Understanding how current flows

[JoeNorm]

Here is a theoretical question to try to understand how current "flows" on a wire when a PV system is present.

Let's say I have a 200 amp main panel. In the main is a 100A breaker feeding a sub panel outside. Subpanel supplies power to 2 heat pumps and a hot tub.

A PV system is installed with a feeder tap into the sub-panel. PV system has potential to back-feed 50 amps.

Now let's say the Heat pumps and Hot tub is on, drawing 60 amps total. It s a sunny day so the PV system is making 50 amps.

What is the amp flow through the feeder to sub panel? The tendency is to think the PV will reduce the demand on the feeder to sub panel, is this the right way to look at it?

This situation comes up a lot, of course. I'm trying to wrap my head around how the current flows in these cases. Is it actually directional?

thanks

 

[synchro]

What is the amp flow through the feeder to sub panel? The tendency is to think the PV will reduce the demand on the feeder to sub panel, is this the right way to look at it?

This situation comes up a lot, of course. I'm trying to wrap my head around how the current flows in these cases. Is it actually directional?

Yes, current flow is directional.
Loads (such as the heat pumps and hot tub in your example) can only draw current and cannot supply any current. The PV system is the opposite and it can only supply current. The feeder you describe is connected to the POCO through the subpanel and main panel, and so it can either supply an output current or accept an input current flowing the other direction. In your example all of the 50 amp PV output will flow into the loads, and the feeder will supply the remaining deficit of 10 amps to the loads. If the load dropped to 40 amps, then 40 amps of the PV system's 50 amp output will flow into the subpanel from the feeder tap. And then the remaining 10 amps of the PV system output will flow into the main panel and POCO through the feeder tap.

 

[Hv&Lv]

If I have the mental picture correct the 60A hot tub and heater consume the full 50A production of the PV panel and the main is feeding 10 amps to the sub panel to make up the difference.

 

[Hv&Lv]

This current flow is also the reason the code has the article regarding bus amperage.
You have a 200 amp panel with a 100 amp breaker feeding a 100A rated sub panel..
Let’s say there are 150A worth of breaker totals in the sub panel not including the 50 amp to the PV system. Put this at the top of the sub panel..

On a bright day the potential is there to have every breaker in the sub panel maxed out with loads and the PV also maxed out. The BUS now has 150 amps on it. The breaker in the main “sees”100A, so it doesn’t trip out to help protect the 100 amp rated sub panel..

 

[rambojoe]

Someone is getting a dc ammeter for christmas...

 

[paulengr]

If it’s AC depends on power factors. The power flow is what you expect because most inverters output unity power. But vars would not be offset and the hot tub has a motor so the flux current is unaffected.

 

[jaggedben]

Someone is getting a dc ammeter for christmas...

I don't think that would help him.

 

[Carultch]

Let's say I have a 200 amp main panel. In the main is a 100A breaker feeding a sub panel outside. Subpanel supplies power to 2 heat pumps and a hot tub.

A PV system is installed with a feeder tap into the sub-panel. PV system has potential to back-feed 50 amps.
Now let's say the Heat pumps and Hot tub is on, drawing 60 amps total. It s a sunny day so the PV system is making 50 amps.

What is the amp flow through the feeder to sub panel? The tendency is to think the PV will reduce the demand on the feeder to sub panel, is this the right way to look at it?

This situation comes up a lot, of course. I'm trying to wrap my head around how the current flows in these cases. Is it actually directional?

The PV system will supply local loads in the subpanel, before its current propagates to higher ranked panels. It will also feed on-site loads before it exports through the service meter to the immediate neighbors. Given 50A of PV, and 60A of load, it will supply its power to the local loads first, and those loads will now draw 10A from the main panel. If you populated the subpanel with 150A of load, it will draw 50A from the PV and 100A from its main supply. This is why the NEC has busbar protection requirements in 705.12. Because 150A of load in this panel would go "unnoticed" by the 100A main supply as an overload condition.

If you supply the panel from the same side, the current "adds up" on the busbar, and it goes without saying that the OCPD's of the sources are limited to the busbar amps. But if you strategically feed it from opposite ends, current cancels rather than adds. It may diminish to zero at some cross section of the busbar, and the current will never exceed either side's source. You might think that feeding it from opposite ends would allow you to interconnect 100A of PV and 100A of main supply on a 100A busbar, and feed 200A of loads within it. But reality is, the famous 120% rule doesn't allow this. 120% is an industry compromise to avoid 200A of heating among the breakers in a 100A panelboard.

The current is AC, but the reasoning would be the same as if it were DC. Just pause the waveforms of voltage and current at an instant in time, when the black wire's voltage is at its positive peak and assign that as the direction of current. What's really happening in AC, is the product of instantaneous Volts and Amps that flows in the same direction. I.e. the power (or apparent power).

 

[Ravenvalor]

If you supply the panel from the same side, the current "adds up" on the busbar, and it goes without saying that the OCPD's of the sources are limited to the busbar amps. But if you strategically feed it from opposite ends, current cancels rather than adds.

If a residential electrical sub-panel is getting fed from both a pv system as well as the utility will the power coming in from the pv system be backfed into a circuit breaker located at the opposite end of the bus system?

 

[Hv&Lv]

If a residential electrical sub-panel is getting fed from both a pv system as well as the utility will the power coming in from the pv system be backfed into a circuit breaker located at the opposite end of the bus system?

It will as long as the loads on the panel are lower than the PV output.

 

[rambojoe]

I don't think that would help him.

Why on earth not? He is certainly asking a fair question, and with the right meter he could see...

 

[jaggedben]

Why on earth not? He is certainly asking a fair question, and with the right meter he could see...

Because he is asking about AC current.

 

[rambojoe]

Very true...and im not even sure where the inverter is in the op's situation, but there sure is a great spot on his line to check...either way, those meters aint cheap, and they also read a/c amps...ill take a new one from santa!

 

[JoeNorm]

I have an ammeter, just haven't given the time to sit and watch a house while the loads turn on with the solar running.

I was pretty sure I knew how the current flowed in this case, just wanted to be sure I was right.

Thanks

 

[Hv&Lv]

Why on earth not? He is certainly asking a fair question, and with the right meter he could see...

Well, if it’s a DC ammeter being used on AC it’s going to read 0 trying to average the AC current sine wave

 

[rambojoe]

No kidding....its clear you know which is which... why bring up a bottomless pit of scenarios? If he plays around a bit (with the right setting) then he will get his "ah haa" moment...have confidence that i could do it. Again, my multi clamps on both dc and ac- but the cheaper ones only do ac...

 

[jaggedben]

Actually, using just an ammeter isn't a really complete way to view power flow in these systems. For one thing, an AC ammeter doesn't tell you the direction of powerflow. You need a meter that combines amps with a voltage reference for that. For another thing, you need at least two meters simultaneously to accurately observe the interaction of two sources with loads. Which is how it's done with battery systems that are setup to follow the load.

 

[Ravenvalor]

It will as long as the loads on the panel are lower than the PV output.

Therefore the service panel feeding this sub-panel is being backfed from the sub-panel and that sub-panel feeder breaker needs to also be on the opposite end of the bus as the breaker feeding it?

 

[Hv&Lv]

Therefore the service panel feeding this sub-panel is being backfed from the sub-panel and that sub-panel feeder breaker needs to also be on the opposite end of the bus as the breaker feeding it?

Wow, that reads almost like “who’s on first”...

I would set it up that way to make sure the bus never”sees” more amps than it’s rated for regardless of which source the energy comes from.

 

[rambojoe]

Actually, using just an ammeter isn't a really complete way to view power flow in these systems. For one thing, an AC ammeter doesn't tell you the direction of powerflow. You need a meter that combines amps with a voltage reference for that. For another thing, you need at least two meters simultaneously to accurately observe the interaction of two sources with loads. Which is how it's done with battery systems that are setup to follow the load.

Its ok, they make meters with the built in thumb rule now! Or you just look at the loads and assume, "Yuppers, its probably flowin' that-a-way"...