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Assume a 200 A service with a 40 A PV and a sub panel with a 20 A breaker. If it is mid-day and if there is excess PV (say 35 A) in the main service panel and light loading the load subpanel, why does the excess current (36 A) not flow towards the load (20 A) and damage it or trip the breaker, but instead flows back to the utility? Is there a NEC code for protection here?

ocrdu
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boneym
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  • Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Feb 17 '24 at 19:32

2 Answers2

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why does the excess current (36amps) not flow towards the load (20amp) and damage it

A load, supplied at a specific voltage, will determine how much current it will draw. It will not draw more, even if more is available. It (usually) will draw a different amount of current at a different voltage. But that is usually not under the control of the load.

but instead flows back to the utility?

It wouldn't necessarily, but the inverter is designed to feed excess power back to the utility. That is part of it's job. How does it do it? By acting like it has a higher source impedance than the mains. (Mains actually has a very low source impedance, which makes the technical problem easier.)

Math Keeps Me Busy
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What happens here is governed by the principles of electric circuits. The first thing to note is that the components are connected in parallel. The PV, the house, and the utility are all tied together. The rule for parallel connections is that the voltage will be the same across each component. Next, Ohm's law tells us that voltage and current must rise together. When the Sun comes up and the PV starts working, in order to put current into the system, its voltage has to be greater than the voltage at the connection point. So it will try to raise the voltage, which means current will flow into the other circuits. It may be able to raise the voltage on your house loads, by pushing current into it. V=I*R, so the house side responds based on the load, R, which is a small resistance. It's small, but it's not zero. And the same is true for the utility. But trying to raise the voltage of the utility will be almost impossible. Its equivalent "R" is about as close to 0 as you can get. You can pump lots of current into the utility lines, but you won't be able to change the voltage more than a tiny bit. But since the voltage for each part can't be different, the PV must raise everyone's voltage together. And it will do this by trying to pump all its current into the power lines. Not because of rules, or regulations. That's just how electricity works.

gbarry
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