Wire Sizes and what they mean

One of the most important factors to consider when designing and installing a solar system (or doing any electrical) is the wire size. The wire size determines how much current can flow through the wire safely and efficiently. The current capacity of a wire is also known as its ampacity.

Choosing the right wire size can help you avoid several problems, such as:

• Overheating and melting the wire insulation, which can cause fire hazards and damage to your equipment
• Voltage drop and power loss, which can reduce the performance and efficiency of your solar system
• Exceeding the code requirements and safety standards, which can result in fines, penalties, or rejection of your permits and inspections

To choose the right wire size for your solar system, you need to consider the following factors:

• The maximum current that will flow through the wire
• The length of the wire run
• The temperature rating of the wire
• The type of conduit or cable that will enclose the wire

How to Calculate the Maximum Current

The maximum current that will flow through the wire depends on the type and configuration of your solar system. For example, if you have a grid-tied solar system, the maximum current will be determined by the output of your inverter. If you have an off-grid solar system, the maximum current will be determined by the output of your charge controller or battery bank.

To calculate the maximum current, you need to know the power (in watts) and the voltage (in volts) of your solar system components. Then, you can use the following formula:

Maximum Current (in amps) = Power (in watts) / Voltage (in volts)

For example, if you have a 3000-watt inverter that operates at 240 volts, the maximum current will be:

Maximum Current = 3000 / 240 = 12.5 amps

How to Calculate the Voltage Drop

The voltage drop is the amount of voltage that is lost due to the resistance of the wire. The voltage drop depends on the length of the wire run, the size of the wire, and the current that flows through it. The voltage drop can reduce the power and efficiency of your solar system.

To calculate the voltage drop, you need to know the resistance (in ohms) of your wire. The resistance of a wire depends on its material, size, and temperature. You can use a wire resistance chart to find out the resistance of your wire per 1000 feet.

Then, you can use the following formula:

Voltage Drop (in volts) = Current (in amps) x Resistance (in ohms) x Length (in feet) / 1000

For example, if you have a 12-gauge copper wire that has a resistance of 1.588 ohms per 1000 feet, and you have a 50-foot wire run that carries 12.5 amps of current, the voltage drop will be:

Voltage Drop = 12.5 x 1.588 x 50 / 1000 = 0.99 volts

The acceptable voltage drop for a solar system is usually between 1% and 3% of the nominal voltage. For example, if you have a 12-volt solar system, the acceptable voltage drop range is between 0.12 volts and 0.36 volts.

To reduce the voltage drop, you can either shorten the length of your wire run or increase the size of your wire.

How to Choose the Wire Size Based on Temperature Rating

The temperature rating of a wire is the maximum temperature that the wire can withstand without degrading its insulation or performance. The temperature rating of a wire depends on its material and type. You can use a wire temperature rating chart to find out the temperature rating of your wire.

The temperature rating of a wire also affects its ampacity. The higher the temperature rating, the higher the ampacity. However, you also need to consider the ambient temperature where your wire will be installed. The ambient temperature is the temperature of the surrounding air or environment. The ambient temperature can affect the heat dissipation and resistance of your wire.

If your ambient temperature is higher than 30°C (86°F), which is the standard reference temperature for most wires, you need to apply a correction factor to adjust your ampacity. You can use a correction factor table to find out the correction factor for your ambient temperature and wire type.

Then, you can use the following formula:

Adjusted Ampacity (in amps) = Nominal Ampacity (in amps) x Correction Factor

For example, if you have a 12-gauge THHN copper wire that has a nominal ampacity of 20 amps at 30°C (86°F), and your ambient temperature is 40°C (104°F), which has a correction factor of 0.91 for THHN wires, your adjusted ampacity will be:

Adjusted Ampacity = 20 x 0.91 = 18.2 amps

To choose the right wire size based on temperature rating, you need to make sure that your adjusted ampacity is higher than your maximum current.

How to Choose the Wire Size Based on Conduit or Cable Type

The conduit or cable type is the protective covering that encloses your wire. The conduit or cable type can affect the heat dissipation and ampacity of your wire. The conduit or cable type can be either metallic or non-metallic, and can have different fill factors and temperature ratings.

The fill factor is the percentage of the cross-sectional area of the conduit or cable that is occupied by the wire. The fill factor depends on the number and size of the wires that are enclosed in the conduit or cable. The fill factor can affect the heat dissipation and ampacity of your wire. The higher the fill factor, the lower the heat dissipation and ampacity.

You can use a fill factor table to find out the maximum fill factor for your conduit or cable type.

To choose the right wire size based on conduit or cable type, you need to make sure that your fill factor is lower than the maximum fill factor, and that your conduit or cable temperature rating is higher than your wire temperature rating.

How to Use a Wire Size Chart

A wire size chart is a handy tool that can help you choose the right wire size for your solar system based on your maximum current, voltage drop, temperature rating, and conduit or cable type. A wire size chart shows the ampacity, resistance, and voltage drop of different wire sizes and types.

You can use a wire size chart to find out the recommended wire size for your solar system by following these steps:

• Find your maximum current in the left column of the chart
• Find your voltage drop percentage in the top row of the chart
• Find the intersection of your maximum current and voltage drop percentage in the chart
• Find the wire size and type that matches or exceeds your intersection value in the right column of the chart
• Check if your wire size and type meets your temperature rating and conduit or cable type requirements

For example, if you have a maximum current of 12.5 amps, a voltage drop percentage of 2%, a temperature rating of 90°C (194°F), and a non-metallic conduit type, you can use the following steps to find your recommended wire size:

• Find 12.5 amps in the left column of the chart
• Find 2% in the top row of the chart
• Find the intersection of 12.5 amps and 2% in the chart, which is 0.8
• Find the wire size and type that matches or exceeds 0.8 in the right column of the chart, which is 14-gauge THHN copper wire
• Check if 14-gauge THHN copper wire meets your temperature rating and conduit type requirements, which it does

Therefore, 14-gauge THHN copper wire is a suitable wire size for your solar system.

You should always consult with a professional electrician or solar installer before deciding and installing your wires. You should also follow the local codes and standards that apply to your project. By doing so, you can ensure that your solar system is safe, reliable, and efficient.

This table provides you with the allowable ampacity (maximum current the cable can carry) that you can use with wires in conduit, raceway, cable or directly buried, assuming an ambient temperature of 86°F (30°C).

*National Electric Code specifies that the overcurrent protection device (e.g. a fuse or breaker) must not exceed 30A for 10 AWG wire, 20A for 12 AWG wire and 15A for 14 AWG wire.

Ambient Temperature Above 86°F (30°C)?

If your ambient temperature is higher than 86°F (30°C) then multiply the ampacity you found in the table above by the correction factor listed under the cable insulation temperature rating below: