(206) 949-5683, Top Rated Plumber, Seattle
Results 1 to 9 of 9

Thread: Running cottage tank at 120 VAC

  1. #1

    Default Running cottage tank at 120 VAC

    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. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009
    Location
    01609
    Posts
    2,720

    Default

    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. #3
    Homeowner Thatguy's Avatar
    Join Date
    Aug 2008
    Location
    MD
    Posts
    1,460

    Default

    Quote Originally Posted by cmh View Post
    Is it acceptable to operate these 240V elements at 120V?
    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 by Thatguy; 06-23-2009 at 03:14 PM.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    Location
    New England
    Posts
    21,399

    Default

    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.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Homeowner Thatguy's Avatar
    Join Date
    Aug 2008
    Location
    MD
    Posts
    1,460

    Default

    10 gals of water (83#) heated 50F will take 415 BTU and will take 121w-h. At 750w it will take 10 minutes.

  6. #6
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    Location
    San Diego
    Posts
    9,001

    Default

    Quote Originally Posted by Thatguy View Post
    10 gals of water (83#) heated 50F will take 415 BTU and will take 121w-h. At 750w it will take 10 minutes.
    But lets look at real world...how long would it take to heat 25 gallons 80??

  7. #7
    Homeowner Thatguy's Avatar
    Join Date
    Aug 2008
    Location
    MD
    Posts
    1,460

    Default

    Quote Originally Posted by jimbo View Post
    But lets look at real world...how long would it take to heat 25 gallons 80??
    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 by Thatguy; 06-24-2009 at 09:38 AM.

  8. #8
    In the trades Dana's Avatar
    Join Date
    Jan 2009
    Location
    01609
    Posts
    2,720

    Default

    Quote Originally Posted by Thatguy View Post
    Half the voltage is 1/4th the power, assuming a constant element resistance of (240^2)/3000 = ~19 ohms.
    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. #9
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Location
    Cave Creek, Arizona
    Posts
    25,633

    Default 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.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •