Wire size calcualtion

System · March 2007

Hello ALL
I am trying to increase my solar panel maximum amp output from 15 to 25 amps. The distance from the panels to the battery is about 30 feet, 15 feet each direction. The charge controller is mounted about in the middle. I can put in larger wire from the charge controller to the batteries but I can't do anything about the cables from the charge controller to the panels, they are built in to the walls of the RV. The current cabling is 12 AWG. So can I get away with 12 AWG or should I increase the size from the controller to the battery or is an upgrade not doable without instaling the addtional panels in a separate aray with seperate wiring.
Thanks for the advise

niel · March 2007

Re: Wire size calcualtion

i'm not fully understanding what it is you mean by: "The distance from the panels to the battery is about 30 feet, 15 feet each direction.". does that mean the distance from the pvs to the controller is 15ft and from the controller to the batteries another 15ft?
what is the controller you are using and what are the pvs you are using? what do you want for the pv upgrade?

crewzer · March 2007

Re: Wire size calcualtion

How did you decide to resolve this issue: http://www.wind-sun.com/smf/index.php?topic=2097.0

Jim / crewzer

mike95490 · March 2007

Re: Wire size calcualtion

12G wire is rated (usually in ROMEX) for about 20 Amps as in household wire.

14G is rated for 15A
10G romex is for 30A

So at higher currents, you will have more voltage drop (loss) in the wires, but will you really be seeing the higher currents, except for shome short period of the day ? Say you purchase a set of panels, rated at 32AMPs. They will only give you 32A under ideal conditions (cold, bright sun, perfect alignment)
It's almost a universal given, that you will not see 32A from them, instead you will see about 70-80% of nameplate rating of the panels. That may save your butt, and keep you from from torching your RV. I doubt insurance will cover intentional electrical overloads.
Maybe, if you change your controller, and rewire your panels to 48V, AND have the controller downconvert that to 24 or 12 V. you can safely achive the wattage you are seeking.

System2 · March 2007

Re: Wire size calcualtion

The wire run is 15 feet from panels to batteries but I understand you have to double that to account for the positive and the negative wire. That is why I said 30 feet total. I am trying to get more non-peak condition amperage but am concerned that at peak times there may be a heat problem from the resistance in the 12 AWG wire. If I replace the wire from the charge controller to a 8 AWG then the distance covered by AWG 12 wire will be only about 7 feet times 2. As I understand these things, the shorter the distance the lower the resistance will be and the less heat generated and the less voltage drop. I am going to replace the current charge controller with one rated for 30 amps and expect to run a maximum amp rating of 25 amps at peak output.

System2 · March 2007

Re: Wire size calcualtion

I wouldn't worry about it, George.

The current rating of building wire is based on the ability of the insulation to withstand heat.

The 12tw (or THHN) inside your wall will pass 20A indefinitely. It will probably pass 30A indefinitely.

The insulation will easily handle the load you are going to put on it for the length of time your 30A will be flowing.

This violates the code and you will incur some losses with this smaller wire, but it will work.

niel · March 2007

Re: Wire size calcualtion

george,
as it stands if you were to run 25amps through 30 feet of #12 copper wire you will have a vdrop of about 1.38v. the percentage would be about 11.5% for a 12v pv system. your present 15amp system to the controller has a vdrop of .83v for a percentage of 6.9%. i don't know if your future controller can downconvert, but having a 24v pv system downconvert to 12v would afford cutting the losses in 2 and going 48v to 12v cuts it in 4.
changing the wire to #8 and using a total of 14ft would make the vdrop be .254v and the % would be 2.12%.
if this is only the wires going from the controller to the battery you'll be replacing then sure go with a good heavy wire for it, but there will still be a large loss from the pvs due to the 15ft run of #12 wire. as to the heat from that i don't think that is going to be too much of a problem as the real problem is losing power from expensive pvs. the losses from the pvs to the controller and the losses from the controller to the battery are additive i might add so whatever measures you can do will help the situation.

crewzer · March 2007

Re: Wire size calcualtion

niel,

George answered the question that was intended for him, not you:

"I am going to replace the current charge controller with one rated for 30 amps and expect to run a maximum amp rating of 25 amps at peak output."

Regards,
Jim / crewzer

crewzer · March 2007

Re: Wire size calcualtion

George,

niel’s helpful wiring analysis contains glimmers of two important points to consider:

(1) The combination of low voltage from hot PV modules and a collection of sequential voltage losses in the PV-to-controller wiring, the controller itself, and the controller-to-battery wiring may result in a charge voltage at the battery terminals that’s too low to reach the ~14.4 V required to transition from bulk to absorb stage. For example, if the hot PV modules can supply 15.5 V, and there’s a ~1.4 V drop in the PV-to-array wiring, a 0.2 V drop in the controller, and another 0.4 V drop in the wiring from the controller to the batteries, then the maximum voltage at the battery terminals will be ~13.5 V. This type of deficit recharging leads to both reduced battery capacity (fewer useful Ah) and shorter battery life (fewer charge/discharge cycles).

(2) Using a DC-DC converter-type charge controller (i.e., 24 V to 12 V) would not only cut the voltage drop loss in the array-to-controller wiring in half, but, because the array current would also be cut in half, the power loss in the wiring from the array to the controller would be cut by ¾. This reduced power loss in the wiring would result in both higher voltage at the controller input and cooler wires. However, the down side of DC-DC type controller is that they’re not as efficient as series-type PWM controllers.

Separately, I understand that the existing PV wiring is “hidden” in the RV walls. Would it not be possible to use that existing wiring to pull some new heavier wire to run between the larger array and the new controller? Or, what about abandoning the old wire and installing some new wire. The “dead space” between the back of the fridge and the outside wall is a popular route, with the wiring exiting via the roof vent for the fridge.

Finally, it’s true that your 30 A PV array will rarely operate above 25 A. However, there will be times, such as during “edge of cloud” events, when the array current may well exceed 35 A for short periods. Your wiring and fuses/breakers should be sized to handle such a worst case scenario.

HTH,
Jim / crewzer

halfcrazy · March 2007

Re: Wire size calcualtion

also think of code and insurance companys if something goes wrong and you have a 30-35 amp breaker on 12 gauge wire well they are going to say sorry it isnt covered. Better to take the time and make it right

Wire size calcualtion

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