240 vs 208 (water heater, etc)

[Richard Moore]

Had a small, 2br/1ba, all electric, condo in a 32+ unit building today. New construction, no apts occupied yet.

When I tested the dryer receptacle, I measured 208 volts instead of the expected 240. Ok...I understand that the units' regular panels would be fed from a 3 phase system (no access to the electrical room in the underground garage) but this is a first for me (or at least the first time I've measured 208 at a vacant dryer receptacle).

Here's my "problem"...When I first arrived, the dishwasher was on the end drying cycle, so someone had obviously been in the unit an hour or so before. I test the hot water temp before restarting it (I want to see for myself it doesn't leak) and only get about 101F. Hmmm...seems low. At the end of the inspection I test it again and still only 101. Thermometer is working OK and WH connections are OK. Seems to me that the "factory set" thermostats are screwed or the recovery rate sucks. I can't imagine the builder resetting the thermostats to a lower setting (this is not a retirement home).

Looking at the label of this brand new water heater I see that it is 240/208 but the wattage is reduced almost 22% when used on 208 volts. I did check the AO Smith install manual and found no special wiring for 208.

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I'm passing this back to the builder to investigate, repair as needed, and/or explain to the satisfaction of my client but, my questions...

Surely the WH thermostats should work the same at both voltages...yes?

Are there more "efficient" 208v water heaters that you would typically install when you knew the service would be derived from 3-phase?

How much slower would the recovery rate be? 22%? More?

Does this cause any "issues" for the other 240/208 appliances?

I've probably "missed" measuring 208 volts before in occupied condos. So, finally, does anyone measure voltage across the main lugs at the unit's panel? I think I'm going to start doing that when I have no other convenient outlet.

[Jim Katen]

Originally posted by Richard Moore

. . . Surely the WH thermostats should work the same at both voltages...yes?

Yes. And stop calling me Surely.

Are there more "efficient" 208v water heaters that you would typically install when you knew the service would be derived from 3-phase?

I don't know. Every residential water heater I've seen can run on single phase or three phase, but the wattage varies -- just as in your example. There are real 3-phase water heaters, but they use all three legs, not just two.

How much slower would the recovery rate be? 22%? More?

About 22%.

Does this cause any "issues" for the other 240/208 appliances?

If I remember correctly, resistance loads will see a greater drop in performance than motor loads will.

I've probably "missed" measuring 208 volts before in occupied condos. So, finally, does anyone measure voltage across the main lugs at the unit's panel? I think I'm going to start doing that when I have no other convenient outlet.

I don't.

The low water temperature that you measured probably has nothing to do with the 208 voltage.

- Jim Katen, Oregon

[Chad Fabry]

If I remember correctly, resistance loads will see a greater drop in performance than motor loads will.

Surely will correct me if I'm wrong so I'm throwing this out there:

Resistance loads depend on, uh, resistance to produce heat. When volt amps are reduced there isn't as much resistance...so less heat. The heat output dropped 22.5% with a 13.5% drop in voltage because the conductor capacity is more closely matched with the load.

Motor performance drops but not at that rate because the motor will draw more current in an attempt to spin at its design rpm. This will make the motor run hotter than it would at a higher voltage.

Reduced voltage means motors run hotter and resistance loads are cooler.

I also doubt that the reduced voltage is the culprit for low hot water temps.

Maybe there are tempering valves in the faucets? Maybe the plumber actually installed those stupid travel limiters?

[Richard Moore]

Chad and Surely,

Thanks. No tempering valves, at least at the vanity and kitchen sinks. Very basic set up with standard "cheap" Delta faucets.