I have a 30 watt heater 240 Vac

What size fuse should I use?
Is there a formula?

What is the branch circuit conductor line size feeding the outlet the heater is going to be connected to?

A 30 watt heater? Or is that a typo?

In any case, to conform to NEC code for building wiring, the setting for a fuse/breaker and branch circuit wire size shall be no less than 125% of the current draw of a fixed electric heater.

What is the branch circuit conductor line size feeding the outlet the heater is going to be connected to?


The wire is 18ga small panel heater to keep out moisture. Set for 32 deg f

30 watts/240 volts X 1.25 = 0.15625 amps

My old Radio Shack catalog shows nothing less than a 1/4 amp. Should at least protect the wires.

The wire is 18ga small panel heater to keep out moisture. set for 32 deg f

Do not jury rig this heater. It must be connected with UL or CSA qualified fixture cable and connection outlet device to an NEC compliant branch circuit connector interface. A two (2) amp fuse is adequate to hold the load on 6 amp max rated 18 awg 105 deg C wiring. Fuse may blow if using fan motor of unmentioned type and size. Need specs.

I am only giving my amateur guess from a distance without seeing the components that you are using and I suggest you consult a professional to oversee your project and environment settup for safety.

Are you sure this is an 240 Vac heater? Radient? Forced air fan? A 750 watt 120Vac variable speed/heat hair dryer would do better.

Since this is for a panel heater, you may be interested in checking NFPA 79 Electrical Standard for Industrial Machinery, 2007 Edition. (http://www.nfpa.org/freecodes/free_access_agreement.asp?id=7907)