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Old Today, 11:17 AM   #1
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Default Solar PV system sizing based on electrical service

Hello,

I'd appreciate some feedback on the following PV system sizing scenario:

The main electrical service for the building is 400A, 1-phase, 120/240V. The main CDP is rated for 400A, 1-phase, 120/240V. Based on the 20% power feedback rule, I'd be permitted max. of 80A (19.2 kW PV system) of feedback into the electrical distribution system.

The CDP has limited number of spare spaces to install circuit breakers, so I intend to install a 100A,1-phase, 120/240V sub-panel via 100A/2P circuit breaker in the CDP. Would this configuration meet the 20% feedback rule or do I need to size the sub-panel to 400A so I'll be compliant with the feedback rule?

Thank you.
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Old Today, 11:28 AM   #2
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Who is limiting the amount of energy you're allowed to backfeed the grid?
Government or the utility? Or are they the same?
That's hogwash! The snowflakes down here would all melt if they heard that!
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Old Today, 12:34 PM   #3
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Mike has a point, what is this "20% feedback rule"? Is that Max. or Min.? If you have "spare" space in the CDP for a 2-pole brkr. why not use it, (isn't that what it's for?) instead of installing a whole new panel? If they need more ckts. later, they can add a sub-panel later.
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Old Today, 12:36 PM   #4
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Quote:
Originally Posted by katwalatapan View Post
Hello,

I'd appreciate some feedback on the following PV system sizing scenario:

The main electrical service for the building is 400A, 1-phase, 120/240V. The main CDP is rated for 400A, 1-phase, 120/240V. Based on the 20% power feedback rule, I'd be permitted max. of 80A (19.2 kW PV system) of feedback into the electrical distribution system.

The CDP has limited number of spare spaces to install circuit breakers, so I intend to install a 100A,1-phase, 120/240V sub-panel via 100A/2P circuit breaker in the CDP. Would this configuration meet the 20% feedback rule or do I need to size the sub-panel to 400A so I'll be compliant with the feedback rule?

Thank you.
Once your over, your always going to be over. It doesn’t mater how far downstream you tie in the solar at. It’s time for a line side tap.

I don’t understand how adding a small sub panel would fix anything. A 100A sub panel would only allow 20 amps of back fed solar. How big is the system? Do you need a sub panel to combine multiple inverters? If that’s the case, the panel just has to be sized for the total output of the inverters. It would be called an AC combiner. But when the single tie in point is a breaker in a panel, it has to be at 20% of the panel.

Not sure how the CEC is compared to the NEC.
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Old Today, 12:48 PM   #5
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The utility limits the backfeed into the electrical panel to a maximum of 20% of the electrical panel / service ratings. So the PV system would have to be sized accordingly.

For a 19.2 kW system, the panel and micro-inverter manufacturer design permits max. of 13-300W solar panels on a 20A/2P circuit breaker. The system would require min. 5-6, 20A/2P circuit breakers which the CDP doesn't seem to have space for. So a sub-panel would make this configuration work, but then my question is about should the sub-panel's bus bar be rated accordingly to comply with the "20% backfeeding rule"?
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Old Today, 05:01 PM   #6
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I’m not a solar expert but here’s what I think I know about the NEC not the CEC.

The NEC requires a single tie in point between the two systems. So multiple two pole twenty amp breakers in the CDP is not an option. So you would have an AC combiner. That would look exactly like a sub panel, but wouldn’t have the 20% rule because there are no “loads” in the panel. You are just combining sting inverters. Not sure how it’s different with micro inverters. I guess you are combining micro inverters on the roof until you get to twenty amps, then run a home run down?

Now when you tie the two systems together in a panel, with loads between the normal and solar that’s where the 20% comes in. For the US it’s not a power company rule but an NEC rule. The power company doesn’t care. You can do a line side Tap to get around back feeding the buss from two ends.

The rule for us, is that they think that there is a scenario, where the total load in the panel could reach the melting point of the buss, if it were to draw 400 amps from one end and over 80 amps from the other.

In a combiner box you would not have the same scenario.
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