That’s a hard one to answer without necessary info. The wire size and count(2wire or 3wire) would depend on if the heaters are 120volt or 240volt to know wire count and the heaters listed amperage to know wire size. 120volt needs a two wire and 240volt needs a three wire. If under 12 amps would be 15amp breaker and use 14gauge wire, and if 12 to 16amps would be 20amp breaker and use 12gauge wire, and 16 to 24amps would be 30amp breaker with 10gauge wire.
Great question! Handy J did a great job answering it too! I would add this one additional comment. Most jurisdictions (City/County building departments) require a licensed professional do the work. (I know, sounds sort of self-serving, but it is the truth). If you were to have a fire due to a baseboard heater failure, curtains too close, bad wiring, etc., understand this, your homeowners insurance will contend it wasn’t done with a permit and according to NEC (National Electrical Code) standards and you could very well not be covered. Most insurances are pretty clear that all work needs to be done properly and inspected by the authority having jurisdiction. This is why most contractors have to have large insurance policies to cover their work and why we go through regular testing on the NEC.
The internet can be our friend for finding answers and do-it-yourself type stuff. Electrical work is not one I recommend you try your hand at. Many homeowners want to save a few dollars and think contractors prices are too high. Qualified contractors have to charge more to “Play by the rules” and keep licenses and insurance up to date. Saving a few dollars to end up losing thousands (or even life!) just isn’t worth it in the long run.
Your baseboard heaters if using 115 volts, would typically use less than 15 amps, and 12/2 wire with a ground is what you would need. However, if your heaters are using 208/230 volt power, you can still use the 12/2 wire with a ground for unit baseboard heaters starting at 275 - 375 watts, and up to 600 watts safely, before having to upgrade to 10/2.
It should be noted that baseboard heaters require a line voltage thermostat, which are notoriously always sold separately.
Jim from All Temp Refrigeration, Inc. Santa Clara, CA.
The first thing you do is look at the wattage. You have to apply the NEC guid lines for wiring heating equipment, which is to add 25%. Then, you divide the wattage by the voltage. This answer will be your amperage. If its 20 amps or less, you use 12-2 romex. If its from 21to 30, use 10-2 romex. Remember to add up the wattage of all 3 heaters if you are going to wire them on the same circuit. I do not recommend asking unlicensed persons when it comes to baseboard heater wiring and installations.
we can used 12-2 for heater
Keep in mind circuits need to only be loaded at 80% never 100%, if you but a 15a heater don’t use 14awg wire use a 20a wire unless you have a long cable run , then you have to consider voltage drop the longer the wire to lower the voltage so the higher the amps. There are many voltage drop and wire size calculator apps out there. When in doubt ask a contractor a simple double check could save your house
Keep in mind circuits need to only be loaded at 80% never 100%, if you but a 15a heater don’t use 14awg wire use a 12awg wire unless you have a long cable run , then you have to consider voltage drop the longer the wire to lower the voltage so the higher the amps. There are many voltage drop and wire size calculator apps out there. When in doubt ask a contractor a simple double check could save your house