Hi,

I have a 50x20 garage which the previous owner heated with propane. I'm trying to evaluate whether I can switch to two floor forced air industrial type electric heaters, drawing about 25 amps each. The garage is supplied by #8/3 wire and is located about 75' from the house.

Any help?

Thank you.

#8 copper will easily handle 25 amps.

Since you mention TWO heaters at "about" 25 amps each, the total load will be "about" 50 amps, and the #8-3 is only rated for 50 amps maximum.

The feeder wire needs to be rated at 125% of the total actual amp load of the heaters.

A #4 copper wire feeder on a 70 amp breaker will be needed to serve both heaters running at the same time.

When sizing the circuit, find the actual nameplate data listing the actual wattage or amperage.

"About" does not cut it.

Why not just use the existing #8 wire for one side and run another wire for the other heater?
If the amp draw is 25A, you could use #10 copper for the other side.

Why not just use the existing #8 wire for one side and run another wire for the other heater?
If the amp draw is 25A, you could use #10 copper for the other side.
Because it is not legal to run more than one circuit to a detached structure.
The correct thing to do is run a larger feeder as TK suggested.
A #4cu feeder would be good for 90 or 100 amps depending on your location.

Energy saving thought for you. Since you have propane available consider a propane fired radiant tube OR panel heater.
They heat only objects for the most part just like the sun.
They are the cheapest way to heat large areas and very simple to install.

Gas Fired Infra-Red Radiant Heaters (http://www.reverberray.com/)

Ambi-Rad - Gas Fired Low Intensity Infrared Heating Systems (http://www.ambi-radusa.com/whatis.htm)

Infrared Radiant Gas Heaters by Space-Ray - Charlotte, NC USA (http://www.spaceray.com/)

A cheap way to find heaters

Radiant Unvented Commercial Gas Heater by Dayton - NR - eBay (item 160236788472 end time May-05-08 22:28:43 PDT) (http://cgi.ebay.com/Radiant-Unvented-Commercial-Gas-Heater-by-Dayton-NR_W0QQitemZ160236788472QQihZ006QQcategoryZ26218QQ ssPageNameZWDVWQQrdZ1QQcmdZViewItem)

Sunstar 45K Nat Gas Radiant Infrared Tube Garage Heater - eBay (item 140229337666 end time May-11-08 02:22:33 PDT) (http://cgi.ebay.com/Sunstar-45K-Nat-Gas-Radiant-Infrared-Tube-Garage-Heater_W0QQitemZ140229337666QQihZ004QQcategoryZ205 98QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)

Some are available vented or non vented. My shop years ago installed many thousands of feet of this type of heat and there were never complaints. We usually replaced standard ceiling blower type gas or electric heaters and the energy saving was fantastic. The occupants also liked it because it heated them more than it heated the air which kept them warm in the warehouse or shop.

The money you will spend on the electric wire,electric panel,grounding and bonding of the detached metal building if it is metal can be used for the purchase of the heaters and the install. The heaters use very little electric usually less than one amp per unit just to control the gas valve so the electric you have in the building now can be used. You will have to run a gas line to the units and the copper/black iron will cost some $$ but the use of radiant gas fired units will quickly pay back not only that cost but will save you $$ every time you use them.

I know this is not a direct answer to your question but I thought I would offer this info just in case you have not considered this energy saving option.

Good luck with your project.