# measuring 220 volt power outlets

#### showtime

Joined Jun 2, 2005
3
how come when i measure each leg of a 220 volt outlet i see a a reading of 117 for example but when i measure both hots together i only see a reading of 209. is it not supposed to read exactly double when measurin both legs compared to measuring each leg on its own?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

That is kind of low between the hot wires. If you think about it, reading each hot to common coming up 117 VAC should mean that hot to hot would be 234 VAC. It's almost as if you have two legs of three-phase power. If you're feeling brave, take the cover off your breaker panel and measure the busses. If you get the same result, you're pretty well stuck with it. Be careful, though.

#### Bruce

Joined Jun 2, 2005
2
Originally posted by showtime@Jun 2 2005, 02:00 PM
how come when i measure each leg of a 220 volt outlet i see a a reading of 117 for example but when i measure both hots together i only see a reading of 209. is it not supposed to read exactly double when measurin both legs compared to measuring each leg on its own?
[post=8164]Quoted post[/post]​
hey dude,

your two lines in theory are 180deg out of phase, which means when you inspect them using an oscilloscope, one is opposite in phase with the other. probing them together should give you exactly the sum of each line. in reality, it is not exactly 180deg difference, there is some error in the phase difference, and that explains why you are not getting 117x2. to further explain it, probing those two lines together is like adding up algebraically each point in each signal at the very same time instance. thus, if two signal are exactly opposite in phase having the same amplitude, that should make it exactly double when you measure it together. the error in the phase difference however would make it a smaller value.

#### showtime

Joined Jun 2, 2005
3
Originally posted by Bruce@Jun 2 2005, 10:15 PM
hey dude,

your two lines in theory are 180deg out of phase, which means when you inspect them using an oscilloscope, one is opposite in phase with the other. probing them together should give you exactly the sum of each line. in reality, it is not exactly 180deg difference, there is some error in the phase difference, and that explains why you are not getting 117x2. to further explain it, probing those two lines together is like adding up algebraically each point in each signal at the very same time instance. thus, if two signal are exactly opposite in phase having the same amplitude, that should make it exactly double when you measure it together. the error in the phase difference however would make it a smaller value.
[post=8181]Quoted post[/post]​

thanx for the help
can you also tell me reasons that would make the lines not 180deg out of phase?

#### Bruce

Joined Jun 2, 2005
2
Originally posted by showtime@Jun 3 2005, 02:19 AM
thanx for the help
can you also tell me reasons that would make the lines not 180deg out of phase?
[post=8184]Quoted post[/post]​
it is the systems that generates the power and transmits it that is imperfect, but what you were seeing at your home is rather very much acceptable. ordinary applications doesn't really need a perfect 220v/110v, and the power provider wouldn't bother to give us perfect 220v because that would entail them an out-of-proportions cost. besides, i don't think it will be achievable to maintain a perfect 220v. our appliances still "process" this 220v power to fit in to its specific requirements, and generally these "imperfections" are already taken cared of by them (the appliances).

#### n9xv

Joined Jan 18, 2005
329
Not to split hairs but it really should be 240 (each leg 120-Vac). if its not 240 (or 120/buss) at the breaker panel then call the utility Co. There short changing you!

#### showtime

Joined Jun 2, 2005
3
Originally posted by n9xv@Jun 3 2005, 01:26 PM
Not to split hairs but it really should be 240 (each leg 120-Vac). if its not 240 (or 120/buss) at the breaker panel then call the utility Co. There short changing you!
[post=8190]Quoted post[/post]​
its actually not my home power im concerned about. i run audio/visual out of different venues using 220 outlets
my question about measuring the outlets is more out of curiousity into why i get different measurements

#### Erin G.

Joined Mar 3, 2005
167
The secondaries of a lot transformers are wound to put out 208 / 115VAC. It's not very common in residential: You usually find this in commercial applications. Like n9xv says, call the utility company and find out what it's actually supposed to be.

Plugging a 220VAC appliance into a 208VAC outlet wont hurt it, as long as it's not for prolonged (months) periods of time. It's only a 5% difference in nominal voltage.