I am in climate Zone 4 (Colorado foothills) and according to my figures for a 700 square foot poorly insulated garage,

If you really spend a lot of time in there, insulate it well. Then recalculate your heater size.

I would consider keeping the garage heated to a base temp full time. Then boost the temp for the times that I was occupying it. It takes a long time to heat a near or below freezing garage up to a comfortable level.

If I had NG on the property, I'd use it, not electric.

 

I can insulate the walls easily enough, but for this upcoming winter I won't have time to finish/insulate the ceiling before I'll be wanting some heat in there. The 3rd bay of the garage is not insulated, but the rest of the garage is R30. Also, I don't always spend a TON of time in there, it just depends on what projects I have going. This winter will be fairly light occupancy.

I do have NG, the meter is right outside the garage... and know that I can get WAY more BTUs that way, but - an equivalent gas heater costs about $200 more, my winter gas bill is already more than double the electric bill, and I'd rather not have to install an exhaust vent or start unscrewing gas pipe. I'd also have to replace that circuit with a 120V since I can't find a gas heater that runs on 240. Seems like for my situation, the extra labor of installing gas and 120v on top of the $200 price difference in heater options makes electric more attractive to me.

 

Lets see if I can post a picture now that I have more than one post on the forum:

Yes! So the 240V 50A circuit in question is visible here. The red box is the desired heater location. The 200A service panel is right outside, behind the drywall. Natural Gas meter is on the ground to the left.

If I go electric, I suppose it wouldn't be a bad idea to put a sub-panel inside the garage for extra safety...

The natural gas line runs inside the garage, heads to the right along the foundation behind the car in the picture, takes a right angle (out of picture frame), and then follows the foundation/wall until it angles into the crawlspace to the furnace. Since the pipe elbow is not a tee, I would have to unscrew pipe from the corner of the garage back to the meter so that I could install a tee to supply the heater.

 

A subpanel inside the garage for the single circuit doesn't add any safety.

It doesn't even save you a walk. The breaker in the main panel will often trip before one in the subpanel on large faults.

 

I was getting the sub-panel notion from here:
http://m.ecmweb.com/code-basics/article-424-fixed-electric-space-heating

The disconnecting means must be capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means must be on the switch or circuit breaker and remain in place with or without the lock installed.
If the fixed electric space-heating equipment has supplementary overcurrent protection, the disconnecting means must be within sight from the supplementary overcurrent device. “Within sight” means the specific equipment is visible and not more than 50 feet from one to the other [Art. 100].

But then a little later is says:

You can also use the switch on the heater unit itself as a disconnect if it has a marked “off” position that is an integral part of the equipment, and the switch disconnects all ungrounded conductors of the circuit [424.19(C)] (Fig. 1).

 

Also do a load analysis (formulas at the back of the NEC) to determine whether your service, even 200 amps, is sufficient to support the heater.

By the way, the 700 sf garage, becoming living space, adds its own 3 watts per sf or 2100 more required watts to the load analysis calculation to cover lights, radio, portable tools, coffee maker, etc.

 

Using this form, I added up all my electrical loads. I used a Power Factor of 1.0 when tallying up the wattage of all the "fixed appliances" and "other loads", and included the square footage of the garage plus the new 15kW new load. I get a total "Service Load" of 165.333A

 

Interesting, is it still considered living space even if I am just working intermittently in there? I should clarify that I won't be in there all that often. Basically, if it is cold and I *IF* I feel like working on a project car over a weekend (4-8 hours a day), I will be happy as long as I can flip a switch and have warm/hot air blowing across the work area so as I won't see my breath or grow snot-cicles or have my fingers stick to frosted-over wrenches

 

The disconnecting means must be capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means must be on the switch or circuit breaker and remain in place with or without the lock installed.

You can do that on the heater breaker in the main panel. The cost of the lockout kit is usually less than $15, often less than $10, depending on mfg/model.

 

A 15kw heater draws 15 kwh per hour of use. If you use it even as little as six hours per day, that is 90kwh per day. Where I live, a kwh is about $0.25, no doubt less where you are, but my bill would be north of $20 per day to run such a heater. You are very quickly going to exceed the cost differential of buying a natural gas heater, since natural gas is much less expensive per BTU than electric.

 

Oh man, didn't realize there would be that big of a difference. Is this math right?
Assuming 120 kWh for 8 hours of heat.
My winter electric rate is $0.04604 per kWh, so about $5.50 per day.
My NG rate is $0.08444 per US Therm. If I am Googling correctly, 120 kWh = 4.1 Therms = $0.35.

Either way, the energy cost to do some occasional hobby wrenching won't break my bank, but gas is definitely much much cheaper.

 

So can I do this?

Upgrade that 240V wire and make it a 240V sub-panel instead, feeding the existing 50A 240 receptacle as well as providing a 120V breaker for a gas heater? At the moment, I don't have any more 120V ampacity left in the garage for the gas heater's blower, and I sure would like to keep that 240V receptacle for welding and whatnot.

Up front costs will be a lot more, but you guys are right, gas is the way to go - not only for energy cost savings but for guaranteed comfort. 15kWh electric is the top of the residential range and it is still barely enough output for this particular garage and climate.

 

Next issue: Gas meter flow enough? 175 CFM, it says. Currently my house has a water heater with input BTU of 34,500 and a furnace with input BTU of 117,200.


:vs_mad:

 

You should be good up to about 3-400 cfm depending on pressure. (the cfm rating is the lowest pressure level which is no longer typical) So you should be good with plenty of room to spare. (approx 3-400,000 btu/hr for 7-14" WC working pressure)

If you're really worried, be like us, we just ask our utility to upgrade. They ignore us, then show up anyways to change the meter, with the same model. Yes they are odd.



Cheers!

 

Service load of 165 amps at 240 volts given by the load calculation, then you don't need to upgrade your electrical service.

We cannot rule out the possibility you will need a larger gas service to supply the garage heater if it is a gas heater. There is probably a comparable load calculation in cubic feet per minute for gas appliances based on BTU of the various appliances.

If you did go with electricity even temporarily, you may not connect other, smaller, things to the 80 amp heater circuit even during the summer when the heater is not in use. Each tool and each appliance has a maximum branch circuit amperage which is implied by the power plug. With a single incoming feed for the garage a subpanel is needed to provide proper breaker protection for various smaller amperage branch circuits for other appliances.