What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system install

rollandelliott · June 2012

I have a project where the home has a 200amp load center panel.
Homeowner wants a 10KW system installed which means two 5KW GT inverters each with a 30amp back feed, total of 60a. However according to NEC max I can install is 120% or 40a. I've read about the 120% overcurent issue so the existing main breaker needs to be downsized to solve the issue.

So I looked around lowes/home depot & they only seem to carry 3 sizes of load centers, 200a, 125a & 100a.
Even looking online I can only find a 150a. I guess they don't make a 175A one which would of been ideal!

So my plan is to simply buy a 150a load center made by the same manufacturer of the exsisting one and have electrician swap out the old 200a main breaker with the new 150a main breaker. I'd buy just the main breaker if I could but finding it has proven difficult.

That would save the hassle of ripping out the old service and rewiring everything which would probably take several hours.

Anything wrong with this plan? Seems like any residential load center breakers over 125A are very hard to source and very expensive. I've read past threads about this issue but all of them are theoretical suggesting one put in a 180a breaker, (well they don't exsist as far as I know!) So any real world feed back would be great.

icarus · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

Forget HOme Depot and Lowes. Try the wholesale electrical houses in your area,, CED, Platt, Westburn, Etc. most will sell over the road, if not they will certainly sell to you electrician. Generally, if you walk in to a whole sale house, looking like you know what you are doing, they are very helpful,, if the counter is not too busy.

Tony

rollandelliott · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

Yeah I figured as much, but it's wierd that I can't even find a 175amp load center online to buy though I did find one place that would sell just the 175A breaker for $200 http://www.pioneerbreaker.com/ProductDetails.asp?ProductCode=THQMV175

After looking for a good hour on the internet I found ONE review of how a downgrade was performed at
http://www.mysolarreview.net/
Never heard of this way to make it work but here's the quote:

Great News - Sungevity pushed through a Variance with TEP. The variance is allowing the bus bar to be upgraded to 225 Amps and still use the same 200 AMP box. This is another "Woo Hoo". I will get my larger system and no major modifications have to be done with the electrical. Thank You Sungevity / Phil - and Thank You TEP.

turns out he wanted to downgrade his 200A panel too, but the Arizona electricity company wouldn't approve a 175a downgrade because the utility company tested his home and his load rating of exsisting circuits was too high.

solar_dave · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

Mine uses a standard 200 amp panel with a 175 amp main breaker installed for my 12.5 kW system. The backfeed is a max 46 amps on a 60 amp breaker. The 2 inverters are combined in a sub panel with a pair of 30 amp breakers feeding the 60 amp panel breaker.

rollandelliott · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

so you or your electrican just swapped out the 200a for a 175a breaker? was the 175a breaker hard to find? They dont' seem that easy to come by at least ordering over the internet. did your building dept or utility dept require a load test for you to downgrade?

solar_dave · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

rollandelliott wrote: »

so you or your electrican just swapped out the 200a for a 175a breaker? was the 175a breaker hard to find? They dont' seem that easy to come by at least ordering over the internet. did your building dept or utility dept require a load test for you to downgrade?

The code guys did require a listing of the loads to determine if 175 was enough, actually in my case it was not acceptable to downgrade my single 200 amp panel because of the existing loads. I was required to upgrade my service entrance to a 400 amp line from the transformer with dual 200 amp panels one of which the electrician used a 175 amp breaker. He didn't seem to have any issue finding a 175 amp breaker at the supply house. You have to remember my existing loads were pretty large to begin with, dual 3ton AC units, electric range, electric dryer, 2 swimming pools pumps, 2500 sq ft house worth of other stuff. I really didn't battle them too much as I knew that I was adding the new workshop/office building (man cave) and it would require a 100 amp sub at least for that. The second panel with the 175 amp main only has 2 loads on it, the new building sub panel @ 125 amps and the Volt Chargers sub panel @ 60 amps along with the 60 amp Solar back-feed breaker. All in all it was a $4300 upgrade to the property for the service entrance work and line pull. Interesting item is the peak net load from the utility I think I have seen with the TED is 13.5 Kw which at 240 volts is 56 ish amps.

rollandelliott · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

"Interesting item is the peak net load from the utility I think I have seen with the TED is 13.5 Kw which at 240 volts is 56 ish amps.
"

Thanks for the answer, well I guess you can sleep well at night knowing you are no where close to reaching the load limits of your service! That's a pretty big safety factor. I guess I better make a list of loads in the house and submit it with my building plans. You're the second person to tell me that this was required,

SolaRevolution · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

rollandelliott wrote: »

... but the Arizona electricity company wouldn't approve a 175a downgrade because the utility company tested his home and his load rating of exsisting circuits was too high.

It seems funny to think of the utility encouraging you to upgrade your service main breaker rating when they so severely under supply the service KVA ratings. If you think about it, 200amp 240vac has a 48 kva rating. Check out a grid fed neighborhood. I often see three 200amp services fed from a single 37.5kva transformer.

I've had a county inspector reject a down grade after my load calculations showed the need for 225amps. They accepted connecting the solar through a tap between a 200amp (main-breaker-only) service entrance and a 200amp main breaker in a subpanel based on NEC 240.21. John Wiles has said that the Tap rules do not apply to a power source. As far as I know there is no rule against using it. Baiscally, it is up to the AHJ.

It may not be convenient for many installations but it is another "tool for the box".

Alex Aragon

rollandelliott · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

http://www.electricalknowledge.com/SFDLoadCalc.asp

found this handy load calculator, I'm going to use it to calculate the home load requirements.

solar_dave · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

Yeah we tried to get the AHJ to accept a line side tap as well. Told us no for a residential install but they would consider it for a commercial install. Really the first thing to do is get the load calc done and then ask the code guys for next steps. BTW you should be able to get a 175 amp breaker for the main, what kind of panel is it?

SolaRevolution · June 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

solar_dave wrote: »

Yeah we tried to get the AHJ to accept a line side tap as well.

Are you talking about the "line side" of the service entrance?

What I was talking about was not a line side tap. I'm talking about a tap between a main breaker and the main breaker of a sub panel. The current in any part of the tapped-conductors is limited by overcurrent protection, there no bus bars involved. And as required by NEC 240.21, all conductors are in metallic raceway.

Alex Aragon

rollandelliott · August 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

Finally got some more information
The panel is a Siemens G2040B1200 load center
Apparently it uses the same breakers as Murphy Panels.

After spending hours looking around, calling electrical supply houses that were clueless, I kind of figured out that no one makes a 175amp panel.

You have to buy a 200amp panel with main LUGS only and then install a 175 main breaker kit.

Some panels can be converted from a main breaker to lugs. Like this pdf file explains:
http://www3.sea.siemens.com/step/pdfs/load_centers.pdf

I cannot find a siemens 175a main breaker kit, anyone know if it exsists?

I may have to buy this one from squareD: QOM2175VH
and install it in it's own panel, then run jumper cables to exsisting panel

ggunn · August 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

solar_dave wrote: »

The code guys did require a listing of the loads to determine if 175 was enough, actually in my case it was not acceptable to downgrade my single 200 amp panel because of the existing loads. I was required to upgrade my service entrance to a 400 amp line from the transformer with dual 200 amp panels one of which the electrician used a 175 amp breaker. He didn't seem to have any issue finding a 175 amp breaker at the supply house. You have to remember my existing loads were pretty large to begin with, dual 3ton AC units, electric range, electric dryer, 2 swimming pools pumps, 2500 sq ft house worth of other stuff. I really didn't battle them too much as I knew that I was adding the new workshop/office building (man cave) and it would require a 100 amp sub at least for that. The second panel with the 175 amp main only has 2 loads on it, the new building sub panel @ 125 amps and the Volt Chargers sub panel @ 60 amps along with the 60 amp Solar back-feed breaker. All in all it was a $4300 upgrade to the property for the service entrance work and line pull. Interesting item is the peak net load from the utility I think I have seen with the TED is 13.5 Kw which at 240 volts is 56 ish amps.

Does this make sense? Any contribution from your PV system would reduce, not increase, the current draw from the transformer serving your home.

inetdog · August 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

ggunn wrote: »

Does this make sense? Any contribution from your PV system would reduce, not increase, the current draw from the transformer serving your home.

Sounds to me like the only solution which they could find to the bus loading limit in the main panel was to use a larger main panel, and therefore a larger service. Connecting the existing 200 amp service to two separate 100 amp panels (with 150 amp busbars maybe) might have done the same thing but would not have given any extra revenue to the utility.

If OP could have gotten a 250 amp main panel with a 200 amp breaker, for example, that should have kept everyone happy. Unless the utility was afraid that he would "upgrade" the main breaker later.

solar_dave · August 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

ggunn wrote: »

Does this make sense? Any contribution from your PV system would reduce, not increase, the current draw from the transformer serving your home.

Your preaching to the choir, but it is all about bus bar rating and current sources, I do have lots of high amp draw sources in the original panel, electric range, dual 3 ton AC units, 2 hp pool pump at that time, electric washer and dryer... I knew at the time the plan was to add a EV charger, and other building with 125 amp sub and a 3 ton mini split, some machine tools ... I just shrug and took the path of least resistance, a 400 amp service entrance line split into dual 200 amp panels, one with a 175 amp derate main breaker. I though I was going to get a battle adding the 125 amp building sub and a 60 amp sub for the garage EV chargers on that derated main panel, not a peep out of the code guy. The replacement main panels installed was $4000 and the 400 amp service line pull from the transformer was $300 from the utility. At least I got to write off 30% on the federal tax credit as it was part of the solar project.

rollandelliott · August 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

found another solution
eaton carter ecb2225R metal box enclosure
Bw2175 main breaker 175a
lug tree to attach hot wires to main panel NCBK225

problem with this set up is the box is as big as a main load center. Wish I could find a very small box for the single breaker. Might have to get a 125a box and gut it.

rollandelliott · September 2012

Re: What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system ins

I went into two electrical supply stores today.
Neither one of them had 175 amp breakers.
I asked them for a a simple Main lug kit as well.
Neither one of them had them.
One store I poked around their shelves and found some main lugs, but they only had 3 not 4
so I had to go online and order some
https://www.gordonelectricsupply.com/index~ID~,Burndy,Mechanical.Lugs~path~find~ds~mfr

The other store said I could order them online and the sales person online told me the lugs cost $5 each and I needed to buy a minimum order of 500. I said that was ridiculous. What kind of company would need that many! That's about 125 solar installs at 4 per job.

Moral of the story is 175A main breaker is very rare, you're probably not going to find it locally and you will probably have to buy it over the internet. Same is true with 150A,

125, 200 and 225A can be found anywhere.

rmk9785e · December 2016

A friend is having PV system installed at his home and the installer is downgrading his 200A main service panel to 150A for a 6.4kW single inverter system with no load increase. I'm curious why?

BB. · December 2016

I do not know... Assuming a 6.4 kWatt AC inverter (and a large enough array to drive it):

6,400 Watts * 1.25 NEC derating * 1/230 VAC nominal line voltage = 33.4 Amps minimum branch circuit rating

A 200 Amp panel * 120% capacity increase for solar = 240 amp maximum rating for box.

So--It could take up to a 40 Amp Solar Branch circuit without issue (as far as I know).

If he was trying to allow for a 10 kWatt maximum--Sizing the AC main down to 150 Amps would work.

If, for some reason, the installer cannot put the AC solar branch circuit at the "bottom of the box" (away from the AC mains breaker), then the box would have to be derated by the solar branch circuit.

It could also be his experience in dealing with the inspector for that city/county that things "go easier" if he derates the panel.

That is about all I can think of.

-Bill

rmk9785e · December 2016

BB. said:

I do not know... Assuming a 6.4 kWatt AC inverter (and a large enough array to drive it):
6,400 Watts * 1.25 NEC derating * 1/230 VAC nominal line voltage = 33.4 Amps minimum branch circuit rating
A 200 Amp panel * 120% capacity increase for solar = 240 amp maximum rating for box.

So--It could take up to a 40 Amp Solar Branch circuit without issue (as far as I know).

Thank you for great explanation. I also thought it shouldn't have required a downsizing.
You mentioned another thing that I have heard off and on. having "a large enough array to drive" the inverter. In this particular case, the array is 6.4kW and the inverter is SE6000. Excuse my novice status in this field, I am curious. I've been told by some installers that oversizing an inverter may result in its not turning on. I've asked the manufacturer (Solar Edge) tech support to clarify and was told that other than being more expensive than needed, there may be a very small drop in inverter efficiency which may not be noticeable. Is it more serious in your experience? Would a 7kW array not drive a 7.6kW inverter?

BB. · December 2016

Some people oversize the inverter (for future expansion, run cooler at lower than rated power). So, if your array is only 1/2 the inverter's rating, then you would get 1/2 the rated power/current from the inverter.

In general, you can put rated panels at rated inverter power (i.e., 6.4 kW array on 6.4 kW inverter)--But you can go as high as 1/0.77 of rated inverter power for array power:

6.4 kW inverter / 0.77 panel+gt inverter derating/losses = ~ 8.31 kWatt maximum array

In general, for most people will find that the array+inverter will output a typical maximum output power of 77% of the array's name plate rating (panels get hot, their output power drops, roughly 3-5% losses in the GT inverter, etc.).

Usually, the critical function requirement is the Vpanel input voltage range of the GT inverter. For an inverter that runs 200-600 VDC input, you want the minimum input voltage >200 volts (Vmp-hot), and not to exceed 600 VDC (Voc-cold)--That works out to, roughly, ~400 VDC Vmp array (STD/Name Plate) rating.

-Bill

rmk9785e · December 2016

I spoke to tech support engineer at SolarEdge today. Initially he also suggested the SE6000 inverter for 7040w STC panel array. When I asked him to identify how much production one would lose by going with the larger SE7600 inverter, he did some simulations and concluded that production loss will be less than 1% kWh/year. I can change the inverter to the smaller model without any cost difference but should I?

BB. · December 2016

Power harvest wise--1% is a "don't care" situation (you are doing well if you can measure your power output within 2-5% accuracy). And the variability in weather/etc. swamp any meaningful harvest variation numbers.

Running the larger AC inverter will (probably) require a larger AC circuit breaker than the smaller inverter. Guessing:

7,600 Watts * 1/240 VAC nominal * 1.25 NEC breaker/wring derating = 39.6 amp minimum branch circuit rating

Electrical/electronics wise, it is always better to run stuff at lower temperatures for longer life (every 10C increase in operating temperature is a 1/2 reduction in life).

Of course, keeping the inverter shaded from direct sun/well ventilated will help a lot too.

-Bill

rmk9785e · December 2016

Thank you, Bill, for explaining so clearly.

What's best way to DOWNsize 200A load center to 150A one for a 10KW solar system install

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