Figuring out service panel derate vs PV system size?

[HX_Guy]

Not sure if this is the right section to post this since it's more of a technical nature but I'm looking for some information on how to figure out if we can derate our 200A service panel to 175A for a 12.5kW system using a Fronius 11.4 inverter or if we need to derate further to 150A and if that's even possible, or if we need to upgrade to 225A instead.

What information do I need to know from the panel to figure this out?

[inetdog]

Not sure if this is the right section to post this since it's more of a technical nature but I'm looking for some information on how to figure out if we can derate our 200A service panel to 175A for a 12.5kW system using a Fronius 11.4 inverter or if we need to derate further to 150A and if that's even possible, or if we need to upgrade to 225A instead.

What information do I need to know from the panel to figure this out?

1. Most important you need to know the ampacity (rating) of the bus in the panel in question. Some service panels with 200A in their description have a 225A bus and just come with a 200A main breaker.
2. What year version of the NEC are you operating under. This area changes significantly in the [2014] edition.
3. What size breaker would you be feeding the Fronius in through?
4. Does the panel allow you to put the backfed breaker at the opposite end of the bus from the main? If not, then you are up against the 100% rule instead of the 120% rule.

With only a 1.25 multiplier on the output amperage of the Fronius, you would need a 60A breaker. That would mean that for a 200A bus the main breaker would have to be (240 - 60), 180A or smaller. With a 225A bus in the panel you would have more room. 225x1.2= 270, so you could combine a 200A main and a 60 or 70A backfeed breaker.

If you end up having to use a larger backfeed breaker (say 60x1.25 = 75A) then adjust the above numbers accordingly.

All of this varies significantly depending on the NEC year in force in your area.

[HX_Guy]

Thanks inetdog for the detailed reply!

1. How would I figure out the ampacity of the bus in the panel?
2. Not sure, is this determined by the year the house was built? If so, it was built 2013.
3. The installer said they would be feeding it through a 65A breaker.
4. I've had several companies that came out to do quotes say that they can use the 120% rule so it does seem that we can do what you're saying.

What I'm not sure is about all the other stuff we currently have...the pool, the two A/Cs, etc and how derating to 175A would affect those.


FULL SIZE PHOTOS:

http://www.nitnelav.com/panel1.JPG

http://www.nitnelav.com/panel2.JPG

http://www.nitnelav.com/panel3.JPG

[inetdog]

Thanks inetdog for the detailed reply!

1. How would I figure out the ampacity of the bus in the panel?
2. Not sure, is this determined by the year the house was built? If so, it was built 2013.
3. The installer said they would be feeding it through a 65A breaker.
4. I've had several companies that came out to do quotes say that they can use the 120% rule so it does seem that we can do what you're saying.

What I'm not sure is about all the other stuff we currently have...the pool, the two A/Cs, etc and how derating to 175A would affect those.

1. Unless you can get additional information from SquareD, you would have to assume based only on what is on the label that it is a 200A bus. The fast that the optional neutral lug has 225 in its part number implies very weakly that it might be a 225A bus.
2. It is determined by what code is in effect in your state now, since that is when the new work is being done.
3. OK.
4. As long as the backfeed breaker occupies the two slots at the bottom left of the panel (currently occupied by spares?), you can apply the 120% rule.

So, you will be in the position of having an allocation under the 120% rule of 240A. A 65A backfeed breaker means you need to use a 175A or smaller main.
Whether that will be large enough for your actual house load, not counting PV, depends on the actual load calculation, not just the sum of the load breakers, so we do not have enough information to say whether you could get by with a 175A main. Chances are very good that you could, but a local electrician would have to make that calculation.

[HX_Guy]

I've been getting some info on another forum and did a quick walk around the house adding up the loads of all major power users and it's not looking too good. The total number does not include any calculation for outlets or lights, just major appliances/AC/pool.

Washer: 12A
Cooktop: 38.3A
Dryer: 24A
Oven: 17A
House A/C : 26.4A
House A/H: 7A
Casita A/C: 9A
Casita A/H: 12.5A
Pool Equip: 14.2A
Disposal: 6.7A
Fridge: 8.5A
Microwave: 8.3A
2nd Fridge: 1.3A

Total of 185.2A not including any of the outlets or light breakers. Not looking too good. From what I understand, the total amps must be at or below the rated panel's amps which now it is, but with adding a PV system, and going by the 240A rule, seems the biggest breaker I could add is 55A.

(Not sure why the House A/H (air handler) is much smaller than the Casita A/H when the House A/C has a double 50A breaker and the Casita has a double 20A, they are probably mis labeled on the panel).

[inetdog]

I've been getting some info on another forum and did a quick walk around the house adding up the loads of all major power users and it's not looking too good. The total number does not include any calculation for outlets or lights, just major appliances/AC/pool.
...
...
Total of 185.2A not including any of the outlets or light breakers. Not looking too good. From what I understand, the total amps must be at or below the rated panel's amps which now it is, but with adding a PV system, and going by the 240A rule, seems the biggest breaker I could add is 55A.

(Not sure why the House A/H (air handler) is much smaller than the Casita A/H when the House A/C has a double 50A breaker and the Casita has a double 20A, they are probably mis labeled on the panel).

I have given you a more detailed answer on the other forum, but I will note here that you are adding apples to oranges with your amp numbers for 120V and 240V loads and that the actual NEC calculation could come out either a lot higher or a lot lower than your simple list of numbers. More info and on-the-spot examination is needed, as well as running the NEC calculation two different ways and taking the one more favorable to you.
You could still be OK, but getting in an electrician would be worth the cost to avoid surprises at inspection. The inspector will usually want to see actual formal load calculation results and I do not trust the solar installer to get that right.

[Ian S]

I've been getting some info on another forum and did a quick walk around the house adding up the loads of all major power users and it's not looking too good. The total number does not include any calculation for outlets or lights, just major appliances/AC/pool.

Washer: 12A
Cooktop: 38.3A
Dryer: 24A
Oven: 17A
House A/C : 26.4A
House A/H: 7A
Casita A/C: 9A
Casita A/H: 12.5A
Pool Equip: 14.2A
Disposal: 6.7A
Fridge: 8.5A
Microwave: 8.3A
2nd Fridge: 1.3A

Total of 185.2A not including any of the outlets or light breakers. Not looking too good. From what I understand, the total amps must be at or below the rated panel's amps which now it is, but with adding a PV system, and going by the 240A rule, seems the biggest breaker I could add is 55A.

(Not sure why the House A/H (air handler) is much smaller than the Casita A/H when the House A/C has a double 50A breaker and the Casita has a double 20A, they are probably mis labeled on the panel).

Since you don't list a water heater, I assume it's gas. If so, I'd be inclined to ditch the electric dryer in favor of a gas one. I did this years ago long before I had even thought of solar and it's one of the best money savers I ever put into effect. Of course I had a convenient gas line for the dryer but as long as you have gas in the house, it shouldn't be a big deal to get it to the dryer.

[HX_Guy]

Ian, as luck would have it, there is already a gas connection at the dryer! It's just that we had an electric one already when we moved to the new house but you gave me a good idea...may be worth to take out the electric dryer and replace it with a gas one, would save a minimum of 20A I would think and that might be enough to get us where we need.

We also have a gas connection for the cooktop but my wife prefers the glasstop vs the grates of a gas cooktop.

[Ian S]

Ian, as luck would have it, there is already a gas connection at the dryer! It's just that we had an electric one already when we moved to the new house but you gave me a good idea...may be worth to take out the electric dryer and replace it with a gas one, would save a minimum of 20A I would think and that might be enough to get us where we need.

We also have a gas connection for the cooktop but my wife prefers the glasstop vs the grates of a gas cooktop.

At the time I put in a gas dryer, I was on a demand/TOU rate and it was a real pain trying to run the dryer load at the best time of day and without other major electric loads at the same time. With a gas dryer, I hardly even notice the effect on my gas bill.