11:14PM | 05/03/07
Member Since: 05/03/07
2 lifetime posts
How do, folks? Please bear with me, this being my first post. If my question has already been answered somewhere that I couldn't find during my searches, please be so good as to direct me there.

OK, enough preliminaries. Here is my problem. The house which I just bought has a 200-amp service. I hope to add a 20-amp heater in the garage, and a 20-amp heater/AC in the 3-season porch. Sadly, the breaker box is maxed out (I'm exaggerating, but it seems like every outlet and light in the house has its own dedicated breaker.) There is a 20-amp circuit feeding the stove/oven. I figure I can pull this and put it in my soon-to-be-added subpanel, along with the two 20-amp heater thingies, for a total of (just a sec, let me get my calculator...) 60 amps of potential load. When it gets cold (as it is wont to do here in Iowa for about 6 months a year) both of the heaters are likely to be running together much of the time. My question is this: Will my breaker and subpanel need to be rated for at least 60 amps, or will a 50 be sufficient?


05:58AM | 05/04/07
Member Since: 04/25/05
1915 lifetime posts
going by breaker size is not usefull. Trying adding up all of the breakers in 200 amp panel and you will probably end up with 400 to 600 amps.

This is for several reasons.

You need to use the actual nameplate rated current or amperage.

Also need to know the voltage. Most likely the heater and AC unit 240 volts. but some are 120. For the AC/heater it might have two ratings, one for AC and one for heating. You need to figure the wattage for each case and then get the highest combination.


AC alone

or standalone heater, plus heater in AC.

Now if this is a gas cooktop/oven then and the 20 amp circuit only feeds those then you need to look up the data on those units. I am guessing about 5 amps. But that would be 120 volts. If this also serves some of the countertop receptacles then treat it as a full 20 amps.

But this it will only be one pole. You need two pole (240 volt) to feed the sub-panel.

If this is 240 volts feeding electric cooktop/oven then it something is drastically wrong. It needs to be at least 40 amp and more likely 50.

Then figure the wattage (for those not marked in wattage) by multipling the voltage by the current.

Then add of all of the wattages and divide by 240.

Then multiple by 1.25 (that is a "reserve" that is needed).

That will will give you the minimum szie of the sub-panel (and feeder).


12:05PM | 05/04/07
Member Since: 05/03/07
2 lifetime posts
Thanks a bunch for your reply, Billhart. Obviously, I will have to wait to install the feeder until after I have purchased the heaters.

By the way, you would be perfectly justified in considering me to be feeble-minded, for the following reasons:

1. I forgot to mention that these will all be 240-volt circuits.

2. After taking a closer look, I realized that the oven/stove breaker is actually 50 amps, not 20. That upsidedown 5 sure looked like a 2 when I first saw it.


12:10PM | 05/04/07
Member Since: 01/09/07
198 lifetime posts
One possibility is to look into getting tandem breakers to use in your main panel. These are just thinner breakers, so that a pair of them fits into a single breaker space in the panel. If you're only going to add a couple of circuits, and don't expect to do much more in the future, these might work for you. They let you avoid all the effort & expense of adding a subpanel, wiring that in, moving circuits there, etc.

Also, a 20A circuit is really small for a stove/oven. Unless it's a gas stove, and the electricity is just to power the clock and a light in the oven? Are you sure that's what it is? (On many old main panels, the descriptions for circuits are no longer accurate.)

Far a subpanel, 50A would be sufficient only if you were not running all 3 devices at full wattage at the same time. (Which isn't likely when they are heaters.) Even then, you'd be just on the edge. Most of the work (and expense) is in installing the subpanel. So if you're going to do that, why not make it large enough that you have spare capacity if it's ever needed in the future? I would go with 75A or even 100A supply into the subpanel. Won't cost much more, and gives you lots more options in the future.
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