Hi cindy here
We have and old fuse panel. And we want to hook up baseboard heaters. What size fuse would we use in the box.:confused:

The fuse size depends on the amount of current the circuit is expected to draw.

To compute baseboard heater current draw, use Power's Law:

P=E x I Where P = power in WATTS (kiliwatts divided by 1000) E=voltage of the circuit, and I= current in amps.

For example, if you have 5 KW heater operating at 240 volts, the current will be:

5000Watts/240 Volts=20.8 amps

Next, the NEC requires you to multiply the current by 125%, so you need to design the circuit for 26 amps.

So, you will need No. 10 AWG wire, and a 30 amp fuse.

Note if you use 120 volt heaters, you need to design for 52 amps, which requires a 60 amp fuse and No. 6 AWG, which is quite heavier, that is why you use higher voltages when you need more power.

However:

Since you said you had a fuse panel, I am assuming you have an older house. There are a couple other things you have to consider:

1. Do you have enough power available at the service (power coming into the house from the utility) to add heaters?

2. A 240 volt circuit must be fused on BOTH sides of the line, so you need to have TWO fuse spaces available in the panel.

3. You must also provide a way to disconnect ALL sources of power to the heaters. Removing one fuse will cause the heater to stop operating, but there will still be power available from the other fuse, which is dangerous if someone tries to work on the circuit. You need to integrate a device that will separate both sources of power with one action

You must consider all these factors for a safe installation.

Thanks