# photovoltaic 101

#### jaggedben

##### Senior Member
I appreciate the responses fellas , thankfully there's no law against educating a redneck spark

But here's one that turns my head around ......

the PV guys come in and do their thing, then determine the panel is too small, by too small i mean the buss

they'll have me place a 200A MLO into a 100A (meter/100A main) system for this

So we end up with a 200A MLO off a 100A service , i just can't 'get' why the focus is on the panel being inadequate when the the rest of the serive is ok?

~R (brain fried in vermont)J~
The basic concept is that since you have two sources, a panelboard could be overloaded if the customer somehow draws more load than the service would carry. When does anybody's main breaker trip simply because they used to much power? Hardly ever, but the code is extra conservative on this. 705.12(B) [2017 NEC reference] contains various rules on how you can do it.

200A on a 100A service seems like overkill for most situations in my experience, since a 100A OCPD in a 125A panel leaves room for 50A of solar under one rule, which is usually way more than enough. But maybe you've encountered some solar bigger systems, or maybe they're just trying to give themselves maximum flexibility.

#### Carultch

##### Senior Member
That's simplified, of course. A grid tied PV inverter is a current source because PV modules are current sources. It doesn't need to raise the voltage to "push out" current; it could just as easily deliver current into a superconductor where there would be no voltage drop.
How does a superconductor "know" which direction to conduct electrical current, if there is zero electric field in it (due to zero voltage difference across it)? Is it just the inertia from current flow in other parts of the circuit?

#### ggunn

##### PE (Electrical), NABCEP certified
How does a superconductor "know" which direction to conduct electrical current, if there is zero electric field in it (due to zero voltage difference across it)? Is it just the inertia from current flow in other parts of the circuit?
I believe that the mechanism is diffusion.

#### GoldDigger

##### Moderator
Staff member
How does a superconductor "know" which direction to conduct electrical current, if there is zero electric field in it (due to zero voltage difference across it)? Is it just the inertia from current flow in other parts of the circuit?
If you have a closed superconducting loop, the current is 100% determined by the magnetic field (flux) cutting the loop. That is the only way to cause a current to flow.

If you have a segment of superconductor attached at either end to a normal conductor, then yes, the electric field in the normal conductor will start the electrons moving and they will just drift without losses from one end of the superconductor to the other.

Superconducting magnets typically use a "switch" in the form of a superconducting segment rendered temporarily normal by temperature or magnetic field. After the desired current is flowing in the bulk of the winding the switch is closed and the charging source is disconnected.

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#### wwhitney

##### Senior Member
Superconducting magnets typically use a "switch" in the form of a superconducting segment rendered temporarily normal by temperature or magnetic field. After the desired current is flowing in the bulk of the winding the switch is closed and the charging source is disconnected.
Can you briefly explain how the energy accounting works out for a superconducting magnet? The stored energy in the closed loop superconducting state is some function of current and other parameters? And I assume if the magnetic field generated does work (my electromagnetics is poor, I forget when/how/if that happens), then the circulating current is reduced?

Thanks,
Wayne

#### GoldDigger

##### Moderator
Staff member
Can you briefly explain how the energy accounting works out for a superconducting magnet? The stored energy in the closed loop superconducting state is some function of current and other parameters? And I assume if the magnetic field generated does work (my electromagnetics is poor, I forget when/how/if that happens), then the circulating current is reduced?

Thanks,
Wayne
Very briefly:
As you increase the current in a superconducting magnets you create a corresponding magnetic field. Since there are no I**2R losses in the magnet, that energy remains stored in the magnetic field.
You can use that field just as you would a permanent magnet to induce voltage in a moving wire loop and extract energy from the wire loop. That energy comes from the work done moving the wire, not from the superconducting magnet.
If you do something to actually reduce the magnetic field cutting the superconducting loop (as with a moving permanent magnet or a coupled electromagnet), you can extract energy from that field and reduce the current.

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Thanks! Wayne