How best to wire a 220V Heater

[rabler]

Yeowser, I am so glad I asked.
I'll tear down the unit (once I get a bench cleared off) and get some picts and take some measurements.

Thanks all for responding.

Dan,
Since there are several options, I think the other important question is what do you want to achieve? Are you trying to replace the existing electrical heat? Add another heater? If you're trying to add another heater, what is the breaker size and wire size for the supporting circuit? Or are you looking to add a circuit?

 

[stupoty]

No, the amps would be reduced. Ohms Law. E = I x R. (E [volts], I[amperes], R[ohms]) The resistance is fixed, at say 7 ohms.

Solving for I, we have I = E/R = 120V/7ohms = 17A. Power = E x I, or E^2/R or I^2 x R. P = 120V x 17A = 2040 watts. (2.04 KW). So you will get less heat running at 120 volts than 220. But the current will go down as compared to the 220V case.

I think he (RJ Sakowski) means the motor amps not the heating element. Hence running the motor at the high voltage and heating elements at lower.

I believe a simplified explanation would be that it's trying to draw the same power and to do this the amps go up , which can happen due to being an inductive load.

(I feel I may have over simplified or be slightly inaccurate )

Stu

 

[middle.road]

Dan,
Since there are several options, I think the other important question is what do you want to achieve? Are you trying to replace the existing electrical heat? Add another heater? If you're trying to add another heater, what is the breaker size and wire size for the supporting circuit? Or are you looking to add a circuit?

I was thinking of just having 'space' heating when I'm out in the garage and am not going to try to heat the entire shop.
There's no insulation in the walls. Ceiling yes, walls nada. I've been using a propane heater this season, but with the cost of propane. . .

Shop sub-panel is a kluge. I should have re-wired the entire garage and insulated it when we were moving in, however there were pressing issues with the house itself that I had to remedy.
It's a detached garage, that was supplied by an underground 10ga Romex to a (8) slot sub-panel, until we dug up the septic several years ago.
The Romex was running out of the basement, under a concrete patio then 10' to the garage.
When that got messed up, I replaced it with a run of 8ga SO under the attached covered porch deck to the garage.
30A at the main panel in the basement out to the sub panel's 30A cut-off breaker, and from there I run 15A or 20A breakers.
The lathe and the ceiling heater are on the same 20A circuit, so I make sure never to run both at the same time.
(7) circuits total currently, (2) 220V, (5) 110V. 24' x 24' garage.

This is the ceiling heater I have up currently:

It's not really a good source since it's mounted up on the 8' ceiling, doesn't get down to 'knee' level.

 

[rabler]

Any reason you don't just put the other heater on the floor?

That heater is drawing just a bit over 20A, so it is actually quite a bit more load than you should technically be running on a 20A circuit. Breakers aren't always exact, but I'm surprise you haven't tripped your 20A breaker running that. It really should have a 30A circuit. Anyhow, you're not going to be able to get MORE electrical heat than that (without rewiring), so all that is left is getting it where you want it.

 

[middle.road]

Any reason you don't just put the other heater on the floor?

That heater is drawing just a bit over 20A, so it is actually quite a bit more load than you should technically be running on a 20A circuit. Breakers aren't always exact, but I'm surprise you haven't tripped your 20A breaker running that. It really should have a 30A circuit. Anyhow, you're not going to be able to get MORE electrical heat than that (without rewiring), so all that is left is getting it where you want it.

No not really. I put in the NEMA 6-15 receptacle above the sub-panel with leftover 8ga close to the ceiling and then mounted the TPI to a rafter.
I was keeping it short, thinking along the lines of a clothes dryer
I really had not thought about taking it down and putting a longer pigtail on it until yesterday when the neighbor gave me the other.
I just ran out to the shop and checked the circuit. The heater is on two 30A breakers. That can't be good. 30-30-30?
I do believe that I need to re-work the sub-panel.

 

[rabler]

The heater is on two 30A breakers. That can't be good. 30-30-30?

Not ideal and may not be to code, but I’d be more worried that I don’t think SO cable is intended to be used as a feeder for a sub-panel.

 

[RJSakowski]

To clarify, if the resistance is 7.7 ohms, it would draw 31.2 amps at 240 volts. Running on 120 volts, the current draw would be 15.6 amps. The resultant power would be 120 volts x 15.6 amps = 1870 watts. The fan motor would have to be run on 240 volts.

This calculation is made with no knowledge of the internal configuration. If it is indeed wired for either single or three phase operation, in all likelihood, there are three separate heating elements and one or two of them could be disconnected to reduce the current draw with a corresponding decrease in heat output.

One consideration is that there is a significant increase in resistance as the heating element heats up so the current draw at 120 volts will be more the half that at 240 volts. The actual draw will be dependent on thermal properties of the heater and the actual alloy used for the heating elements. Nichrome is a commonly used alloy for heating elements.

 

[middle.road]

Not ideal and may not be to code, but I’d be more worried that I don’t think SO cable is intended to be used as a feeder for a sub-panel.

No probably not. I believe it is to only be used for temporary feeds. This hunk came off a construction site where it provided power to the site.
It ran from the mains to a temporary outdoor panel, and also fed the trailer, which had 220V HVAC.
It came down to a matter of cost at the time. I had this on hand. 8ga at the BigBox retailer would have ran ~$260. It wasn't in the budget and I needed power in the shop to run the equipment to repair the house - Catch22. I didn't want to run a bunch of extension cords out of the house into the garage.

To clarify, if the resistance is 7.7 ohms, it would draw 31.2 amps at 240 volts. Running on 120 volts, the current draw would be 15.6 amps. The resultant power would be 120 volts x 15.6 amps = 1870 watts. The fan motor would have to be run on 240 volts.

This calculation is made with no knowledge of the internal configuration. If it is indeed wired for either single or three phase operation, in all likelihood, there are three separate heating elements and one or two of them could be disconnected to reduce the current draw with a corresponding decrease in heat output.

One consideration is that there is a significant increase in resistance as the heating element heats up so the current draw at 120 volts will be more the half that at 240 volts. The actual draw will be dependent on thermal properties of the heater and the actual alloy used for the heating elements. Nichrome is a commonly used alloy for heating elements.

I need to get this puppy opened up and see what's inside.