I am planning to install a tankless hot water heater. I was doing fine until I got to the fine print. The unit I am interested in says that it is rated for 240V which is what I expected. My problem is that the manual says it works with less throughput at 220V and 208V. That makes sense-but now I am confused. What voltage DO I have (in the US)? I expect that it is 240V peak-to-peak and the power should be calculated with the RMS value, but what do the specifications of this heater mean for me?
There are two different ways that a house can be wired. The most common way by far these days is that you get a center tapped transformer coming off of a single phase. This gives you 120 volts line to neutral and 240 volts line to line.
The other way is that you would get two phases off of a three phase line. This gives you 120 volts line to neutral but only 208 volts line to line. Most of these systems have been replaced over the years, but there are still a few out there.
Appliances are generally designed to handle both systems, though your clothes dryer may take a bit longer to dry clothes on a 208 system and your hot water tank won’t heat as quickly.
I’m not sure that 120 volts is standard all across the US as well. Some places might be 110 or 115. 120 isn’t even guarnateed to be 120. It’s usually 120 +/- 10 percent, which is anywhere from 108 to 132.
And the voltages in the US are always RMS, never peak. And peak to peak voltage is really fictitious since the voltage between the lines never exceeds the peak voltage.
Thanks!
Am I correct in assuming that the decision to use a center-tap transformer and provide one phase=240v versus 2 phases=208V is made by the power company and what transformer/wiring scheme they use? This isn’t something that can be determined by the individual house?
You are correct.
Why not call your local electric power distributor for the information concerning the voltages supplied to your residence.
Since this appears to be a DIY job you already KNOW HOW TO SAFELY open the fuse/breaker panel and measure the voltages to ground of each line and the line to live voltage.
There is that and there is also the fact that the heater is going to be hooked up and it will be hooked up to whatever arrangement the utility provides. You will take whatever you get from that arrangement, whether or not you know what it is.
It is definitely not a DIY job! All I am doing is making the decision whether to proceed. I live in Katrinaland and one does not get a lot of time with any skilled worker here. I finally got an electrician to stop by my house on his way from one job to the other and he looked at the unit in it’s shipping box, looked at my panel, told me how much to install, and was gone. I am just trying to figure out what is involved in the process so that I can judge whether I have made the correct choice. I have plenty of time-it will be weeks before he can get back to me.
True, but an understanding of the arrangement is critical in selecting the unit. If my house only has 208V, that means there is 25% less hot water available. So I don’t want to make a mistake and over/under size the unit.
OK, I see what your are after now. In that case I would follow springears advice and call the utility or better yet go down and talk to one of their technical people. Chances are you have 240/120 but it’s better to make sure.
rbroome:
Buy a $10 DVM and measure the hot-to-hot voltage at your dryer or range outlet. And then get back with us.
Here, you cannot legally install such a system without the help of a licensed electrician or an apprentice under the supervision of one.
The OP aalready said it’s not a do-it-yourself in post #8.
Different municipalities have different practices when it comes to this. Sometimes the homeowner can sign-off on the work that they do and have the inspector come through and verify that all is OK. It varies from city to city, but in general you can’t perform your own electrical work.
Although I get to, nya, nya! 