I am wiring a 30A, 230v circuit for a 30,000 BTU ductless heat pump. There is not a wattage number on the plate of the pump, so I am unsure of what size of wire to select. I imagine that 8 gauge wire would be the safe bet, but 10 gauge would be significantly cheaper. Would 10 gauge work? Thanks

If the circuit breaker or fuse you are connecting this to is rated 30 amps then #10 AWG wire is correct. If it's 40 amp then #8 AWG...

Converting btu's to watts:
BTU to watts (W) conversion calculator (http://www.rapidtables.com/convert/power/BTU_to_Watt.htm)

Get a qualified, licensed electrician to determine the wire and breaker size.
Don't wait to call an electrician when your heat is inoperative and your house is freezing up in mid-February.

You must use the labeling on the equipment...
If the MOCP (maximum Over Current Protection) is 30 amperes then the wire
Size is #10 awg.

I am wiring a 30A, 230v circuit for a 30,000 BTU ductless heat pump. There is not a wattage number on the plate of the pump, so I am unsure of what size of wire to select. I imagine that 8 gauge wire would be the safe bet, but 10 gauge would be significantly cheaper. Would 10 gauge work? ThanksHow did you determine it requires a 30A circuit in the first place?

Mac,

There are several criteria that goes into the correct selection of a cable. It is not as simple as 30 amp circuit breaker so use a 10 AWG cable. Although it is a pretty good bet that you will need at least a 10/2 with ground cable.

You need to know the insulation type of the cable (THHN/THWN) for example. The distance of the cable run.

By NEC requirement, 10 AWG cable must be protected by a 30 amp breaker. So if this heater is designated as a continuous use, which it probably is, would mean that the cable will have to be sized for 37.5 amps which would mean you need a 8 AWG cable.

If the unit is only going to use 20 amp, then a 10 AWG conductor would be fine (25 amps).

By NEC requirement, 10 AWG cable must be protected by a 30 amp breaker.

This is a heat pump circuit, so this rule of thumb goes out the window.

It is entirely possible for #10 wire in this application to be protected by a 50A breaker.

Without knowing the specs on the unit everything is speculation.

Agreed, however, the OP is stating 30 amp. Of course without more accurate information we are stuck with what the OP gives us.

My post was speaking to the need to consider more that just the cable size.