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What Size Wire for a 50 Amp Sub Panel? (According to NEC)

Writen by Edwin Jones

Fact checked by Andrew Wright

what size wire for a 50 amp sub panel

“What size wire for a 50 amp sub panel?” is a common question, even though 50 amps fall a bit below the current ratings of most properties.

This article will give you the best answer to this query based on the NEC’s wire size recommendations.

It will also discuss the adjustments you need to reduce voltage drop, which may arise from the wire’s total length.

Continue reading to get the clearest, most verifiable answer to your sub-panel issue.

The Correct Wire Sizes for a 50A Sub Panel

According to the size chart recommendations from the 2002 edition of the NEC, the minimum wire for 50 amps should be 6 AWG for copper. For aluminum, it should at least be 4 AWG.

However, these answers assume that there’s no voltage drop to consider. The NEC considers a 5% voltage drop as the maximum safety limit. Some electricians don’t even go past 2% voltage drop on the feeder and 3% on branch circuits because these percentages are considered beneficial for performance.

Anything over 5% may need bigger wires, which can reduce voltage drop. This requirement usually applies if your run exceeds 100 feet, given that this length results in a significant drop that will require you to increase the amp rating of the wire by 20%.

Unless you’re installing the sub panel relatively close to the service panel (i.e just 50 feet away or at 75 feet), we’ll likely have to account for that drop.

Theoretically, yes, you can still consider a 6 AWG copper or 4 AWG aluminum as the right size for 50 amp sub panel 100 feet away, as long as you confirm that your voltage drop doesn’t exceed 5%. However, that may be less than optimal.

Calculating With Voltage Drop Considered

50-amp-sub-panel-wire-size

The golden rule is: once a significant voltage drop enters the picture, adjustments are necessary.

Below are the recommendations for 50 amp service wire sizes, with a focus on 50 amp 240 volt panels, as they are more common than 120V models.

  • If the sub panel needs to be located at 150 feet distance from the main panel, then the most suitable 50 amp sub panel wire size would be 4 AWG copper or 3 AWG aluminum, but a 6 AWG copper wire size for 50 amps may still work.
  • If the sub panel needs to power appliances and power tools 200 feet away from the service panel, it’s also advisable to use 4 AWG copper or 3 AWG aluminum
  • On the off-chance that your panels are rated at 110V/120V, the right wire size for 50 amp sub panel should be bumped up to 2 AWG copper or 1/0 AWG aluminum for 200 ft. The sizes for 100 ft will be 4 AWG copper and 3 AWG aluminum.

If you’re wondering how these recommendations are made, you can get technical by calculating for resistance per 1,000 feet of each wire. Let’s take the 110V/120V sub panel as an example.

We first need to get the maximum allowable resistance with regard to the panel’s voltage rating, amp rating, and voltage drop. For that, we get the following equation:

Maximum Allowable Resistance = 5% x 110 = 5.5; 5.5 / 50A = 0.11 ohms

  • The resistance of a 2-gauge copper wire is 160 per 1,000 feet. With a 200-foot distance, we’re going to need double that, we’ll be working with exactly 400 feet of wire.

0.160 x 0.40 = 0.064 ohms

That doesn’t go over the maximum allowable resistance, so it’s a great option.

  • But if we use a 4-gauge copper wire instead, which has 0.253 ohms of resistance per 1,000 feet, then we arrive at:

0.253 x 0.40 = 0.1012 ohms

Already too close for comfort, huh? Hence, we play it safe (i.e. keep voltage drop low) by upgrading to the next size!

You can use the same formula for the other wire size recommendations made here if you want to confirm.

What Size Wire Do I Need for 50 Amp at 110-240v?

50-amp-wire-size

Voltage has no bearing on wire size. It only determines its insulation and thickness, so it doesn’t matter whether it’s 110V or 240V when figuring out the correct wire size. This is especially true when we don’t account for distance and possible voltage drop.

Normally, a 6 AWG copper or 4 AWG aluminum will do for this amp rating, but you can’t take voltage drop out of the equation, especially if it’s a sub panel that’s situated a bit far (100 feet or more) from the service panel.

In such cases, it’s better to use a wire that’s one or two sizes larger.

What Wire Do I Use for a 50 Amp Sub Panel?

For this type of sub-panel, the 50 amp wire size would be 6 AWG or 4 AWG for copper and aluminum, respectively. This sizing is confirmed by NEC wire charts.

Will 8 Gauge Wire Handle 50 Amps?

Theoretically, it can. But this amount of current will likely heat the wire up to 75 degrees Celsius.

It’s also an impractical setup that results in 10% power loss, based on calculations, so don’t be surprised if plenty of electricians don’t use 8 AWG wires for 50 amps. Keep in mind that 8-gauge is rated at 40 amps only.

Conclusion

I hope I’ve given you a satisfactory answer to “What size wire for a 50 amp sub panel?” When in doubt, look at your local building codes first, then use the answers I’ve provided here to confirm your choices.

These are the safest answers, simply because they’re based on the latest National Electric Code guidelines and how most electricians do their calculations when sizing sub panel wires.

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