Running cottage tank at 120 VAC

Discussion in 'Water Heater Forum, Tanks' started by cmh, Jun 23, 2009.

  1. cmh

    cmh New Member

    Messages:
    2
    We inherited a cottage with a small electric water heater. I still have to check the thermostat, but I believe the element requires replacement.

    I am comfortable working around household wiring, so I removed the 15 amp fuse protecting the unit, and opened the access cover. The single element is a Stillman model 140, labeled for 240V and 3000W. Our cottage does not have 240V electrical service, and so this element is connected to 120V power.

    Is it acceptable to operate these 240V elements at 120V? Should I look to replace it with a 120V element?

    And currently it is wired with 14 gauge wire and protected by a 15 amp fuse. Is this appropriate?
  2. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    The primary drawback to running a 240V element on 120V is that the tank's recovery rate roughly doubles. (The 3kW element is only dissipating ~ 1.5kW at half the rated input voltage.)

    Replacing a 3kW 240V element with a 3kW 120V element would put you near the amp limits chassis wiring for 14AWG wire, and way over the 15 amp code limit for 14AWG house wiring. If there's a 1500watt 120V element available, it'll be nearly identical to the 3kw 240V unit, and preferable for your setup.

    If not, replace it with the same element that you started with (and keep the same fuse.) In short runs 14AWG wiring can take more current, but 15 amps is still the limit for what would actually meet code. It'll be drawing ~13amps, which is already close enough- don't push your luck.
  3. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Half the voltage is 1/4th the power, assuming a constant element resistance of (240^2)/3000 = ~19 ohms.

    How many gallons is it and
    what recovery time do you need and
    what is your incoming water temp and
    what is your heater 'stat set at?
    Last edited: Jun 23, 2009
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,330
    Location:
    New England
    Running the thing on 120vac is sort of like having a lamp on a dimmer...it still lights, but not anywhere near as bright or as hot as it would be if it was up full. 15A for a WH isn't very much, so while it should work, it will take a long time to recover.
  5. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    10 gals of water (83#) heated 50F will take 415 BTU and will take 121w-h. At 750w it will take 10 minutes.
  6. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    But lets look at real world...how long would it take to heat 25 gallons 80º??
  7. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    25 gals of water (208#) heated 80F will take 16,640 BTU and will take 4876 w-h. At 750w it will take 6.5 hrs.

    Correction: (10 gals)(50F)(2.43)/750w = 1.6 hrs.
    Last edited: Jun 24, 2009
  8. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    DOH! Of course it is! (What was I thinkin'? or rather, why wasn't I thinkin'? :) )

    So @ 120VAC with the old element he's running 750W, but could comfortably (legally) tolerate up to 1500W given his wiring/fusing configuration. Anything 120V element between 1000-1500W would be a noticeable difference in recovery time compared to simply replacing the 3kW/240V element.

    But as with most electric tanks, it takes awhile. Even were he running at the fully rated 3kW it takes ~100 minutes to raise 25gallons 80F. At half power it takes ~200 minutes, etc.

    Basically it means be sure to let your girlfriend shower first... :)
  9. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,267
    Location:
    Cave Creek, Arizona
    element

    1650 watts is the maximum 120 v. element because of amperage draw limitations. If you can get it in a "low watt density" format it will last longer than the more common high watt density one.
Similar Threads: Running cottage
Forum Title Date
Water Heater Forum, Tanks Running a gas line for gas range.... May 27, 2014
Water Heater Forum, Tanks GE Hybrid water heater, Tips to get it running again? Jul 14, 2012
Water Heater Forum, Tanks Power vent tank blower constantly running Dec 29, 2011
Water Heater Forum, Tanks AO Smith FPS 50 Powervent Water Heater: Burner won't stay lit, blower keeps running Nov 26, 2011
Water Heater Forum, Tanks Hot water running out too quickly Jul 29, 2011

Share This Page