basic load calcs

[Ponchik]

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]

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.

 

[Ponchik]

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.

Please explain.

Thanks

 

[Dennis Alwon]

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]

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]

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]

Got it. Loud and clear.

Thank you.

 

[Ponchik]

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

 

[GoldDigger]

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

 

[Ponchik]

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

Thanks :thumbsup:

 

[Ponchik]

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

 

[david luchini]

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

705.12

 

[YourElectricianOC]

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:

 

[Ponchik]

705.12

Awesome. thanks.

Thanks for all of the inputs

 

[kwired]

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]

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]

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.

 

[Smart $]

... if you have more production then load - you better at least have supply equipment that can handle the production ability.

The grid becomes the load.

 

[kwired]

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]

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.