Confused on Wire Gauge from Panels to Charge Controller

[mrybas]

Hi,
I am adding two Sunpower 245's in parallel to my 12V electrical system. It is roughly 30' one way from the panel to the charge controller (OutBack Flex 60). The panel is rated at 40.5 volts. I should be looking at a wire gauge chart for 40.5Vs, not 12V, right?
Here is my math: 2 x 245W panels= 490W
490W/40.5V=12 Amps
One way distance 30' x2= 60'

According to a wire gauge calculator website @ 3% loss I need 8G wire. Does this sound correct?

From the charge controller to the batteries is 15'. This voltage would be stepped down to 12V.
490W/12V=40.8 Amps
15' x 2= 30'
@ 3% loss, I would need 4G wire.

Does this sound correct?

[djmarchand]

Well, you are approximately correct and there is nothing unsafe about your wire sizes. Is your panel rated at 40.5 V at its maximum power point (Vmp)? I would have thought that the voltage would be lower and the current higher. It is the current at the maximum power point (Imp) that you should size your wiring from the panel to the controller. Take the Imp and double it for two panels and use that in your calculations.

Make sure that you put a fuse or circuit breaker near the battery sized for 50 amps. There is no need (contrary to a lot of bad advice on this site) for another fuse at the panels. 8 gauge wire is plenty big to handle all of the current that the panels can put out.

David

[mrybas]

Originally Posted by djmarchand

Well, you are approximately correct and there is nothing unsafe about your wire sizes. Is your panel rated at 40.5 V at its maximum power point (Vmp)? I would have thought that the voltage would be lower and the current higher. It is the current at the maximum power point (Imp) that you should size your wiring from the panel to the controller. Take the Imp and double it for two panels and use that in your calculations.

Make sure that you put a fuse or circuit breaker near the battery sized for 50 amps. There is no need (contrary to a lot of bad advice on this site) for another fuse at the panels. 8 gauge wire is plenty big to handle all of the current that the panels can put out.

David

Hi David,
The Vmpp is 40.5V. The Imp is 6.05A. So I need to size for 6.05A @ 40.5V, right?

[s/v Jedi]

Originally Posted by mrybas

Hi David,
The Vmpp is 40.5V. The Imp is 6.05A. So I need to size for 6.05A @ 40.5V, right?

No, you just need to size for 6A times 2 (panels) = 12A. Voltage has no meaning for determining cable diameter as long as the cable insulation value is high enough (but that is 600V normally so fine).

12A with 3% voltage drop puts you at AWG6, but only just so you could get away with AWG8 with may be 4-5% voltage drop.

If you have no other panels connected yet, you could go for a series connection of the two panels which gives 6A @ 81V. Your controller can take that and the required cable goes down to AWG8.

[David M]

Jedi is correct. You gauge wire for the current, the run, the percentage drop and sometimes the temperature. The insulation must be rated for the voltage and the type of environment.

[mrybas]

Ahhhh, it all makes sense now. Thanks for the help!

[mrybas]

Originally Posted by s/v Jedi

If you have no other panels connected yet, you could go for a series connection of the two panels which gives 6A @ 81V. Your controller can take that and the required cable goes down to AWG8.
[/IMG]

Yes, it is just the two panels. Is it correct that the two panels wired in series makes them more susceptible to the effects of shading? They are mounted on top of the bimini, so its fairly clear, except for a backstay and mast shadow.

[FlyingCloud1937]

Originally Posted by David M

Jedi is correct. You gauge wire for the current, the run, the percentage drop and sometimes the temperature. The insulation must be rated for the voltage and the type of environment.

Originally Posted by s/v Jedi

No, you just need to size for 6A times 2 (panels) = 12A. Voltage has no meaning for determining cable diameter as long as the cable insulation value is high enough (but that is 600V normally so fine).

12A with 3% voltage drop puts you at AWG6, but only just so you could get away with AWG8 with may be 4-5% voltage drop.

If you have no other panels connected yet, you could go for a series connection of the two panels which gives 6A @ 81V. Your controller can take that and the required cable goes down to AWG8.

I'm sorry, but you are both wrong.

The percentage of voltage drop, is in fact related to the Volts in the circuit.

A 24 volt circuit with a 12 amp load, # 6 wire, with a run of 30 feet one way is 1.22%

A 12 volt circuit with a 12 amp load, # 6 wire, with a run of 30 feet one way is 2.44% double what the 24 volt % drop is.

A 48 volt circuit with a 12 amp load, # 6 wire, with a run of 30 feet one way is .69% half what 24 volt % drop is.



Lloyd

[rebel heart]

I don't think his 12v panels are going to push at 48 volts.

[mrybas]

Originally Posted by rebel heart

I don't think his 12v panels are going to push at 48 volts.

No, but close.... Vmpp= 40.5V. This is from the panels to the charge controller, before its stepped down to 12V by the charge controller.

[s/v Jedi]

Originally Posted by FlyingCloud1937

I'm sorry, but you are both wrong.

The percentage of voltage drop, is in fact related to the Volts in the circuit.

A 24 volt circuit with a 12 amp load, # 6 wire, with a run of 30 feet one way is 1.22%

A 12 volt circuit with a 12 amp load, # 6 wire, with a run of 30 feet one way is 2.44% double what the 24 volt % drop is.

A 48 volt circuit with a 12 amp load, # 6 wire, with a run of 30 feet one way is .69% half what 24 volt % drop is.

Lloyd

Ah, the mad scientist, telling us we're wrong again

Mr Ohm dictates us that the voltage drop over the wire equals the wire resistance times the current, regardless of voltage of the source. Mr Ohm does not allow for opinion being different than his law

When we operate the panels at a higher voltage, the current goes down (again Mr Ohm) and with it the voltage drop.

[goboatingnow]

Not to mention that the power loss , which is actually stuff thrown away , is I squared R


Dave

[Zil]

Currant does not necessarily go down with higher voltage. Size your wires based on the demand of your controller. Use a wire chart for the distance.

[s/v Jedi]

Originally Posted by Zil

Currant does not necessarily go down with higher voltage. Size your wires based on the demand of your controller. Use a wire chart for the distance.

It does for solar panels which is the topic here.