# How to Calculate Maximum Amps for a Cable over a certain Distance?

Different calculators all seem to give me different results. And What am I supposed to put for Voltage for the standard 240 V North American Power? in NA power one wire is 240 volts relative to the second wire, but only 120 volts relative to neutral.

I have 4-4-4 Aluminum Underground Direct Burial with an Ampacity of 125 amps.

I want to use it to move standard 240 V power about 150-180 feet away.

I cannot figure out the maximum amperage.

The only time you're allowed to use 3-wire for 120/240V is on the service entrance -- that's from the utility drop, through your electric meter, to the very first disconnect switch or breaker.

Beyond that, you must assign 1 wire to safety ground. With 4-4-4, that leaves only 2 wires, so you'll either have 120V only, or 240V only (no neutral).

If you run in 240v only mode, then at wire redline (65A) you'll have 4.5% voltage drop. That's "not great, not terrible" - I wouldn't sweat it. In the US you are not required to sweat it.

In Canada, you are required to limit to 3.75% voltage drop (literally: 3% drop at 80% ampacity, but, same thing). That calls for a 55A circuit (3% happens at 44A).

If you run in 120V mode, then you'll be at the horns of a decision. You can run 65A of draw at an alarming 9% voltage drop.... or you can run proportionally less amperage at proportionally less voltage drop.

Canada rules would limit the circuit to 25A. Ouch.

• Ya, I will have to add a wire for ground. Thanks for all the information. Do you know what the ground wire would have to be? It can be a smaller gauge than the main wire, right? Commented Jul 30, 2021 at 0:13
• @Jonathon Grounds, for up to 100A #6Al should do it. For up to 200A #4Al. Commented Jul 30, 2021 at 5:11
• I have been thinking over this, because I am still confused at the theory behind this and how the power knows it is 240v when the individual separate wires are not 240 V. Are you saying: If I were to add a ground, and sent standard residential 240v power over these wires. By code I could use a total of 55A in 240V power, but only ever use a total of 25A per per hot wire in 120V mode (totaling to 50A)? Commented Jul 30, 2021 at 18:57
• @Jonathon by Canadian Code. Because voltage drop is decided by current and resistance only (VD = C x R) and is absolute: it does not know what circuit voltage is. If you have 12V voltage drop on a 120V circuit, that is 10% drop. If you have 12V voltage drop on a 240V circuit, it is only 5% drop. Commented Jul 30, 2021 at 23:44

## The limit's ampacity, not voltage drop

In your case, voltage drop is insigificant for 240V power over a mere 180' of distance, but you are limited to a 70A breaker (65A actual ampacity) on 4AWG Al by the NEC Table 310.16 ampacity chart, assuming your terminations are rated for 75°C. (The 125A number you gave for the cable is likely only for electrical utility use.)

• I think you read the copper number? It looks like it is 55-65 in the chart to me. Commented Jul 29, 2021 at 1:04
• How do we know that 180' is insignificant? These table does not list the distance I can draw these amps over, which from my understanding is an important aspect of the calcuilator. Commented Jul 29, 2021 at 1:07
• @Jonathon -- the rule of thumb is that if the distance is less than the voltage, you don't have to worry about the voltage drop. The 55A number you're seeing is for 60degC terminations, by the way Commented Jul 29, 2021 at 1:21
• "if the distance is less than the voltage" or less than half the voltage? Neat rule of thumb btw! Commented Jul 29, 2021 at 5:05
• @P2000 "if the distance is less than the voltage" -- IIRC, Harper gets the credit for introducing me to it Commented Jul 30, 2021 at 1:06