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.

Quote:

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Originally Posted by nmwirez

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

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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.

Quote:

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Originally Posted by ffurlong

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

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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.