Figuring out service panel derate vs PV system size?

HX_Guy

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?

solar_dave

Re: Figuring out service panel derate vs PV system size?

You need to know the manufacturer's rating on the panel bus bar, most likely in writing for the AHJ. One of the caveats to derating is will the AHJ allow a lower main breaker? Your load profile might not allow them to OK that, this was the case here. Also increasing the input size requires the proper wire size from the transformer, but I bet they wouldn't whine about that with a derated breaker. Although they could as the next owner could make the derated breaker go away.

The 175 amp derate should suffice, as the rule is 120% of bus bar - main breaker for max back-feed amps.

My little story was that the AHJ would not allow a 200 --> 175 amp derate on the main panel because of the load profile, two 3 ton AC units, electric range, electric clothes dryer, pool pump ... The only way I could add 12.5 Kw solar is to upgrade the service entrance to 400 amps from the transformer to the meter and then split there to two 200 amp panels with one derated to 175. In my case I didn't whine to much because I knew I was adding a 125 amp sub for the new constuction building and a 60 amp sub for electric car charging. The upside was it was all rolled into the solar project so the tax credit would apply to the $4300 modification.

I would take a picture of the panel, and get the model number of the panel then list all the major load items and ask the AHJ.

HX_Guy

Re: Figuring out service panel derate vs PV system size?

Thanks Dave. Real quick, that is "AHJ"?

Here are some pictures of the panel and breakers we already have in place. Like you, we have quite a few things in there like 2 A/C units, pool, two fridges, etc.


FULL SIZE PHOTOS:

http://www.nitnelav.com/panel1.JPG
http://www.nitnelav.com/panel2.JPG
http://www.nitnelav.com/panel3.JPG

solar_dave

Re: Figuring out service panel derate vs PV system size?

AHJ = Authority Having Jurisdiction AKA code/permit monkeys

Me thinks you are in the realm of an upgrade, just my opinion and I am not a electrical engineer or code guy. In any case you will need a double slot at the bottom of the main to add that 60 amp breaker in if allowed by that AHJ.

I remember someplace reading that Square D will NOT tell you that the rating is any greater than 200 amps. There is one company that will but I forget who it is right now.

HX_Guy

Re: Figuring out service panel derate vs PV system size?

I do have the slots available, the picture was just cut off...

solar_dave

Re: Figuring out service panel derate vs PV system size?

In my case the code guy actually came to the house and looked it over before saying NO! You might be luckier, I don't think I have ever seen the house draw more than 15K watts, which is 62.5 amps @ 240 volts.

Then there is the possibility of a line side tap. Again the code guy here said only on commercial installs, no residential.

BTW just the 240 breakers there add up to 220 amps, I am not sure on where they decide it is an overload.

Cariboocoot

Re: Figuring out service panel derate vs PV system size?

Note: having the slots for breakers available is not the same as having capacity for additional breakers available. In the case of back-feeding from a GTI this is not an issue, as it will contribute current not take it away. Some AHJ's do not comprehend this.

A 200 Amp service under NEC can have 40 Amp back-feed breaker: 9.6 kW worth. A 12.5 kW GTI will be 52 Amps, requiring a 70 Amp breaker (good luck - they can be hard to find). As such your main breaker needs to be derated to 170 or less. The AHJ may well have a problem with such a small main and all those branch breakers on it - and so would you if the combined demand popped the main at 2:00 AM. Add them all up, with 120 VAC breakers counting as 1/2 current, and see what you get.

solar_dave

Re: Figuring out service panel derate vs PV system size?

Cariboocoot wrote: »

Note: having the slots for breakers available is not the same as having capacity for additional breakers available. In the case of back-feeding from a GTI this is not an issue, as it will contribute current not take it away. Some AHJ's do not comprehend this.

A 200 Amp service under NEC can have 40 Amp back-feed breaker: 9.6 kW worth. A 12.5 kW GTI will be 52 Amps, requiring a 70 Amp breaker (good luck - they can be hard to find). As such your main breaker needs to be derated to 170 or less. The AHJ may well have a problem with such a small main and all those branch breakers on it - and so would you if the combined demand popped the main at 2:00 AM. Add them all up, with 120 VAC breakers counting as 1/2 current, and see what you get.

The Fronius 11.4 is rated at 47.5 amps @ 240 volts. It is slightly under the 80% rule for a 60 amp breaker.
http://www.fronius.com/cps/rde/xchg/SID-3F3E667D-66EC76EC/fronius_usa/hs.xsl/2714_1483.htm

Still requiring a derate but the 175 amp main should suffice. 200amps X 120% = 240amps - 175 main = 65 amps max back feed.
Coot is right the AHJ is going to have heartburn over your load profile to derate the panel.

jcheil

Re: Figuring out service panel derate vs PV system size?

Cariboocoot wrote: »

Note: having the slots for breakers available is not the same as having capacity for additional breakers available

Just like the "I still have checks left so I must have money available"

Cariboocoot

Re: Figuring out service panel derate vs PV system size?

solar_dave wrote: »

The Fronius 11.4 is rated at 47.5 amps @ 240 volts. It is slightly under the 80% rule for a 60 amp breaker.
http://www.fronius.com/cps/rde/xchg/SID-3F3E667D-66EC76EC/fronius_usa/hs.xsl/2714_1483.htm

Still requiring a derate but the 175 amp main should suffice. 200amps X 120% = 240amps - 175 main = 65 amps max back feed.
Coot is right the AHJ is going to have heartburn over your load profile to derate the panel.

That's good!
I did the calcs based on the 12.5 spec the OP had given.

solar_dave

Re: Figuring out service panel derate vs PV system size?

Cariboocoot wrote: »

That's good!
I did the calcs based on the 12.5 spec the OP had given.

Ah the problem with STC rears its ugly head! LOL hehehe

HX_Guy

Re: Figuring out service panel derate vs PV system size?

solar_dave wrote: »

BTW just the 240 breakers there add up to 220 amps, I am not sure on where they decide it is an overload.

Cariboocoot wrote: »

. Add them all up, with 120 VAC breakers counting as 1/2 current, and see what you get.

As Dave pointed out, just the 240 breakers already add up to 220 amps. If I add in all the the 15A 120V breakers (12 of them X 15 / 2 = 90) plus the 20A 120V breakers (7 X 20 / 2 = 70) I get a total of 380A? Umm something can't be right.

DanS26

Re: Figuring out service panel derate vs PV system size?

Just adding up breaker sizes is meaningless......it's actual load that counts. That requires a load analysis.

Cariboocoot

Re: Figuring out service panel derate vs PV system size?

HX_Guy wrote: »

As Dave pointed out, just the 240 breakers already add up to 220 amps. If I add in all the the 15A 120V breakers (12 of them X 15 / 2 = 90) plus the 20A 120V breakers (7 X 20 / 2 = 70) I get a total of 380A? Umm something can't be right.

They don't expect full power on all branch circuits at once. Try 50% power: 190 Amps.
In other words they'll see the service as maxed out with a 200 Amp breaker and logically would not expect a 175 to work.

HX_Guy

Re: Figuring out service panel derate vs PV system size?

DanS26 wrote: »

Just adding up breaker sizes is meaningless......it's actual load that counts. That requires a load analysis.

Dan, any tips on doing a loan analysis? Or is that really something for the pros to figure out?

SkiDoo55

Re: Figuring out service panel derate vs PV system size?

Agree need to do a load analysis.
i have over 400 amp s of breakers in a 200 amp main panel (40 amp backfeed) and if I turned everything on,dual convection ovens, dryer, A/C 3 ton, all the lights, fired up the air compressor , and MIG welder wouldn't exceed @75 - 80 amps max.
But yes an individual load survey and if the local codes people will allow a breaker down-size will determine end result.

solar_dave

Re: Figuring out service panel derate vs PV system size?

HX_Guy wrote: »

Dan, any tips on doing a loan analysis? Or is that really something for the pros to figure out?

The installer and the AHJ did a load analysis on mine with about the same situation you have, permitting required a service entrance upgrade but my AHJ is pretty fussy. That is when the installer brought up line side taps and was rejected outright as not on residential. They rejected my final inspection 3 times, once for some really minor stuff. You may have a more reasonable department.

Good luck on the AHJ sign off, just trying to point out you have a risk that needs mitigating by getting the AHJ to buy off on your design. Not saying it won't happen as it depends on your inspector/permitting department.

HX_Guy

Re: Figuring out service panel derate vs PV system size?

SkiDoo55 wrote: »

Agree need to do a load analysis.
i have over 400 amp s of breakers in a 200 amp main panel (40 amp backfeed) and if I turned everything on,dual convection ovens, dryer, A/C 3 ton, all the lights, fired up the air compressor , and MIG welder wouldn't exceed @75 - 80 amps max.
But yes an individual load survey and if the local codes people will allow a breaker down-size will determine end result.

Yeah, just a quick walk around the kitchen and found that the fridge draws a max of 8.5A (has a 20A breakers), drawer fridge is 1.3A (20A breaker), washer is 12A (20A breaker) and dryer is 24A (has 60A breaker).

So is that what's needed basically, go around and look at all the appliances and add up the amps? How much overhead needs to be allowed, 20%? So something pulling 8.5A should have at least a 10.2A (round up to 15A) and a 24A dryer should have 28.8A? Which I would guess to round up to 30A, but it has 60A allocated for it?

How about things like lights and outlets? Do those just get calculated at the actual breaker size (15A for example) or do you just half those for calculation purposes?

BB.

Re: Figuring out service panel derate vs PV system size?

I should stay away from building code--I don't get it right....

<sub>It is really difficult to walk around and just add up circuits...

As I recall, for residential you are allowed to install more breakers than the main breaker/bus is rated for under the assumption that nobody turns everything on in the home at the same time.

For commercial, the main breaker/bus bars need to be rated for the total of all the breakers added up together. The assumption being that all circuits are used at the same time during the 8 hour work day.

What you are designing for off grid--You are the power company. How much money do you want to dedicate for being able to run X number of appliances at once.

For residential panel size--My parent's home originally had a 30 amp 120/240 VAC service. Was more than enough in the mid 1950's.

My house (no AC, no electric heating devices) has a 125 Main panel (probably should have been 200 amp--But it is a 75 year old home). I have never come close to popping a 125 Amp breaker:

125 maps * 240 VAC = 30,000 Watts of loading...

There is no way I could use that much power in my home at once.

For grid connected homes--You are a mix of what is practical for your needs, vs what code (always changing) is required by your AHJ. And a mix of what they will let you get by when remodeling (AHJ may let you squeak buy without a panel upgrade because it is an old house and you are not spending that much money).</sub>

-Bill

Cariboocoot

Re: Figuring out service panel derate vs PV system size?

HX_Guy wrote: »

So is that what's needed basically, go around and look at all the appliances and add up the amps? How much overhead needs to be allowed, 20%?

The big problem is this:

solar_dave wrote:

Good luck on the AHJ sign off, just trying to point out you have a risk that needs mitigating by getting the AHJ to buy off on your design. Not saying it won't happen as it depends on your inspector/permitting department.

inetdog

Re: Figuring out service panel derate vs PV system size?

BB. wrote: »

It is really difficult to walk around and just add up circuits...

As I recall, for residential you are allowed to install more breakers than the main breaker/bus is rated for under the assumption that nobody turns everything on in the home at the same time.

For commercial, the main breaker/bus bars need to be rated for the total of all the breakers added up together. The assumption being that all circuits are used at the same time during the 8 hour work day.

Sorry Bill, that is 100% not true.

For both residential and commercial the main must be sized to satisfy the calculated load. That number comes from a procedure set out in the NEC involving the ratings and type of each of the actual connected loads, with some explicit interactions like only counting the larger of electric heat or A/C because they will never both be used at the same time.
It just happens that for commercial use the calculated load is likely to be closer to the sum of the breakers than for a residence. The rules do not change.
(The heating versus cooling applies equally to both.)

The problem for a residence is that the calculations were done once when the house was built and the service installed, and they are not likely to be lying around anywhere unless the owner can find the electrical sheets from the construction plan set. The inspector would have been justified in asking to see the calculations when signing off on the permit. So we do not know how much extra headroom is available given that the 200A main would be used regardless just for cost and future expansion reasons.

For commercial, the load information is more likely to be kept in a known safe place for later reference.

HX_Guy

Re: Figuring out service panel derate vs PV system size?

I guess we won't know until the inspector comes out but I did a quick run through and this is what I counted up:

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. Cariboocoot's was pretty spot on with his guess of 190A.
(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

Re: Figuring out service panel derate vs PV system size?

HX_Guy wrote: »

I guess we won't know until the inspector comes out but I did a quick run through and this is what I counted up:

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. Cariboocoot's was pretty spot on with his guess of 190A.

1. You are mixing 120V and 240V loads. That is working to your disadvantage!
For example, if you have two 120V refrigerators at 8A which are connected to opposite sides of the 240V feed, that will only count as a single 8A.
Or, looking at it from a different angle, the 200A 240V main gives you 400A at 120V to work with.
Most of your large loads (A/C, Cooktop, etc.) listed are 240V though.

2. You are not including general lighting load or general receptacle load, which could be substantial or not.
The NEC allows you in some cases to use a lower wattage figure than the nameplate rating for Cooktop and Oven.
Just what is included in A/C versus A/H for house and Casita?
The disposal and microwave are unlikely to be used at any given time. Short term overloads are acceptable under the calculation rules.

3. There are two different methods of calculating load under the NEC, the standard method and the optional method. The optional method does not add up all of the known possible loads and usually gives a lower figure than the standard method.
In any case the actual power consumption is likely to be lower than either, but that does not count for main breaker sizing.


The ONLY way to get an acceptable calculation is to spend some (long) time studying the NEC and sample calculations or to get an electrician to do it for you.

BB.

Re: Figuring out service panel derate vs PV system size?

Thanks Inetdog... I should stay away from Building Code. It was not fun working with NEC a decade ago, and I don't have an NEC book to refer to. A bad mix.

-Bill

HX_Guy

Re: Figuring out service panel derate vs PV system size?

inetdog wrote: »

1. You are mixing 120V and 240V loads. That is working to your disadvantage!
For example, if you have two 120V refrigerators at 8A which are connected to opposite sides of the 240V feed, that will only count as a single 8A.
Or, looking at it from a different angle, the 200A 240V main gives you 400A at 120V to work with.
Most of your large loads (A/C, Cooktop, etc.) listed are 240V though.

2. You are not including general lighting load or general receptacle load, which could be substantial or not.
The NEC allows you in some cases to use a lower wattage figure than the nameplate rating for Cooktop and Oven.
Just what is included in A/C versus A/H for house and Casita?
The disposal and microwave are unlikely to be used at any given time. Short term overloads are acceptable under the calculation rules.

3. There are two different methods of calculating load under the NEC, the standard method and the optional method. The optional method does not add up all of the known possible loads and usually gives a lower figure than the standard method.
In any case the actual power consumption is likely to be lower than either, but that does not count for main breaker sizing.


The ONLY way to get an acceptable calculation is to spend some (long) time studying the NEC and sample calculations or to get an electrician to do it for you.

I guess there is the Air Handler and the A/C Compressor? Not quite sure.
Either way, there doesn't seem to be much use to me trying to figure it out, it's way above my head. We'll see when we get there what happens...if we need to upgrade to 225A then I guess we can do it at that time...just have a feeling this is going to take a bit longer than planned, I don't see it being smooth sailing.

DanS26

Re: Figuring out service panel derate vs PV system size?

HX_Guy wrote: »

Dan, any tips on doing a loan analysis? Or is that really something for the pros to figure out?

This is old but still good if you want to do it yourself.

http://www.deanza.edu/es/mediatedlearning/pdf/es70loadanalysis.pdf

jcheil

Re: Figuring out service panel derate vs PV system size?

HX_Guy wrote: »

I guess there is the Air Handler and the A/C Compressor? Not quite sure.
Either way, there doesn't seem to be much use to me trying to figure it out, it's way above my head. We'll see when we get there what happens...if we need to upgrade to 225A then I guess we can do it at that time...just have a feeling this is going to take a bit longer than planned, I don't see it being smooth sailing.

There is usually a breaker for the compressor (outside) part and another for the air handler (inside part).
The inside breaker size may vary depending on the air handler has an electric heat strip installed in it vs if your compressor is a heat pump.
If it doesn't have a heat strip it could be a very small breaker because it is only turning a fan. But add a 15k or 20k (or larger) heat strip and THAT is where all the power goes.

HX_Guy

Re: Figuring out service panel derate vs PV system size?

I see, then the labeling is probably correct. The house actually has a separate gas furnace, so that 15A A/H breaker is probably just for the fan. On the other hand, the Casita is electric heat, which would make sense with the double 20A breaker. Cool info, thanks.

BTW we have a gas connection at the dryer, so we could replace the dryer with a gas model andsave a minimum of 20A I would think and that might be enough to get us where we need. Could be an easier solution than upgrading the whole service panel, and we'd get a new dryer out of it

jcheil

Re: Figuring out service panel derate vs PV system size?

HX_Guy wrote: »

BTW we have a gas connection at the dryer, so we could replace the dryer with a gas model andsave a minimum of 20A I would think and that might be enough to get us where we need. Could be an easier solution than upgrading the whole service panel, and we'd get a new dryer out of it

The electric heating element part of the dryer does consume quite a bit of power (most of it) but the motor on my brand new gas dryer still pulls quite a few amps when running. So it's not like you'll get all that 20amps back, but you'll still get quite a bit of it.

How about switching out to a gas range also?

HX_Guy

Re: Figuring out service panel derate vs PV system size?

The electric dryer now is 24A, so I left 4A for the gas version...is it typically more than that?

My wife prefers the glasstop range over gas with grates.

jcheil

Re: Figuring out service panel derate vs PV system size?

HX_Guy wrote: »

The electric dryer now is 24A, so I left 4A for the gas version...is it typically more than that?

My wife prefers the glasstop range over gas with grates.

I measured mine a few weeks ago and we had a long post about washers and dryers:

[And the other poster was right, regarding my dryer, the 800w it was drawing was just when the igniter circuit was on, after that goes off, it settles down to 275w for the rest of the cycle. I didn't monoitor it the entire time, but on the high setting I don't think it cycled at all. Just full heat all the time so the igniter never had to come back on again. Total drying time, 1 hour, .27kwh.]

http://forum.solar-electric.com/showthread.php?22803-Operating-my-washing-machine-off-solar