What is the right wire size for a 30A 220 Circuit over 40 ft?
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.