electrical question

[rstrosn]

i have a 100amp subpanel in my garage, power by a 30 amp breaker from my home 200amp breaker box. i bought a used electric furnace coleman evon eb15b 51,000 btu, which can be powered by a single 90 amp breaker or one 60amp and one 30 amp breaker. i got a jumper bar for the 90 if i can use it. my question is will this furnace work with this setup or is the 30amp feed from house too small of a breaker? and should i use the 60 and 30 or the 90?

[Black walnut]

First a 30 amp feed will not work to run a 90 heater. Is your garage attach or detached? A 100 amp sub is not big enough to run a 90 amp heater plus a 15amp lighting circuit. With no mention of power tools. If it were me I'd be looking to wire the heater from the main panel.

[cwsmith]

You may have a 100 amp sub-panel in your garage (I presume that's the rating you are referring to), but if it's being fed by a 30 amp breaker from the main panel, that 30 amp is all you've got and therefore any demand bigger than that is not going to work. Changing that 30 amp feed breaker to the garage to a 90 amp (or even a 100 amp) may not work either, as you'd be right at the limit with the furnace requirements with no accountability for anything else.

You didn't mention what the wire feed to the garage was, or whether it is rated at the 100 amps or better to match your panel. Also, what kind of distance between the main panel and the garage sub-panel are we talking?

I had an electrician run a 100 amp service to my garage a couple of years ago. He really didn't want to do that because he said the wire was going to cost me a lot more. He said, "Most guys just run 60 amps". I insisted, as you generally only do this kind of thing 'once', and I knew I'd eventually want to get my shop out there. So, his attitude sort of begs me to ask the question about your wiring. The point is, it isn't just about what the sub-panel might be rated for, and it makes me wonder why you'd only have a 30 amp breaker feeding it.

CWS