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Convert 240 volt outlet to 120

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#1 IronMike

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Posted June 11 2005 - 2:06 PM

Looking to move from a 240 volt supply to 120 on an outlet that powers a thru-the-wall A/C unit, hoping this will allow me more choices in replacement units.
What considerations do I have to keep in mind here?
Wire size OK going this direction? breakers? Don't bother?


#2 Tesla

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Posted June 11 2005 - 4:41 PM

Itis a fairly straight conversion,but you have to do a little investigative work first,and you will have to change the breaker and the receptacle.
The first step is to identify the correct 220v breaker panel in your panel,shut it off,and to be safe shut off your main, and remove the trim cover from the panel.
When you look at the breaker it will have a current value on it probably 20 amps.This will also help determine the wire size as a 20 amp breaker will be fed with #12 wire.
This is where it gets a little tricky, in most cases the 220v breaker will have one black and one white wire attached to each pole.
If this is the case remove both wires from the breaker,and then remove the breaker from the panel. The white wire will the be relocated to the bar with all the other white wires in the panel.The black wire will be installed on a new single pole breaker of the correct amperage.
THe 220volt receptacle must also be changed to a 110volt receptacle.
After you turn the main breaker on test the receptacle for proper voltage and polarity.

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#3 Danno

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Posted June 11 2005 - 8:25 PM

If you are using a thru the wall A.C I would leave the 220 there. A 110 powered unit won't cool a house. There is a reason the recepticle is 220.

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#4 IronMike

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Posted June 11 2005 - 9:08 PM

Thanks to both of you for info. Danno, I'm not sure why it's 220 volt in the first place - I'm pretty sure it was installed by previous owner, not builder. It's a thru the wall in the master bedroom - 20x20, gets fairly warm in there. Existing unit is 10,000 BTU - probably worked great in it's 20 years ago day. I don't think it was meant to cool the house, just this large bedroom, and it seems there are PLENTY of 115volt 10k+ units available.
Is the cooling effeciency/capacity of a 10,000 BTU 115 powered unit not as good as the same BTU rated unit but 220 volt powered? If so, why?
There's also lots of Energy Star rated units in 115volt, but not so many in 220volt, another reason I wanted to convert.


#5 Charleston

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Posted June 11 2005 - 10:50 PM

The 110 volt unit will cost you a fortune to run!!
Leave the 220 volt service in place and get a modern higher SEER AC unit that runs on 220 volts.
The 220 was put there for a reason!

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#6 IronMike

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Posted June 12 2005 - 9:46 AM

Interesting, thanks for the input.


#7 Ed J

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Posted June 12 2005 - 12:16 PM

I'd stick with the 240 volt unit as well. A modern unit with a EER of 10 will consume the same amount of power whether its' 120 or 240 volts though. It will also deliver the same 10K BTUH at either voltage, BTU's are BTU's either way .
EER or SEER numbers are ratio's of work performed vs. energy consumed.
If the unit you have / had was 10, 000 BTUH and it consumed a total of 2000 watts of power it would have an Energy Efficiency Ratio of 5. 10000/2000 = 5.
A more modern unit would have an EER number closer to 9 or 10, so if it's that 10,000 BTUH unit again and it's now only consuming 1000 watts it archives an EER of 10 (10000/1000 = 10).
Double the voltage means half the amperage but = the same in KW's at the meter and out of yer pocket.
But I'd stay with the 240 volt unit as no need to change anything and with this unit drawing half the amperage at start up, there is much less a likelihood of the lights dimming in the house at start up.

#8 BobT

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Posted June 14 2005 - 10:15 PM

Ed, That is a great explaination. Are the motors (fan, compressor) built a little bit beefier on the 240 volt models as well? improving unit lifetime?

#9 Ed J

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Posted June 15 2005 - 8:35 AM


Originally posted by BobT:
Ed, That is a great explaination. Are the motors (fan, compressor) built a little bit beefier on the 240 volt models as well? improving unit lifetime?

Not that I am aware of. The only advantages I can see in this case are what i listed above.
If this was my house and I couldn't find a high efficiency in 240 volt or more importantly, a unit that will fit the hole, I'd be going 120 in a heartbeat .

#10 rguttenb

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Posted April 21 2009 - 10:10 AM

Generally speaking, a 240V device will be more efficient than a 120V device. There are several things that contribute to the increase in efficiency (one of which is the design of the motor itself), but I believe the biggest factor is simply the lower current draw of the 240V device for the same power output. It's current not voltage that develops heat in electrical wiring. For the same power output, the 120V device pulls twice the current as the 240V device. Whereas this does not double the heat created in the wiring, it does increase it and results in a lower net efficiency.

#11 fishn808

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Posted April 21 2009 - 2:53 PM

If you do decide to change the outlet to 120V, be careful. The 240V outlet could be either a 2-wire with ground, or 3-wire with ground or an outlet with no ground (not good). 20-ampere outlets need #12 wire, 15-amp outlets need #14 wire, minimum (including the ground). If you change from a 240V, 3-wire with ground outlet to a 120V, 2-wire with ground outlet, you will have an extra wire that you have to tape off in your outlet box and panel. White should almost always be neutral; green is always ground.

10,000 BTUH A/C units are a lot easier to find in 120V compared to 240V. 10,000 BTUH seems a little small for a 400 sq. ft. room, especially if you like it cold.

Ed J is right - before you do any wiring changes, it might be easier to find an A/C unit that best fits the opening. If the new unit is 240V, great - no electrical work to do.

#12 JAL

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Posted April 21 2009 - 3:49 PM

Correct me if I am wrong, but don't most motors run cooler when drawing less amperage, and heat is the number one enemy to efficiency and longevity.

I've always heard it's better to run at 240 v and half the amps versus wiring for 120 v (twice the amps), even though in theory both consume the same amount of watts.

#13 Ed J

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Posted April 21 2009 - 4:11 PM

E = I x R
P = I x E
^ ohms law, half the amps doesn't mean half the heat, just twice the pressure.
Power consumption is power consumption. The running cooler is an old wife's tail. Efficiency of a motor has to do with how the motor is made, not the pressure (voltage) that feeds it.
If you want more exact info, an inductive motor is less efficient than a capacitive motor and a 3 phase motor beats them both. But this still has nothing to do with voltage.
Voltages are about the size of the wires we run, nothing more.

#14 Ben Lippen

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Posted April 21 2009 - 4:34 PM

Air conditioners... pfft

A good Flux Capacitor beats them all
What's cooler than that Mike? SRSLY?

#15 Wayne Tj

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Posted April 21 2009 - 5:30 PM

You need at least a 15,000 BTU unit to cool a 20'x20' room - maybe more if you have a southern exposure with windows. I too would keep the 240 volt service and buy accordingly.

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