120% rule

[Eros]

i have a 200A main breaker residential panel that needs to accommodate a 20kw solar array. i can not install a 90A breaker in the panel due to code violations and the 120% rule. How can i interconnect the solar with the electrical service and still meet the code guidlines ?

 

[Smart $]

i have a 200A main breaker residential panel that needs to accommodate a 20kw solar array. i can not install a 90A breaker in the panel due to code violations and the 120% rule. How can i interconnect the solar with the electrical service and still meet the code guidlines ?

Tap into the service conductors and install an inline 100AF fused safety switch with 90A fuses.

It's a Supply-Side Point of Connection, covered under 705.12(A).

 

[ggunn]

Tap into the service conductors and install an inline 100AF fused safety switch with 90A fuses.

It's a Supply-Side Point of Connection, covered under 705.12(A).

I'll just add that you'll want to consult with your AHJ and see if they have specific guidelines to address line side connections. Some of them want the grounding to be done a certain way.

 

[pv_n00b]

In this situation you have a few choices, one which has already been pointed out, but there are several and I'll list them in increasing order of pain and expense:

• Supply side interconnection; requires AHJ and utility approval, and you have to be able to physically do it in a code compliant manner which is not a shoe in for a resi panel.
• Check the load to see if you can swap out the 200A main for a 150A. That will get you the 90A allowance you need for the PV back feed CB
• Put in a larger service panel, a lot of people think this kills a project but the big guys do it all the time.

Good luck

 

[ggunn]

• Put in a larger service panel, a lot of people think this kills a project but the big guys do it all the time.

I don't know if our operation qualifies as a "big guy", but we frequently do this, especially in jurisdictions where line side interconnections have a lot of conditions that must be met. The strategy is to leave the main breaker the same size and install larger capacity busbars to give yourself more headroom within the constraints of the 120% rule. Something we do a lot is to swap out a 200A bus for a 225A bus where the main is 200A; this increases our headroom from 40A to 70A. Obviously this is not enough to insert 90A of PV, but if you can increase the bus to 225A and reduce the main to 175A that would get you 95A.

But if your AHJ will allow IPC's (insulation piercing connectors) and let you simply connect the EGC from the PV back to the grounding block in the MDP, that would be the way to go, IMO. It's inexpensive and will allow you to do the job without having to call for a power shutdown from the utility.

 

[jaggedben]

In residential meter-main combos supply side taps are generally not feasible. If you've got a separate meter base then that's a more feasible option to explore.

If it's a meter main, the main breaker downsize is probably the best option (to 150A in this case). We do this often enough we call them MBDs. I've yet to do load calculations on an existing 200A service that came out over 150A, but your mileage may vary. On a 200A service MBD is usually (not always) my go to option over a service panel replacement, but we do those a lot, too (and I don't think we're a 'big guy' either).

Last option that hasn't been mentioned already: downsize the PV system.

 

[cookerobert21]

Why can't you tap in the main panel or under the meter and just put a fused 100 amp disconnect fused at 90amp

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[ggunn]

Why can't you tap in the main panel or under the meter and just put a fused 100 amp disconnect fused at 90amp

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That's one of the options we are talking about.

 

[ggunn]

In residential meter-main combos supply side taps are generally not feasible. If you've got a separate meter base then that's a more feasible option to explore.

I have yet to encounter one of those so I dunno. In these here parts all the meters are separate.

Last option that hasn't been mentioned already: downsize the PV system.

Enough to move from a 90A breaker to a 40A breaker? Ouch.

 

[jaggedben]

Enough to move from a 90A breaker to a 40A breaker? Ouch.

Yup, that's why it's the last option. Just thought I'd put it out there for completeness.

I've had a few customers choose this route, but I think they've all been of the variety where 'downsizing from 18 to 17 solar modules means you don't have to upgrade your 100A service'. Most likely someone who can afford the 20kW system can also afford the electrical work to connect it.