NEC 705.12(D) - Does this apply to main panel if subpanel used?

[Charlied]

Hello,

I was originally planning to do the 120% rule (705.12(D)(2)(3)(b)) and bring the PV supply into the main panel. Due to the fact it will be difficult to run the line into that panel, I am considering alternative options using sub panels. Here is my layout/configuration.

100A Main Breaker outside by meter
To
125A Main Service Panel inside
To
50A Breaker in middle of Main service panel
To
100A Subpanel

The sub panel has breaker's that go across horizontally, and the line feeder comes in on both sides of the panel. So there is no real way to have the PV supply come in on the opposite end of the subpanel. Although according to 705.12(D)(2)(3), I don't think I would need to worry about having the PV supply be on the opposite side of the line feeder for that subpanel. Because the 50A breaker supplying the subpanel and the 40A breaker from the inverter add up to 90A, which is less than that 100A rating of the bus bar of the subpanel.

So far so good, my main concern is the connection location of the 50A breaker supplying the subpanel that I'm going to connect the PV supply into. That 50A breaker in the main panel that is supplying the power to the sub panel is located in the middle of the main panel. Because the subpanel will backfeed into the main panel, I assume I will have to apply the rules to the main panel too. If I have to apply the rules to the main panel, I have to follow the 120% rule unless I upgrade that panel. When following the 120% rule, I would assume I would need to move that 50A subpanel breaker down to the bottom to protect from arcing. I'm not sure I have enough wire to move that 50A breaker to the bottom of the main panel.

What are your thoughts? Would I need to move that 50A breaker on the main panel down to the bottom in order to pass inspection?

Thanks,
Chris

[jflorey2]

I think it would actually be easier to replace the panel than to do a supply side tap, but I could be totally wrong.

You can get a 60 amp meter adapter that allows you to feed up to 60 amps directly to the meter; the panel is then unaffected. Search for "renewable meter adapter" to see some example.

[reader2580]

If I was doing this I would just downrate that main to 175. Ya those breakers are expensive but replacing a main panel is a lot of work. Don't forget that your AHJ is got you on his radar now (from your other thread) so might want to follow K.I.S.S.

Minnesota is under 2014 code and will go to 2017 code on July 1st.

Cost is not an issue for replacing the main breaker as it only costs $84. It looks to me like replacing the main breaker in the main load center will be almost as hard as replacing the entire load center. There is no way to pull the main wires out of the main breaker because they are so stiff. I think I'll have to disconnect everything from the panelboard and unscrew the panelboard so I can pull the panelboard down to release the wires from main breaker.

The garage will need to be brought up to 2014 code when I replace the load center there. There is only a three wire feed from house to garage and I need to replace the feed with a four wire one. I am probably going to have to move the load center in the garage to a new location because a concrete slab blocks running wire to the current location. Running new cable to the garage will probably be the hardest part of the whole solar project.

[bcroe]

Remember, the inverter feed breaker should be at the opposite end of the box busbar, from the AC line
feed. That reg prevents currents from both sources ever adding together at any point on the busbar. If
the garage feed then heads over to your main box, its breaker again should be at the opposite end. So
then there is no way the busbars or wiring can over current. Additional regs may limit the sum of all
power source breakers to 120% of the original house box, this gives you belt and suspenders protection
in that box. Bruce Roe

[tyab]

(assuming your under 2014 code)

So for a 34.2 amp out, X 1.25 gives 42.75 so you round up to a 50 amp OCPD.

Upgrading that garage sub from 60 is required and to a 125 is the best solution. You keep the 60amp OCPD in the main for it, just means you can't pull more than 60 in your garage which is what you have currently. The feeder wire is already sized for that 60. You will be back feeding 50 (ya I know its only 34.2 but you are under 2014 code) into your 200 amp main, so you have correctly computed that you need to down rate that main OCPD to 175. If you are under 2011 code you will have to replace that feeder to the sub with larger wire but under 2014 code you don't.

If I was doing this I would just downrate that main to 175. Ya those breakers are expensive but replacing a main panel is a lot of work. Don't forget that your AHJ is got you on his radar now (from your other thread) so might want to follow K.I.S.S.

[reader2580]

What is stopping me from using a meter-main panel inside is it simply wouldn't fit. It also appears to cost more than a main lug panel and buying a main breaker separately.

[inetdog]

There is nothing stopping you from using a meter-main panel inside and just not using the meter socket!

[reader2580]

I can't find a Siemens 225 amp panel that has a 200 amp main breaker. I would need to buy a main lug 225 amp panel and then buy the 200 amp main breaker separately. Eaton and Square D both make "solar ready" panels that have a 225 amp bus with 200 main breaker. The only Siemens "solar ready" product is an outdoor load center with meter socket. My load center is in the basement.

[reader2580]

I am not sure it would be as much work, especially when you take into account removing and repulling all of the branch wiring (unless you just swap the guts inside the enclosure.)
But the new main breaker may well cost as much as the new panel with breaker as a bundle.
If you change the panel to a 225A bus you may find yourself changing brands and that may mean replacing all of the branch breakers too.

The current panel is an ITE panel. Siemens bought the company and current Siemens/Murray breakers can be used in the panel. Many of the breakers have been replaced with AFCI due to major electrical work to the house. Siemens makes a 225 main lug amp panel that uses the same breakers. The panel is $160 and a 200 amp main breaker is $84. A 175 amp main breaker for current panel is also $84. (Yes, Siemens still sell main breakers for a panel type that was discontinued in 2002.)

The wire from the meter is very stiff aluminum wire. I highly doubt I could pull the wire out of the main breaker lugs to replace the main breaker. I think I would need to disconnect everything from the panel board and then unscrew the panel board and pull it down to get free of the free cables.

The idea of just going with an 7.6 KW inverter and losing 10% of output on the best day is starting to sound better the more I think about it,

[inetdog]

Replacing the main breaker is probably just as much work as replacing the load center.

Another option I am considering is just going with a 7.6 KW inverter that will not require changes to the main load center. With thirty 300 watt panels I will be at 118% of inverter capacity which is within specs. The ideal string size seems to be 10 panels with the panels I am getting.

I am not sure it would be as much work, especially when you take into account removing and repulling all of the branch wiring (unless you just swap the guts inside the enclosure.)
But the new main breaker may well cost as much as the new panel with breaker as a bundle.
If you change the panel to a 225A bus you may find yourself changing brands and that may mean replacing all of the branch breakers too.

[reader2580]

Reader2580 - (all of this may be mute since your AHJ wants you to be a qualified person), but lets step back a moment. How big of a system were you planning on needing. Just want to run the calcs starting with that garage subpanel.

My plans are to use a Fronius Primo 8.2 KW inverter which has output of 34.2 amps at 240 volts. I plan to replace the garage subpanel with a 12 circuit panel that has a 125 amp bus. The feed is 60 amps from the house. The garage subpanel should be fine since the bus bar is 125 amps.

The house panel is 200 amps. The garage is fed from a 60 amp breaker. I will be slightly over the bus bar capacity. The options are a new load center with 225 amp bus and 200 amp main breaker, or putting a 175 amp main breaker in the current load center. Replacing the main breaker is probably just as much work as replacing the load center.

Another option I am considering is just going with a 7.6 KW inverter that will not require changes to the main load center. With thirty 300 watt panels I will be at 118% of inverter capacity which is within specs. The ideal string size seems to be 10 panels with the panels I am getting.

[tyab]

Reader2580 - (all of this may be mute since your AHJ wants you to be a qualified person), but lets step back a moment. How big of a system were you planning on needing. Just want to run the calcs starting with that garage subpanel.

[peakbagger]

FYI , the local utility in my area does not allow any PV wiring in their meter enclosure and does not accept meter adaptors. Apparently other utilities have the same attitude. The local utility requires the line side tap in a separate enclosure downstream of the meter enclosure. If there is room in the main panel upstream of the main breaker that's okay but most folks apparently are cutting a new enclosure into the conduit.

[GRickard]

The tap would be on the load side of the meter base. With either option you will have to have the utility pull the meter and the jacks on the pole, but with the taps all you do is replace the existing lug plates with the new ones and install conduit and wire to the disconnect within 10 feet (of wire). That's a lot easier than a panel change out and with the right prep work and communication could be done in a few minutes with the utility and the inspector there to minimize outage time.

Greg

[reader2580]

I think it would actually be easier to replace the panel than to do a supply side tap, but I could be totally wrong. It appears I could just replace the main breaker with a 175 amp one, but replacing the main breaker will be almost as difficult as replacing the whole load center from the looks of it.