Selecting wire size based on voltage drop for solar systems

Step 1. Work out your power needs

Step 2. Sizing battery bank

Step 3. Sizing solar array

Step 4. Wire sizing
NOTE: Clicking on any link in the body portion of this page will take you to a new window or tab so that you don't lose any information.

The longer your wires are, the more the voltage will decrease due to losses. This is especially important for the wires going from your solar panels to your charge controller since this is usually a long distance and you'll want a certain minimum voltage at the charge controller.

Below is a calculator that will tell you what is the smallest size wire you can use. You tell it things like what voltage and current will be on the wire, how much loss you're wiling to have, how long the wire is and what type of wire you're using if you know the type.

Voltage drop calculator

Fill in the following and click on the Calculate button...
Voltage across the wire: (volts) How much voltage will be across the wires.
e.g. The solar array voltage.
 
Acceptable voltage drop: (%)
or
(volts)
How much voltage you're willing to lose. You can either give it as a percentage of the voltage you gave above or as the amount of voltage you're willing to lose. Typical percentages are 2% or 5%.
Make sure the field you're not filling in is blank. If both are blank then the amount you're willing to lose will default to 2% of the votlage you gave above.
 
Current: (amps) At least the number of amps the wire will be carrying.
e.g. The current on the wires coming from the solar array.
 
Length of wire: (feet) Length the wire will be.
e.g. If the length of one of the wires, either the positive or the negative, between the solar array and the charge controller is 25 feet, then put that.
 
Type of wire: The type of wire. Leave as (unknown) if you don't know or if you don't want to give it.
The size of wire will have to be able to handle the amount of current you specify. If you give a wire type then the ampacity or maximum current that type of wire can handle will be taken into account when selecting a wire size. If you don't give a wire type then a rule called the 700 amps per circular mils rule will be used to find a wire size that can handle the current you specify.

Result:
The wire size needed for the requested pecentage voltage drop is gauge (AWG).
The cross sectional area is circular mils. The ampacity of that wire is amps.

Errors and what they may mean

The calculator may give you one of the following error messages instead of a wire size.

Error: No wire was found that's big enough. Maximum size is N gauge (AWG).

It's possible that it won't be able to find a size, given what you tell it. There are a number of reasons why:

  • The length is so long that to meet your voltage drop requirements needs a wire thicker than any in the calculator's internal table.
  • The voltage drop you want is too small, and to meet it means using a wire thicker than any in the calculator's internal table.
  • The current you've requested is so high that in order to accommodate it requires a wire thicker than any in the calculator's internal table.

Error: No wire was found that could handle the given current. Maximum ampacity is N amps.

Different wire sizes are rated for different maximum amounts of current, called that wire's ampacity. If more than that amount of current runs through them then they will heat up, possibly melting their insulation and causing a fire.

This error happens if the calculator found a wire size that meets your requested voltage drop but the current you gave is greater than that wire size's ampacity. In that case the calculator keeps going through the table until it can find a larger wire whose ampacity is the same or smaller than your given current. If none large enough is found then this error is given.

The solution here is to reduce the current by increasing the voltage. For example, if this is for the wires between the solar array and the charge controller, than you may be able to reconfigure the solar array to have a higher voltage and a lower current. This reconfiguring involves increasing the number of panels in series and possibily having fewer in parallel.

What if the wire size is too expensive?

Copper wire is very costly. If the calculator does give you a wire size to use, it's possible that you find a price for it and it turns out to be too expensive. This is often the case for the wires going between the solar array and the charge controller since that involves a lot of long wire.

In that case you could try a number of things:

  • You could try a higher voltage. This will also result in a smaller current. If the difference is enough then this may move you to a smaller wire size.
    For example, if this is for wiring between a solar array and a charge controller then you could reconfigure the solar array to have a higher voltage. This will automatically result in a smaller solar array current too.
  • You could try a larger voltage drop requirement. Of course this will mean a lower voltage for the charge controller. If that's not feasible because it would be too low for the charge controller then you'd either need to consider the other option above or add more solar panels. It may then become cheaper just to go with the more expensive wire instead.

How the calculations are done

This voltage calculator uses the following formula from www.electrician2.htm:

Voltage drop formula.

However, the formula was rearranged to solve for the circular mils instead:

Voltage drop formula rearranged to solve for the current instead.

The k is the specific resistivity of copper. The value used here is 11 ohms, which is the specific resistivity at between 77F/25C and 121F/50C.

The circular_mils_area, or the circular mils, being solved for in the above formula is a measurement for the cross sectional area of the wire. It's used by electricans as a convenient way to calculate the cross sectional area without using the mathematical constant, pi.

So by calculating the cross sectional area of the wire that meets your given length_of_wire, current and voltage_drop requirements it's actually calcuating the wire size, i.e. which wire you need.

To turn that from a cross sectional area into a wire gauge, the circular mils area is looked up in the following table by finding the next biggest one.

  NEC ampacities for various wire types*
Circular
mils area
Wire size/
gauge (AWG)
60C
TW, UF
75C
RHW, THW, THWN, THHW, XHHW, USE
90C
RHH, RHW-2, XHHW, XHHW-2, XHH,
THHW, THWN-2, THW-2, THHN, USE-2
4110 14 20 20 25
6350 12 25 25 30
10380 10 30 35 40
16510 8 40 50 55
26240 6 55 65 75
41740 4 70 85 95
66360 2 95 115 130
133100 2/0 145 175 195
211600 4/0 195 230 260
* The NEC values were gotten from here.

Once the gauge of the wire is found in the table above, the last step is to make sure that the current you gave doesn't exceed the ampacity of that gauge of wire. When filling out the calculator you are asked for the type of wire. If you don't know it then the ampacity is assumed to be the circular mils divided by 700 amps per circular mil (e.g. 16510 circular mils / 700 amps per circular mil = 23.58 amps.) If you do select one of the wire types from the calculator then that type is looked up in the above table for the ampacity.

In either case, once the ampacity is known, the next check is that the current doesn't exceed that ampacity. If it does then the next entry in the table is examined for its ampacity, and so on down the table until a wire size is found that has an ampacity that is greater than or equal to your current.

The end result for step 4

You've likely used this to figure out the size of wire you'll need for going between the solar array and the charge controller. Click on an icon at the top or bottom of this page to go to another step.

NOTE: Clicking on any link in the body portion of this page will take you to a new window or tab so that you don't lose any information.
Step 1. Work out your power needs

Step 2. Sizing battery bank

Step 3. Sizing solar array

Step 4. Wire sizing
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