2 electrical questions

Discussion in 'Electrical Forum discussion & Blog' started by idoc4u, Jun 28, 2010.

  1. idoc4u

    idoc4u New Member

    Messages:
    83
    Location:
    Indiana
    Hi,

    I have 2 separate questions to ask, if you guys have some input, I'd appreciate the thoughts.

    1. If on a 15A breaker I have 3 - 100W lights + 12 - 65W lights + 1 water softener tranformer on the same circuit, this equals 1080W being used if they are all on at the same time not including the softener. The softener uses I believe .02 kW/h. While you should not exceed 80% of the 15A circuit (1440W) and although I don't know exactly how much draw the softener has but it seems like it would be very little, would it be a problem for any reason to have 15 fixture + the softener on the same circuit as long as the 1440W (80% rule) is not violated?

    2. Does anyone know the maximum ft sq covered by indoor electrical fireplaces and do they require a dedicated 20A circuit like electric base board heat? The big box stores sell units that cover 400 sq ft. and use 400Watts, from what I recall. The blowers seem rather weak, however. I'm considering gas instead, but thought I'd ask.

    Thanks for your input.
  2. Jim Port

    Jim Port Electrical Contractor

    Messages:
    156
    Location:
    Maryland
    For general use circuits like you describe there is no 80% rule. It is a good design idea tho.

    Can't really help on the fireplace without knowing how many watts it draws.
  3. leejosepho

    leejosepho DIY scratch-pad engineer

    1 watt per sq. ft. sounds a bit light. In-floor electrical heating is usually 12 to 15, and a typical space space heater draws over 1000 watts. But as to gas versus electricity: Electric heat is 100% efficient (no $$ floating out the chimney) and never explodes.
  4. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Start with 10 watts per square foot on electric heaters, down to 30º. Add for lots of glass, poor insulation. X2 for Zero degrees, X3 for minus 30º.

    A hair dryer uses 1200 watts, so if some mutt in hd thinks that 400 watts will heat 400 square feet......good luck.
  5. Lightwave

    Lightwave New Member

    Messages:
    98
    Location:
    Vancouver, BC
    There's no such thing as a rule of thumb for heater sizing. You must do a load calculation to estimate how much power you'll need to heat a given room.

    Heating requirements vary hugely depending on room construction characteristics in addition to square footage. For example, in the same house, I have a 120sq/ft room that needs nearly 7,000btu (2kw electric) to heat and a 260 sq/ft room that needs only 4,500btu (1.3kw). The difference is all in the insulation, glass area, orientation, and ceiling height. There's no way to go from square footage alone to heater size--you have to do a proper Manual J load calculation to take into account construction in addition to volume.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,251
    Location:
    Cave Creek, Arizona
    400 watts is less than two red infrared heat lamp bulbs. If the room was tightly sealed and insulated it MIGHT warm it up a little bit, eventually. But, a toaser oven might do just as good, or better, for a lot less money. Watch your television set. I believe the Amish are "giving their fireplaces away", just pay shipping and handling.
    Last edited: Jun 30, 2010
  7. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    The rip off is a little different than that......the fireplace is free...you have to buy the "handcrafted cabinet" which sets you back a bundle.
  8. IAMIR

    IAMIR New Member

    Messages:
    4
    Location:
    SE PA
    You are correct in saying rating a CB at 80%. NEC Sec. 384-16(c). You should not exceed 12 amps for a 15a breaker or 1440 watts. As an Electrical Estimator/Project Manager we run into this often on desighn build projects. The local jurisdictions will often look a load calculations in determining approval of permit drawings.

    The heater thing I am not familiar with so I will plead ignorance.
  9. Furd

    Furd Engineer

    Messages:
    446
    Location:
    Wet side of Washington State
    I don't know what version of the National Electrical Code you have but it sure isn't the 2008 version. There is NO section 384.16 (c) and indeed section 384 has NOTHING to do with branch circuit ocpd ratings but concerns strut-type channel raceway.

    The truth is that the NEC does NOT limit branch circuits to 80% of the ocpd rating EXCEPT for circuits supplying CONTINUOUS loads, defined as being energized for more than three hours, and some specific loads including water heating and space heating. All other branch circuits may have a connected load up to, and often exceeding, the branch circuit ocpd rating. Furthermore, since most residential equipment is cord-and-plug connected it would be utterly impossible to calculate the connected load of most residential branch circuits.


    Idoc4u, you have no problem with the circuit as you described. The actual load from the water softener is minuscule.
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