What size wire and circuit breaker do I need?

You should have a 50 amp breaker for this. The requirements of the attached appliances should not exceed 80% of the rated capacity of the breaker. 40 X 80% = 32, not quite enough. 50 is the next largest.

Wire size depends upon the specific type of wire, in a conduit or not, length of the run. 8 gauge minimum (12 gauge is required if you want to heat the sauna by burning your house around it). I would never use less than 6. For actual requirements, you will need to get specific.

A few points of clarification:

First, I have not worked on many saunas, but the ones I have seen that have an electric heating source are rated for a continuous load as indicated on the ID plate. I suppose it is possible that some are not, in which case you may not be required to derate the circuit but I would still be inclined to do so as a safety factor.

Next, if you do need to apply the continuous load rule, a 45 amp breaker will work (45 x 80% = 36), and a 35 amp will do if you do not need to derate. You will not find either of these at you local Home Depot or Lowes. In some areas you may be able to find them at an electrical supply shop. In this area they aren't available anywhere unless you special order. As a DIY project, it is easier to go up to the 50 (or 40) than to waste time on a scavenger hunt.

Finally, don't overlook the main point -- there is not enough information provided to advise you. 50 amp breaker, #6 wire (#8 minimum) will give you a point to start looking at your costs and how you might put this together, but more information is required to give you a "correct" answer.

I am not an electrician (Electronics Engineering student) but I understand a little bit about basics in electricity. I am building a residential house in the Philippines with an electrical load that require 150A Main Circuit Breaker. I then researched the required wire size for the service line and the recommended wire size was #1 AWG at least. I even considered the next larger wire size 2/0 AWG safety purposes. However, when I purchased the 150A circuit breaker (SQUARE D by Schneider QOM2150VH QO 150A QOM2150HV 2 POLE 120/240V 50/60 HZ) the sticker label indicated #4 AWG [#4 AWG -300 kcmil (25-150mm2) cu/Al]. No offense to the licensed electricians/engineers but the largest wire size that this breaker lugs can take is #4 aw

If this is not a continuous load, as defined by the NEC (A load with a duration equal to or exceeding three hours without shutting off) then the next standard size circuit breaker must be used. You will find this info in NEC 240.6(A). This would be a 35 Amp breaker. Table 310.16 indicates that a # 12 AWG copper THHN conductor should work, but if you reference Section 240.4(D), you will find that you actually need to use a #10 AWG copper conductor with THHN insulation. Check with your local electrical inspector; ask if this is correct, or if they have an amendment that would require you to match the termination temperature of the circuit breaker, that may require an increase. [By the way, any electrical inspector who is designing systems is not doing his or her job – our ONLY function is to review, then approve or disapprove the plans, and then the installation – we are not allowed to design any part of the installation. All we can do is tell you what the Code requires, you have to take it from there.]

If this IS a continuous load, than you must multiply 33 amps by 1.25, gives you 41.25. We are allowed to drop the .25, but you would still require a 45 Amp breaker [240.6(A) lists this as a standard size]. Then a #8 AWG conductor is required.

All equipment grounding conductors must be sized from Table 250.122.

The last "trap" to avoid here is this one: If ANY PART of the circuit wiring for the sauna is exposed to the heat produced, you must know the temperature that it is exposed to, and derate the ampacity of the conductor accordingly (see the bottom of Table 310.16). The maximum ambient temperature that does not require derating is 86 degrees F.

All of the above has considered the installation to be Type NM Cable ("Romex"), if the wiring is installed in a raceway (conduit), you may have to do some additional derating, depending upon how many current carrying conductors you have in the raceway.

If this sounds at all confusing, then apply NEC 90.1(C); and hire a qualified licensed electrical contractor. It’s hard to enjoy a sauna from a hospital bed.

is it inside or outside? how long is the run? is it 120V or 240V ? there are a lot of important electrical codes that pertain to saunas, hot tubs and pools for safety reasons. it needs to be grounded properly, a disconnect may be required near its location, you may need to upsize the wire for long runs. that is why it should be done by a licensed and insured el and inspected. DIY could null your home insurance policy. minimum 8 AWG ; 40 amp GFCI breaker

edit; yes the 80% rule, 6 AWG copper wire 50 amp GFCI breaker

dmja45 is wrong on size wire

you need #8 wire and a 45 or 40 amp breaker.

8 wire and 40 amp breaker

DMJA-----NOOOOOO.

12 gauge wire is only rated for 20 amps.

He needs 6 gauge wire. 6/3 romex and a double-pole,

50 amp, GFCI circuit breaker.

8 awg w/ 40 amp breaker. 80% does not apply to a sauna as it is not a continuous load

...NEC2005 section 240.4(d) "overcurrent protection shall not exeed...30 amperes for 10AWG copper"

10AWG on a 35 amp breaker is violation of the the NEC

A 50 amp breaker and 12 gauge, 2 wires with ground(yellow colored covering).

In my city you can go to the planning dept at City Hall and one of the inspectors will tell you (for free- your tax dollars at work!)