What is the wire size for a 30 amp line at 300 feet?

In my article below, I will tell you what size wire you need to operate an electrical appliance that draws up to 30 amps of current and is located 300 feet from the main power source.

I hate to say "it depends", but you need to consider power and voltage drop.

In general, however, 30A service at 300 feet required usING wire No. 6 AWG, but if the actual continuous current is below 24A, you can use #8 AWG wire.

This article explains how I solved this using calculations. It is assumed that you will use copper wiring and normal environmental conditions.

current strength

The first thing you need to know is whether the 30A requirement is the strict or the theoretical maximum.

In other words, will the device continuously run at around 30A, or will the current typically be below 25A? Amperage is the maximum current that a conductor can carry continuously and safely without causing excessive heat. At this stage, we take into account only this factor, and not the distance.

Strict system 30A

If the instrument is to be operated at about 30 A, you will need a wire with a current of at least 37.5 A. This is in accordance with the rule (NEC 220-2) that the maximum load must not exceed 80% of the wire's rated load. I calculated it like this:

You might be tempted to use #10 AWG wire, which is rated up to 35 amps, but that's slightly less than the amperage required. To ensure safety by reducing the risk of fire, the next thicker wire must be used. This is #8 AWG wire, rated up to 50A.

Thus, to work strictly at 30 A, wire # 8 will be required, because wire # 10 will not be enough. This requirement is the same whether the mains voltage is 110-120V or 220-240V.

However, voltage affects how much power a wire can provide. A 110-120V system will be limited to 3.3-3.6kW, while a 220-240V system can deliver 6.6-7.2kW.

System up to 24 years

If the instrument will be operated at 24A or less (which is 80% of 30A), 30A wire will suffice. In this case, you can use #10 AWG wire rated up to 35A.

Distance and voltage drop

The next thing to consider after considering capacitance is voltage drop, which is voltage drop due to distance. The voltage drop for 30 A copper wire at 10 feet will be negligible (less than 3%). This means that at 300 feet you definitely need to consider voltage drop.

As a rule of thumb (NEC 310-16), for 30 amps you need to increase the current by 20% for every 100 feet away from the main power source. For a 30A mains at 300 feet, this is 60%, making distance an important factor when considering voltage drop.

Consider both cases of current strength 30A and 37.5A:

• Current 30A: 30A x 1.6 = 48A
• Current 37.5 A: 37.5 x 1.6 = 60 A

Taking voltage drop into account, the above calculations show that we will have to drastically change the choice of wire of the correct size. #10 AWG wire would not work for the first case, and #8 AWG wire would also not work for the second case.

Correct wire size

Taking into account both power and voltage drop, we can now answer the question of what wire size would be appropriate for a 30 amp cable at 300 feet:

For a strict 30A requirement at 300 feet, you must use #6 AWG wire, which can handle up to 65A. Otherwise, to cover 48A, you can use #8 AWG wire, which is good for 50A.

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