Load side tap

[SunNinja]

2020 NEC
I have a new situation. I believe I will need to do a Load Side Tap, and I want to see if I'm interpreting the rules around it correctly.
Existing:
Meter main combo, with 2 x 100A breakers. One supplies a hot tub. The other supplies a manual transfer switch for their backup generator, which then supplies MSP.
No room between meter and 100A breaker to do a line side tap - the rules surrounding that I'm comfortable with.
Solar is only 20A, we are protecting and sizing wire at 30A (microinverters can handle up to a 30A string, future expansion possiblility).

I need to interrupt either 100A feeder and tap off for the solar connection.
I'm reading 705.12(B)(1&2) to say that the portion of the run between your solar connection and the Load needs to be (125%solar) + (OCPD for Feeder). Opinions?
Makes sense, as the house could potentially pull 100A off the grid and SolarAmps at the same time.

Also, I am trying to figure out why I believe ANY tap will need a fusible 60A rated disconnect, even if fused down to 30A. It would seem that it is only required on supply-side tap because the disconnect can be perceived as subject to service disconnect rules. So - can a load side tap just be a 30A fused disconnect?

 

[jaggedben]

Yes, a load side tap can be a 30A fused disco.

If you do a tap then the feeder and transfer switch and anything downstream need to be 120A rated or greater at least until you land on a 100A OCPD. That's also part of 705.12(B)(1).

At this amperage it might be easier to insert a subpanel than to do a tap.

 

[SunNinja]

Yeah, downstream of the tap 130A rated till you hit a breaker.
Have to look at more detail about what’s actually onsite to figure out what direction to go.
Extra device on the wall vs demo & repull wire.
$ cost?
Carbon cost?

 

[wwhitney]

I'm slightly confused--doesn't it suffice to add a 100A main breaker to the MSP downstream (away from the grid) from the MTS, and leave the feeder at 100A? The MTS isn't a load, so the 100A breaker at the MSP should satisfy 705.12(B)(1)(b).

Cheers, Wayne

 

[jaggedben]

I'm slightly confused--doesn't it suffice to add a 100A main breaker to the MSP downstream (away from the grid) from the MTS, and leave the feeder at 100A? The MTS isn't a load, so the 100A breaker at the MSP should satisfy 705.12(B)(1)(b).

Cheers, Wayne

Correct. In post #2 I should have said 'unless' instead of 'until'. But if the feeder, switch, and panel it serves are all 120A or more, no need for the breaker.

('MSP' here is an incorrect usage for the downstream panel as the 'S' stands for 'service', which is the meter main. Call the downstream panel a subpanel or, if you like, main distribution panel - MDP).

 

[jaggedben]

Yeah, downstream of the tap 130A rated till you hit a breaker.
Have to look at more detail about what’s actually onsite to figure out what direction to go.
Extra device on the wall vs demo & repull wire.
$ cost?
Carbon cost?

Not sure why you'd have to demo and repull anything without seeing details. I've done many new subpanels like this where I brought the wiring back to the existing panel and spliced it to the existing feeder there. Or, if the length was long enough, into an adjacent jbox. You just have to be prudent and only do this where there's enough space for splicing, or you can make it. Dealing with a 100A feeder this way is usually doable. (200A not so much).

 

[SunNinja]

Not sure why you'd have to demo and repull anything without seeing details.

Yeah - I’m just assuming they used 100A wire, not 130A wire (solar gets 125% cuz continuous - and, in this case we are leaving room to max out the string in the near future).

Aha - I’m used to calling the primary panel in a house the MSP - never caught that I’m misusing the term.

Wwhitney: I think you were asking why I don’t put the solar downstream of the MTSwitch - when the generator is running, you can’t have solar in the system without a bunch of fancy voltage & frequency controls. It works fine w batteries and off grid inverter, but we don’t have that here.

Oh. Yes. If I come off the meter main breaker and come into a sub panel with a 100A breaker to then feed MTS… duh. It can go back to 100A.
Then I wonder if 120% of the main breaker bus needs to be capable of carrying the 130A if done that way.

 

[wwhitney]

Wwhitney: I think you were asking why I don’t put the solar downstream of the MTSwitch - when the generator is running, you can’t have solar in the system without a bunch of fancy voltage & frequency controls. It works fine w batteries and off grid inverter, but we don’t have that here.

No. I was asking if the MDP (which you called MSP) has a 100A main breaker, or can easily have a 100A main breaker added. If yes, then that satisfies the 705.12(B)(1)(b). You can use a 100A feeder from the actual service panel to the solar interconnection to the MTS to the MDP.

Because this is a situation where people before you may have gotten it wrong, check that the 100A feeder is actually #3 Cu or #1 Al; if it's #4 Cu or #2 Al, you'll need to downsize the 100A breakers in the meter main and MDP to 90A.

Then I wonder if 120% of the main breaker bus needs to be capable of carrying the 130A if done that way.

What's the bus rating on the meter main? I was assuming it was 200A because it has 2x100A breakers. 200A would let you do up to 40A (after 125%) of PV interconnection under the 120% rule; the breaker size (100A) doesn't matter. If it's 125A, you're limited to 25A, and for 100A you'd be limited to 20A; either way you'd not have the 30A future expansion you want.

Cheers, Wayne

 

[jaggedben]

It sounded to me like the service panel has 2 x 100A main breakers, no load side bus.