Service size after reducing Main breaker size

Sierrasparky

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
Hello,
I am not a solar contractor however I would like to understand the methodology in calculating new loads when a PV system is installed and the Main breaker size is reduced to accommodate a larger array?
Example is you have a 200a all in one and you reduce the main breaker to 150 in order to have a greater back-feed. I want to know what the new panel rating is for load calculations is. Is it still a 200a or a ?
 
Sierrasparky said:
Hello,
I am not a solar contractor however I would like to understand the methodology in calculating new loads when a PV system is installed and the Main breaker size is reduced to accommodate a larger array?
Example is you have a 200a all in one and you reduce the main breaker to 150 in order to have a greater back-feed. I want to know what the new panel rating is for load calculations is. Is it still a 200a or a ?
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Well, obviously if the PV system is not operating and the loads are drawing more than the rating of the new main breaker, the breaker will trip, but most residential 200A services rarely if ever pull anywhere near 200A. Downgrading the main breaker does not change the size of the service from the the POV of a PV system; likewise reducing the MB rating does not affect the rating of the main panel busbar. You are not limited to the panel rating for the total ratings of load breakers; if the panel rating were to change because of the reduced MB, what calculations would be affected?
 
ggunn said:
Well, obviously if the PV system is not operating and the loads are drawing more than the rating of the new main breaker, the breaker will trip, but most residential 200A services rarely if ever pull anywhere near 200A. Downgrading the main breaker does not change the size of the service from the the POV of a PV system; likewise reducing the MB rating does not affect the rating of the main panel busbar. You are not limited to the panel rating for the total ratings of load breakers; if the panel rating were to change because of the reduced MB, what calculations would be affected?
Click to expand...

You say "Downgrading the main breaker does not change the size of the service from the the POV of a PV system" It does in the sense that the 120% rule of 705 limits the max solar input to 40a ( all other proper sizing done) and when you downgrade to a 150 you can have up to 90amp back-feed. What I would like to know is when doing a load calculation under 220 what would the service size be? As far as residential services rarely pull anywhere near 200a I agree in most situations. However when you have people converting to all electric, install a EV , have a Hot tub, Electric water heat, Electric cooking , Electric HVAC , and a ADU with all the same appliances. All the LED lighting is not going to reduce the current!
 
IMHO reducing the service breaker size doesn't change the calculated service load one bit, and that you would not be kosher reducing the main breaker size below the calculated load.

With that said, I agree that the article 220 calculations are wildly pessimistic; my house calculates out at north of 130A, is a 100A service, and I've never used more than 40 or 50A.
 
winnie said:
IMHO reducing the service breaker size doesn't change the calculated service load one bit, and that you would not be kosher reducing the main breaker size below the calculated load.

With that said, I agree that the article 220 calculations are wildly pessimistic; my house calculates out at north of 130A, is a 100A service, and I've never used more than 40 or 50A.
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My Customer is adding new high current loads and a ADU so I need to calculate the load for plans. I was just observing the current service. It is a 200a panel with a reduced main to 150. If I use the standard methods and tools I come up with over a 150 easily, very close to 200a. Without knowing the rule here I cannot advise the client to eliminate a load or not.

Thanks
 
winnie said:
IMHO reducing the service breaker size doesn't change the calculated service load one bit, and that you would not be kosher reducing the main breaker size below the calculated load.

With that said, I agree that the article 220 calculations are wildly pessimistic; my house calculates out at north of 130A, is a 100A service, and I've never used more than 40 or 50A.
Click to expand...
I cannot count on the fingers of both hands the number of load calculations I've done to show an AHJ that a 200A breaker could be downsized to 175 or 150A. Almost all of them came out under 125A. I did more main breaker downsizes where the AHJ didn't even ask. (But I'm glad we have PCS now and don't have to do that anymore.)
 
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Yes I use the optional method and the regular.

Have you read the requirements?
2 complete kitchens, electric range, 2 kitchen appliance cir each, 2 dishwashers.......
2 complete laundry with electric dryer
11Kw hot tub
They want to add :
EV 50a
Electric hot water heater 4.5 KW

I do not know how you do a load calc but I use the Mike holt as it is easy. I do not fudge numbers
 
If the main breaker is 150 amps that is the size of your service. Ignore that there is PV connected to it. If you are going to be by-the-book with the calculation and its over 150 amps your customer will need to do some upgrades.

What is the current PV breaker size? How old is the panel? If its relatively new the buss is probably rated at 225 amps.
 
curt swartz said:
If the main breaker is 150 amps that is the size of your service. Ignore that there is PV connected to it. If you are going to be by-the-book with the calculation and its over 150 amps your customer will need to do some upgrades.

What is the current PV breaker size? How old is the panel? If its relatively new the buss is probably rated at 225 amps.
Click to expand...

This is an existing 200 amp Murray all in one with the quad breaker. The customer has a 11Kw solar array with a 80 amp back-feed. I know the 80 is double what is allowed. A line side tap is not gonna work as there unless the utility allows some panel modifications.

So back to my question: If the 200a main breaker is swapped out with a 150 what is the size of the panel now? Is the panel now a 150 with a 200 amp buss?
 
Sierrasparky said:
This is an existing 200 amp Murray all in one with the quad breaker. The customer has a 11Kw solar array with a 80 amp back-feed. I know the 80 is double what is allowed. A line side tap is not gonna work as there unless the utility allows some panel modifications.

So back to my question: If the 200a main breaker is swapped out with a 150 what is the size of the panel now? Is the panel now a 150 with a 200 amp buss?
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The current service is 150 amps if that is what the main breaker is. If this is CSED forget about modifying it to do a line side tap, but I think you already know that.
 
Around here (same state and utility?) the ADU would require it's own meter so you'd be upgrading the service anyway?

I agree that 2 complete dwellings and hot tub is not likely to be under 150A.

I never count dishwashers though, those are on the SABC. 😉
 
jaggedben said:
Around here (same state and utility?) the ADU would require it's own meter so you'd be upgrading the service anyway?

I agree that 2 complete dwellings and hot tub is not likely to be under 150A.

I never count dishwashers though, those are on the SABC. 😉
Click to expand...
Most of the ADU's we wire are connected to the same meter/service panel as the main house.
Dishwashers are never permitted to be connected to the SA circuit.
 
curt swartz said:
Combination Service Entrance Device (Meter/Main/Load Center) (All-In-One)
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Oh, In all my years never hear it that way. Agree no way most CSED panels can have a line side tap. Especially this one. Picture is of the same model and part number not the customers.
 

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2 answers to questions you did not ask but might be options:
1. You do have the option of a 175A main breaker. In such a case, many electricians (oh no, not me...:sneaky:.) don't even do load calcs, they figure 175A covers 99.99% of 200A service loads.
You would need to finnagle your 80A PV output down to 65A though. (65A PV + 175A main brkr = 240A)
Remember you are allowed to use individual inverter output amps when adding them..... instead of adding individual inverter breaker sizes. This helps if there is considerable rounding up.
Sometimes a smaller micro inverter does the trick, with surprisingly minimal to no peak shaving losses.
Details.....
2. Do you actually have a 225A bus? That opens up a whole new world. That means 70A PV ......w/o downsizing nuthin.
Or 95A PV with a 175A main breaker.
E.g. many EATONs are 225A today.
 
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