How often will our RV actually pull 40A? That's worth investigating. well, it's saying that will give us 7.15% drop at 40A. So let's see: XHHW in conduit, 6 AWG aluminum (no bump): Remember a 50A breaker (like an RV) you only have to factor for your actual loads, or 40A at most. Your second number is based on voltage drop. That is our first number, the absolute minimum size we can use. THHN/XHHW wires in conduit (and certain cables): 8 AWG copper (50A 75C) or 6 AWG aluminum (ditto).Most cables: 6 AWG copper (55A 60C) or 4 AWG aluminum (55A).(literally they let you go 90C, but, the terminations at both ends are only rated 75C, so you are capped at 75C. Noting that most cable puts you in the 60C column, and wires-in-conduit (because of their THHN or XHHW insulation) let you use the 75C column. You don't need to multiply that by 125%.Īrmed with that number, you find the minimum cable size that exceeds it in Table 310.15(B)(16).

you follow the provisioning rules - for instance an RV stand of 20/30/50A requires 50A provisioned.

start with your actual load and multiply by 125%, so for instance if you have a 39A hot tub, you must multiply by 125% to get 48.75A, which rounds up to 50A.

The two calculations you must doįirst, you must determine the absolute minimum wire size, even if the cable were only 1 foot long.

It's 'legal' to put cable in conduit, but the conduit must be larger for the same current, for a couple reasons: First, most cables are only rated for 60C temperature, so you need larger cable and second, the cable sheath adds a great deal of bulk - the conduit inside diameter must be 138% of the widest part of the cable. Not least, this makes it much easier to pull the wires through, since the individual wires are quite flexible. Otherwise if you're going to use conduit, do that.īut in conduit, you use individual wires - not a cable. If you're going to direct-bury cable, do that. Definitely get a huge panel with lots of spaces, and if that means the 'main breaker' or busing is much larger than your feed, that doesn't matter. Calculating 3% on breaker trip is just throwing away money.Īnd by the way, there's a pattern where people who a) do that and b) also use copper wire without even thinking about aluminum, also c) buy too small a subpanel, and run out of spaces later. Indeed in Canada, where they require 'nanny breakers' to keep your voltage drop below 3%, they only require that based on a) actual load or if not known, b) 80%. Heck no! If your breaker is 50A, the maximum current you're even allowed to plan for is 80% of that, or 40A. They make it sound like if your breaker is 50A, you need to calculate for 50A voltage drop. But it's probably unnecessary.Įven then, the calcs lie. If you want to keep the load a secret, by all means, we'll cheerfully 'spend your money' lol. What NEC admonishes is try to avoid more than 8% cumulatively on all segments from the meter to the final load, but even that is situational to what the load actually is. And NEC even says so it's an informational note that advises 3% is an ideal to design toward if money is no object. Notice they also default to copper :)ģ% isn't that important. That's just the wire size calculators trying to upsell you on wire.

50 amp breaker wire