Wire Size Guide for Solar PV Systems (How To Calculate)

When installing a solar PV system, using the correct wire size is critical. If the solar array pushes too much electrical current through too thin of a wire, the metal conductors get hot and can melt the outer insulation, which becomes a dangerous fire hazard.

Solar wire sizing can be confusing because there are multiple factors to consider, including the size of the solar array, how the panels are wired together, and which solar charge controller you use. There is no one-size-fits-all wiring solution.

This post will help you identify exactly what solar wire sizes you need for your entire solar system, including the solar panels to the charge controller and the controller to the batteries. Your resulting wire gauges will comply with National Electric Code (NEC) standards to help keep your solar system safe from overheating and potentially catching fire.

However, you should also read our solar fusing guidelines after this post for additional safety wire safety. Let’s get to it.

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A Quick Word on Wire Safety

We already know thicker wires can carry larger amounts of electrical current (amps). But did you know that wire ‘quality’ also matters? The materials used for the metal conductors (copper vs. copper/aluminum) and the insulation jacket all factor in the total ampere carrying capacity of the wire.

There are A TON of different electrical wires sold online. Some of these wires are good quality, but some are questionable junk with foreign manufacturers facing no real accountability for misleading marketing. Sifting through all the varieties is a headache. We know, we did it ourselves!

To keep this post simple and safe, we only recommend Windynation-branded solar cables. Windynation is an American-based company, and every wire is UL-tested and 100% made in the USA. In other words, we trust their listed wire ampacity ratings, which, in turn, keep our solar system safe.

Disclosure: We are not affiliated with Windynation and do not make a commission if you purchase wires directly from their website. We do make a commission for sales made on the Amazon platform, but the cost to you remains the same.

Two Solar Wiring Sections To Account For

When calculating solar wire sizes, there are two wiring sections to consider. Sometimes, these two sections will use the same wire size, and sometimes, they will not. This post will help you calculate the correct gauge size for BOTH parts.

• Section 1: Between the solar panels and the charge controller
• Section 2: Between the charge controller and the bus bars

Where are the batteries? We will connect the charge controller to bus bars, which are power collection and distribution centers. This is where the solar system terminates. You can learn how to wire the bus bars to the batteries in our 12V system guide.

Solar Wire Size Calculator (7 Steps)

This seven-step guide will help you determine your solar system’s wire sizes. We’ll start with calculating your solar array’s max current production and work towards getting you the best possible wires for your system.

Step 1: Finalize Solar Array Configuration

The first step to sizing the solar wires is to finalize your solar system setup. To do this, you will need to answer the following four questions before you can move on:

1. What is the total watts of the solar array?
2. Which specific panels will you use?
3. How many panels will you have?
4. Are solar panels wired in series or parallel?

Each question is essential to calculating the maximum amps produced by the solar array. Let’s explore each question further.

1.1 – What Is the Total Watts of the Solar Array?

The total watts produced by the solar system is one of the most critical factors determining solar wire gauge size. The more watts, the more amps produced, and the thicker the wire size you’ll need.

Solar calculator: Unsure how much solar you need? Use our solar wattage calculator.

1.2 – Which Specific Panels Will You Use?

Knowing which specific panels you are using is important because different-branded solar panels will output different amps, even if they are rated for the same number of watts. For example, we list the maximum potential amps – Isc(A) – for three different 200-watt panels below:

Solar Panel	Maximum Potential Amps
Renogy 200W Panel	11.05A
Newpowa 200W Panel	11.47A
BougeRV 200W Panel	12.21A

Solar panel recommendation: We use Newpowa solar panels on our van, and they’ve held up well throughout our travels for the past five years. Newpowa panels are built tough and are generally cheaper ($ per watt) than their competitors. However, you should shop and compare the prices online first.

1.3 – How Many Solar Panels Will You Have?

The physical number of solar panels you have also impacts the total amps produced by the solar array.

If building a camper van, we recommend using fewer – but larger – solar panels.

1.4 – Are Solar Panels Wired in Series or Parallel?

Whether the solar panels are connected in series or parallel will significantly affect the total amps produced by the array. You must decide which wiring method you will use.

Learn more: Pros and cons of series vs. parallel solar panel wiring.

In short, solar panels wired in series produce fewer amps than panels connected in parallel. Therefore, panels wired in parallel tend to require thicker wires to safely transmit higher amperages.

Step 2: Calculate Maximum Amps Produced by Array

Assuming you’ve answered the previous four questions from Step 1, you can now calculate the maximum amps produced by the solar array. Making this calculation involves a two-part process:

1. Identify one of your panel’s short circuit current, or “Isc(A)”.
2. Calculate the total current based on series or parallel wiring.

2.1 – Identify Short Circuit Current – Isc(A) – From Single Panel

The short circuit current – sometimes identified as Isc(A) – is the amps produced by the panel in the event of a short circuit and is the maximum current that the panel can theoretically produce.

Where is this information? You can find your panel’s short circuit current from the panel’s specifications sticker on the backside of the panel or by visiting the panel’s online product page. Below, we locate the short circuit current of Newpowa’s 200W panel from their Amazon product page, which is 11.47 Amps.

2.2 – Calculate the Total Short Circuit Current Produced From the Array

If you only plan to have one solar panel, the short circuit current you identified above will be the total current produced by your solar system. But if you have more than one solar panel, how you connect these solar panels – series or parallel – will affect the maximum amps produced by the array.

Series connections: The total current produced by the solar array is only equal to the short circuit current of one of your panels. This is because, in series connections, the voltage (V) produced by each panel adds up, but the amps stay the same.

Parallel Connections: The total current produced by the solar array is equal to the sum of the short circuit currents of each panel. This is because, in parallel connections, the amps produced by each panel add up, but the voltage stays the same.

Example: You have two 200W Newpowa solar panels each with a short circuit current Isc(A) of 11.47A. Refer to the below table to see how wiring them in series or parallel affects the total amps output by the array.

	Total Amps
Series Wiring	11.47A
Parallel Wiring	11.47 + 11.47 = 22.94A

Step 3: Apply NEC Safety Factors

The National Electric Code (NEC) exists to help keep people and property safe from electrical hazards. This step will show you how to comply with NEC 690.8, which deals specifically with PV systems and solar wire sizing.

In short, you must apply the following TWO safety buffers (125% rules) to comply with the NEC code.

1. Add 25% to the max solar array current from Step 2.2 (multiply by 1.25).
2. Add another 25% to account for continuous load (multiply by 1.25).

3.1 – Add 25% to Max Solar Array Current

The NEC dictates that the maximum current of a PV system to be the “short-circuit current…multiplied by 125 percent” (NEC 690.8(A)(1)(a)(1)). This is to account for the fact that solar panels can sometimes produce MORE watts than what they are rated for. Therefore, the NEC assumes that any solar array has the potential to produce 25% more power than what it is technically rated for.

To comply with NEC code, you must take the maximum current you calculated for your solar array (Step 2.2) and add 25% (multiply by 1.25) to this amount.

Example: If your solar array’s maximum amps is 11.47A, the NEW maximum total current is 14.34A. (11.47A * 1.25 = 14.34A)

3.2 – Apply Another 25% for Continuous Currents

The NEC states that to calculate ampacity for wire sizing, you must add an additional 25% to the new maximum current that you calculated in Step 3.1 (NEC 690.8(B)(1)). This is because the NEC considers PV circuits to be a ‘continuous load’, which is a circuit that experiences a maximum current for three, or more, hours.

A wire that experiences maximum current for long and sustained periods of time can cause the wire to become extremely hot and eventually fail. The NEC wants you to consider this when sizing your solar wires and another 25% (multiply by 1.25) to the maximum current produced by the solar array.

“The maximum currents calculated in 690.8(A) multiplied by 125 percent without adjustment or correction factors.”

NEC 690.8(B)(1)

Example: The new maximum current from the previous example was 14.34A. By adding the 25% safety margin, we get 17.93A. (14.34A * 1.25 = 17.93A)

The 1.56x rule: A faster way to get through Step 3 is to take the maximum current produced by the solar array (from Step 2.2) and add 56% to this amount (multiply by 1.56). This is because multiplying an amount by 1.25 twice is equal to multiplying the same amount by 1.56 one time.

Step 4: Calculate Solar Wire Size for Section 1

Now that you have applied the two NEC safety factors to your solar array’s maximum amps, you can determine the solar wire size you need for Section 1.

Remember: Section 1 represents the wires that connect the solar panels to the solar charge controller.

Take the amps you calculated in Step 3.2 – after the second 25% NEC safety factor – and refer to the table below to identify which wire size you need to complete Section 1. The amps should fall within one of the three ranges.

Total Amps Of Solar Array	Solar Wire Size (Section 1)
30A or less:	12 AWG
30A – 40A	10 AWG
40A – 55A	8 AWG

Where did these ampacity ranges (table column 1) come from? We refer to Windynation’s solar wire specifications, which state that their 12, 10, and 8-gauge solar wires are certified for 30A, 40A, and 55A, respectively.

Below are the three solar wire size recommendations based on the solar array’s maximum current output. If you anticipate the distance between the solar panels and charge controller being longer than 15′, you may want to consider upgrading to the next wire size to prevent voltage drop and power loss.

Step 5: Select Solar Charge Controller

Now we move on to calculating the correct solar wire size to connect the solar charge controller and the bus bars. This is section two.

What about connecting to the batteries? It is good electrical practice to wire the solar charge controller to the bus bars. For information on what you need to connect the bus bars to the batteries, head over to our 12V system wiring.

The first step to calculating your section 2 wire size is to select your solar charge controller. Every charge controller has a ampacity rating, which is the maximum amps that the controller can push through the wires.

Example: A charge controller rated for 30A will output a maximum 30 amps to the bus bars.

The charge controller’s ampacity rating depends on the total wattage of your solar array. Refer to the table below to locate you ideal charge controller size based on your solar watts (for 12V systems). For wattage verification, refer to the Victron manuals listed in the table footnotes.

Total Solar Watts	Solar Charge Controller Size
0-220W	15A Controller1
220-440W	30A Controller2
440-700W	50A Controller2
700-1000W	70A Controller3

Table Footnotes:
1. Victron 15A manual
2. Victron 30A/50A manual
3. Victron 70A manual

Recommended charge controller? Victron solar charge controllers are workhorses. It is what we use. We’ve been using a Victron 30A charge controller for five years, and it works day in and day out without issues. We love its Bluetooth connectivity, sending solar data straight to our phones. Read our Victron charger controller review for more.

Example: Two 200-watt solar panels produce a total of 400 watts. Therefore, a 30A solar charge controller is best.

Step 6: Add 25% To Charge Controller Ampacity Rating

Similar to Step 3.2, we must add 25% (multiply by 1.25) to the charge controller’s amp rating because the controller’s output wires are also considered to be a ‘continuous load’, having to handle maximum current for three, or more, hours.

After adding 25%, the resulting amount is what we will use to determine the solar wire size you need for section 2.

Example: A 30A controller will output a maximum 30A current. After adding the 25% buffer, we get 37.5A. (30A * 1.25 = 37.5A)

Step 7: Calculate Solar Wire Size for Section 2

You can now determine the solar wire size you need to connect the charge controller to the bus bars (Section 2) based on the current you calculated in Step 6.

To do this, take the amps you calculated in Step 6 and identify the correct wire size from the table below. The amps you calculated should fall within the range listed in column 1.

Amp Range	Charge Controller
0-30A	10 AWG Wire
30-55A	8 AWG Wire
55A-115A	6 AWG Wire
115-150A	4 AWG Wire

The below wires (8 AWG to 4 AWG) are Windynation wires, which we trust to carry the ampacity ranges we list in the above table. The 10 AWG wire below is an Ancor marine-grade wire, a brand that we also trust.

Example: From Step 6, we calculate 37.5A from our 30A charge controller. Using the above table as a reference, we should use 8 AWG wires to connect the charge controller to the bus bars.

Summary (TL;DR)

The diagram below shows what wire sizes you’ll need to connect the solar panels to the charge controller and the bus bars.

1. The solar array’s maximum current determines the size of the solar wires between the panels and the controller.
2. The total wattage produced by the solar array determines the amp rating of the solar charge controller.
3. The amp rating of the solar charge controller determines the size of the solar wire between the controller and the batteries.

Now that you’ve selected your solar wires, you might be ready to start wiring everything together. Check out our solar system guide for a step-by-step installation tutorial.

Solar Wiring Diagram (Less Than 440W)

If you prefer to follow a premade design, you can refer to the solar wiring diagram below. This solar system is designed for a max solar array of 440W with panels wired in series.

Free download: Get this wiring diagram as a PDF.

FAQ

Conclusion: Proper Solar Wire Size Is All About Safety

If you’ve gotten this far in the post, congratulations! Calculating the correct wire sizes for your solar system is tedious and, understandably, not particularly exciting. However, the biggest argument for taking the time to do your calculations correctly is for YOUR safety.

You want to keep yourself, your loved ones, and your home safe from electrical faults and fires.

But proper wire size is just one-half of the safety equation. You must also install the correct fuses and circuit breakers to protect the solar wires from over-current events. Read our post “how to fuse a solar system” to learn more about how to determine what you need.

If you have any questions regarding the best solar panel wire size for your system, please comment in the section below.

Happy building!

Appendix

1. Windynation Solar Wire Specifications

Below are the solar wire specifications for Windynation’s 8 AWG, 10 AWG, and 12 AWG wires. These solar connectors are UL-certified, National Electric Code compliant, and are rated to handle the below-specified ampacities.

2. Windynation 8 to 4 AWG Wire Specifications

We inquired with Windynation support staff regarding the ampacity ratings of their wires and below is an excerpt of their reply.

“Attached is our Certificate of Conformance for our cable. The ampacity is indeed continuous in open air (room temp).”

Windynation Support Staff

Their certificate of compliance is the perfect resource because it verifies Windynation’s own wire ampacity ratings in a continuous current environment instead of relying on a generic 3rd party table that encompasses many different wire grades. We will refer to this certificate when recommending your (section 2) solar sizes.

We also like Windynation because the American-based company manufactures premium-quality pure copper wires encased in a tough EPDM rubber insulation jacket for high-temperature resistance and ultra-flexibility. Lastly, these wires are assembled in the USA, which we prefer because this gives us greater reassurance that we can trust the wire quality and their verified ampacity ratings.

3. National Electric Code: NEC 690.8(A)(1)(a)(1)

“The maximum current shall be the sum of the short-circuit current ratings of the PV modules connected in parallel multiplied by 125 percent.”

NEC 690.8(A)(1)(a)(1)

Solar panels are rated by a laboratory under photovoltaic standard test conditions (STC) of 1,000W/m² and a temperature of 25°C. In real life, these conditions are rarely met and the solar panel will output less than their laboratory rating. However, when environmental conditions are extremely favorable, solar panels can output MORE than their lab rating.

The Sandia Report (SAND2004-3535) proves this. The below scatter plot records the wattage output of a 165W solar panel in a January month. From the graph, you can see that the panel does occasionally output more than 165W.

In our van, we can also sometimes harvest up to 15% more watts than our supposed maximum when we are traveling in high-altitude environments during a clear and cool sunny day.

It is for this reason, the NEC includes 690.8(A)(1)(a)(1) to increase the solar array’s maximum potential current output by 25%.