Question: The photo I sent in came from the back of a microwave oven I purchased for my boat. I’ve never had a microwave on the boat, so this is a totally new installation. My plan is to install an electrical outlet right behind where the microwave will be located in my galley. Any tips for rating the circuit breaker for this new microwave? How about sizing the wire going to the electrical outlet?

Dividing 1,200 watts (1.2 kW) by 120 volts means a 10-ampere current demand.

Answer: OK, we need to take this in several steps. First of all, you are making a good choice in deciding to set this up with a dedicated electrical outlet. Microwave ovens can draw relatively high amperage, so it’s easy to overload things when doing something as simple as heating up a bowl of oatmeal and making coffee simultaneously for breakfast. The photo you sent in tells most of the story, at least as far as the microwave is concerned.

You need to know at least two of the three electrical players in this game so that you can determine the third. I’m referring to operating voltage (120 volts here), either wattage (1.2 kW here) or amperage draw. The key specifications here are ultimately amperage and voltage, as knowing these will be the determining factors in deciding what size wire to use and what size circuit breaker to use for your microwave power outlet.  So, with 120 volts known, we can find the amperage draw by dividing volts into wattage, in this case 1,200 (1.2 kW). Dividing 1,200 by 120 equals 10 amps of electrical current to run the microwave. I wouldn’t use a 10-amp rated circuit breaker, however, because you may find yourself using the second outlet in your new duplex receptacle for something else in your galley. Just be careful that it is not another high current draw appliance (like the coffee maker). Limit the current draw to 15 amps for the receptacle. Use a 15-amp circuit breaker at your AC panel board for the receptacle. As for the wire size, ABYC standards talk about sizing a circuit breaker at no more than the “ampacity” of the conductor it is protecting. The wiring you use will be marine-grade stranded copper boat cable in triplex (three wires in one jacketed piece). Based on tables found within ABYC Standard E-11, using cable with a 105ºC thermal temperature rating would dictate that the minimum wire gauge size you would use for this application will be 14 AWG to ensure correct ampacity for up to 15 amps. The actual theoretical ampacity if you use 14 AWG will be 24.5 amps outside engine room spaces. You could use 16 AWG outside engine room spaces, but in general most technicians will keep the size for AC wiring to a 14-gauge minimum. The ABYC minimum is 16 AWG based on old USCG minimum requirements.

So, use a 15-amp breaker and 14 AWG triplex boat cable for your new microwave installation.