I get conflicting information, one person says 8/3, another say 10/3. Just going to be used for space heater.

  • what is the price difference on two sizes of the wire?
    – jsotola
    Feb 15 '18 at 22:57
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    What's the actual nameplate current on the heater? Feb 15 '18 at 23:26
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    Well, especially, which country do you speak about? Because different countries have different electricity code and different rules to apply for the same circuit. One thing is sure, that the length should not be an issue, 40ft is a short wire.
    – yo'
    Feb 16 '18 at 0:08
  • In the US, Colorado in particular. Now I have someone suggesting to run a line to a subpanel, and then create the circuits from there. Now the question becomes how heavy a wire for a 50amp service to the subpanel.
    – Jimmy D
    Feb 16 '18 at 17:36

First, get Amps

You start by looking at the instructions on the heater. If the instructions say to use a specific breaker and maybe wire size, then that's what you do, and you're done with this section.

If the instructions don't say, your next stop is the nameplate on the heater. Either literally a nameplate, possibly behind a cover, or in the documentation. You are looking for either

  • Amperes (amps or A), a number somewhere between 20 and 32 or so. If not listed, try
  • Watts or W, expect a number in the 5000 to 7500 range. You will need to divide by 240 to get Amps (I gather this is a 240V heater). Or
  • VA, which on a resistor based heater will be almost identical to watts. Again you divide that by 240 to get Amps.

Now derate that Amps number by multiplying it by 1.25 or 125%. That is because it is a continuous load and runs the wires pretty hard.

For instance, if the heater was 30 amps, multiply that by 1.25 and get 37.5.

Now size your breaker and wires

Take your instructions or derated amps number and round UP to the next larger size of 30, 40 or 50. So 37.5 becomes 40.

That will be the size of your circuit and breaker.

  • The minimum wire size for 30A is 10 AWG.
  • The minimum wire size for 40A is 8 AWG.
  • The minimum wire size for 50A is 6 AWG copper, or 4 AWG aluminum. Don't use aluminum smaller than 4 AWG.

You can use larger wires if you really want to. There are some good reasons to do this, like the possibility of upgrading heaters later. But you must stay with the breaker size determined by the need of the heater, as we did above.

Upsizing wire for long distances

Not an issue at 40'.

Over long distances, wires can have "voltage drop". It may be desirable to limit voltage drop by using larger wire than required. First, nobody cares about drops less than 3%, and that happens at around 120' length (round trip) on most wire sizes. So below 120' you never have to worry about it. Otherwise it depends what your loads can tolerate -- resistive heaters are very tolerant of voltage drop for instance; motors not so much.

You usually don't need /3 cable for heaters

/3 cable has an extra conductor, allowing 240V and also a middle neutral, making both 120V and 240V available. Dryers and ranges need this. Water heaters, A/C and electric heaters usually do not. Therefore running /3 cable is a waste of money unless a) the heater says it requires it, or b) you expect in the future to fit something that would require it.

Some people believe that running /3 lets you carry more power. That is not true unless you are dealing in 3-phase "delta", but you won't have that in a residence.

  • Code of which country do you use to get the AWG numbers? Remember that this may matter! Also, the 3-vs-2 wires advice can be wrong in many countries. For instance here, each 240V appliance and plug (including lights!) requires a yellow-green GND wire, when the wiring is done completely new from the circuit breakers.
    – yo'
    Feb 16 '18 at 10:41
  • @yo' -- Harper's referring to North American (NEC) ampacities here. Also, NM cables in the US designate the presence of a ground separately from the conductor count, so a /3 cable will have 3 nominally current-carrying wires in it, with the presence of the ground wire indicated by a separate W/G (with ground) designation Feb 16 '18 at 12:43
  • @ThreePhaseEel Ah ok. But is electricity 220 in general in the US? I thought not, that's why I assumed the country is unknown (and probably not the US). Thanks for the information about the /3 designation, I didn't know of it. Here, the designation is including all the wires, so the standard thing is 3C and then the cross-section in mm2, so the standard thing for 30A single-phase is 3Cx6, for 40A it's 3Cx10 (assuming the worst placement conditions).
    – yo'
    Feb 16 '18 at 13:25
  • "30A" as a size, 220 colloquially when that voltage is actually fairly rare, Imperial units, casual use of AWG, casual use of /2 /3 designation -- each individually is a flag for North America or Philippines. All together, sure thing. Euro/rest of world talk differently. If he had said "to the hob" I'd have stopped dead in tracks. Feb 16 '18 at 16:09
  • @yo' You know how construction receptacles in the UK as 110V center-tap, to put each leg 55V from ground? We do the same thing with our 240V (it's actually 240V, it was in fact 220V when electricity was first aggressively marketed to the American public, but has had several bumps since then). That gives us 240V leg-leg and 120V leg-middle, which we carry as neutral. So we can have either one depending on how we punch down a circuit, or both which is what /3 is for. This system precludes 3-phase to the home (Except NYC). Feb 16 '18 at 16:25

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