basic load calcs

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Ponchik

Senior Member
Location
CA
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Electronologist
When doing the optional single family dwelling load calculation, where does the solar come into the calculation? Or does it even come in?

Thanks
 

GoldDigger

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Location
Placerville, CA, USA
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It doesn't. It is not a load. And you cannot count on it to offset loads either.

Although when sizing the breaker panels and the service you should run through the 120% rule totally separate from the load calcs, just in case there is a squeeze there.
 

Dennis Alwon

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Location
Chapel Hill, NC
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Calculations are for loads and the solar does not add loads but takes some load off the service. The problem is you cannot deduct that extra support from the solar panels because it is not always able to reduce the load on the service. For example long periods of rainy weather- no sun.
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
Calculations are for loads and the solar does not add loads but takes some load off the service. The problem is you cannot deduct that extra support from the solar panels because it is not always able to reduce the load on the service. For example long periods of rainy weather- no sun.

I got the "PV is not load" part.
I do not understand the "120% rule and squeeze" part.

Thanks
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
I got the "PV is not load" part.
I do not understand the "120% rule and squeeze" part.

Thanks
For any panels or feeders which carry backfed power from the PV inverter(s) the sum of the backfeed amps and the normal feed breaker amps must not exceed 120% of the feeder or bus bar rated ampacity.
So if you have a 100A panel fed from a 100A main or feeder breaker you cannot have more than 20A of solar backfeed. And the solar breaker must be at the opposite end of the bus or feeder from the normal supply connection.
It is slightly more complicated than that, but that consideration is what i was referring to.
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
So if I have a 200Amp panel the solar back feed breaker can not be more than 40Amps. Correct?

Is it 120% of the rating of the main breaker or 120% of the rating of the buss?

What is the NEC code section on this rule?

Thanks
 

Ponchik

Senior Member
Location
CA
Occupation
Electronologist
It is 120% of the bus rating.
Some 200A panels have a 225A bus, potentially giving you a 7.0A allowance.
(45A + 25A)


What is the NEC section on the rule? I found it in the James Dunlap book but can't find it in the NEC.

Thanks
 
It doesn't. It is not a load. And you cannot count on it to offset loads either.

Although when sizing the breaker panels and the service you should run through the 120% rule totally separate from the load calcs, just in case there is a squeeze there.


I was doing a EVSE install for a local city, and the plan check department insisted I had to count the solar as a load for the load calculation. I wonder how many unnecessary panel upgrades they've helped sell by requiring that... :roll:
 

kwired

Electron manager
Location
NE Nebraska
On site power production is not a load it is a source. However supply conductors and equipment need to factor in any impact it may have on the current those items carry or potentially could carry.
 

GoldDigger

Moderator
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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
On site power production is not a load it is a source. However supply conductors and equipment need to factor in any impact it may have on the current those items carry or potentially could carry.
In particular there will be situations such as solar farms where the generated lower is far higher than any local loads. In such cases circuit sizing will depend, as set out in the NEC, by the generation rather than the loads. But even in those cases the generation will not add to the loads in reaching that result.
 

kwired

Electron manager
Location
NE Nebraska
In particular there will be situations such as solar farms where the generated lower is far higher than any local loads. In such cases circuit sizing will depend, as set out in the NEC, by the generation rather than the loads. But even in those cases the generation will not add to the loads in reaching that result.

I guess my thoughts is load calculations are load calculations. Then if you also have on site production you need to figure any impact it will have on top of load calculations. If you have much more load then production - you may have little or nothing significant to calculate but if you have more production then load - you better at least have supply equipment that can handle the production ability.
 

kwired

Electron manager
Location
NE Nebraska
The grid becomes the load.


But your on site load offsets this, is there anything in art 220 that tells us how to calculate for on site production that backfeeds the grid? I could be wrong but I don't think so. Art 220 is primarily for on site loads. Those on site production loads are covered in 690, 694, or whatever article applies to the type of production.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
But your on site load offsets this, is there anything in art 220 that tells us how to calculate for on site production that backfeeds the grid? I could be wrong but I don't think so. Art 220 is primarily for on site loads. Those on site production loads are covered in 690, 694, or whatever article applies to the type of production.

Other than specific branch and feeder wire sizing requirements (including the legendary 120% rule), the thing specific to service size is that the calculated PV production (including 125% for continuous?) cannot be greater than the nominal POCO service size. What size wire they use at that point in the circuit is up to them.
 
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