load and 48A of PV inverter output (125% factor applies). You could change the panelboard at the shed to a 100A (using the 120% rule) or 125A (no restriction on breaker locations) panelboard, and interconnect there with a 60A breaker. Or you could just do a feeder interconnection on the 60A feeder supplying the 60A panelboard, as long as the shed panelboard has a main breaker. Bear in mind also the various requirements for the 225.31 disconnect(s) required at the shed, you'd need to be able to disconnect both forks of the 60A supply.

As to the utility end of the 60A feeder, if the 100A panel has a 100A main breaker and a 100A rated busbar, and the 60A feeder is supplied via a 60A breaker, that doesn't work for 705.12. So you again have a choice of upsizing the panelboard, or doing an interconnection on the supply side of that 100A panelboard. Which would be a supply-side interconnection in the sense of 705.12 if that panelboard is the service panel.

And then if that 100A panel is not the service panel, you'd have to apply similar considerations to the service panel as well as any other possible panels between the service panel and the 100A panel.

Cheers, Wayne
 
wwhitney said:
Is the 100A panelboard the service panel, i.e. the service size is 100A? If not, you need to specify the full one-line diagram between the shed and the meter.

As for the wires running to the shed, if they have a 60A ampacity, then can support both a 60A load and 48A of PV inverter output (125% factor applies). You could change the panelboard at the shed to a 100A (using the 120% rule) or 125A (no restriction on breaker locations) panelboard, and interconnect there with a 60A breaker. Or you could just do a feeder interconnection on the 60A feeder supplying the 60A panelboard, as long as the shed panelboard has a main breaker. Bear in mind also the various requirements for the 225.31 disconnect(s) required at the shed, you'd need to be able to disconnect both forks of the 60A supply.

As to the utility end of the 60A feeder, if the 100A panel has a 100A main breaker and a 100A rated busbar, and the 60A feeder is supplied via a 60A breaker, that doesn't work for 705.12. So you again have a choice of upsizing the panelboard, or doing an interconnection on the supply side of that 100A panelboard. Which would be a supply-side interconnection in the sense of 705.12 if that panelboard is the service panel.

And then if that 100A panel is not the service panel, you'd have to apply similar considerations to the service panel as well as any other possible panels between the service panel and the 100A panel.

Cheers, Wayne
Click to expand...
Thanks, Wayne! That’s what I thought, I just wanted to double-check in case I missed any exceptions.
I was wondering if I could use the shed feeder, but since the service panel is a 100A panel, it’ll need to interconnect on the supply side.
 
Mr.Y said:
I was wondering if I could use the shed feeder, but since the service panel is a 100A panel, it’ll need to interconnect on the supply side.
Click to expand...
But you can interconnect on the supply side of the 100A service panel, and still use the 60A feeder to the garage. Interconnecting on the supply side is equivalent to setting a second service disconnect. This new service disconnect would have a 60A OCPD and then connect to the 60A feeder to the garage. Then at the garage, you interconnect on the supply side of the existing 60A panel.

Cheers, Wayne
 
wwhitney said:
But you can interconnect on the supply side of the 100A service panel, and still use the 60A feeder to the garage. Interconnecting on the supply side is equivalent to setting a second service disconnect. This new service disconnect would have a 60A OCPD and then connect to the 60A feeder to the garage. Then at the garage, you interconnect on the supply side of the existing 60A panel.

Cheers, Wayne
Click to expand...
1747691418759.png
 
Right, but you could also do the interconnection with the single existing 60A feeder between the garage and shed as in the drawing below. [Not so well shown: the new service disconnect should have a 60A OCPD, and the two branches of the 60A feeder at the shed should have colocated disconnects, and the existing 60A panel needs a main breaker.]

Cheers, Wayne


1747691418759.png
 
apollogrid said:
In this case, the plan is to install the 12kW PV grid-tie system on a shed with a 60A MLO panel that is fed by a 100A panel in the main garage. As the shed panel has a bus capacity of only 60A, it is not possible to meet the requirement for reverse feed through the circuit breakers (the combined rating of the inverter circuit breaker and the main circuit breaker must not exceed 120% of the bus rating as per NEC 705.12). Therefore, it is not possible to connect the PV AC output directly to this panel. The recommended practice is to use a ‘supply-side tap’, i.e., connect the PV system in parallel in the feeder line between the main panel and the shed, using NEC-compliant connections (e.g., tap clamps or terminal boxes) with appropriate disconnect protection and labelling. If permitted by the local electric utility or AHJ (electrical jurisdictions), this is the most cost-effective option because hundreds of additional feet of cable do not need to be laid. If the supply-side tie cannot be approved, then consideration should be given to connecting the PV system back to the meter or in front of the main circuit breaker on a separate lead, which is more costly but the most technically sound, safe, and widely accepted. In summary, it is critical to avoid connecting the PV system directly to the shed panel as it cannot handle the feedback current of a 12kW system, which could be a safety hazard and a code violation.
Click to expand...
Sounds like an incorrect AI based response.
 
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