Measuring AC draw

Discussion in 'The Projects Forum' started by greenhorn1, Dec 22, 2011.

  1. greenhorn1

    Thread Starter New Member

    Dec 22, 2011
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    Hi All! A newbie here, so apologies if this is the wrong forum for this question.

    Looking at installing a whole house generator. Getting wildly confusing/contradicting info from a number of contractors, so I'd like to check one big item for myself.

    I have a heat pump (outside) and an air handler (inside) for my heating system.

    Is there a way, with a clamp meter for instance, I can simply clamp over the large diameter power feeds for these units (240v) to measure what they’re drawing in amperage at startup and steady state? If so, any recommendations on a meter? Thanks much for all thoughts!
     
  2. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    a good ol Fluke clamp on will work for running draw, but start will be harder to pin down. Not that this will do you any good, as the code deals with motors, with special care given to hermetics. What you want/need is your nameplate data, even if you have to go back to the manufacturer.
     
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
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    You probably know but make sure you clamp on meter is clamped over a single wire not over the whole 2 or 3-wire cable.

    I use to use a Fluke (not sure the model) with a peak detector feature that would capture the startup current better than a standard DVM. It has to have a short response time to give you an decent result.
     
  4. greenhorn1

    Thread Starter New Member

    Dec 22, 2011
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    So the nameplate says I have an RLA of 16.63 - am I correct in calculating the running watts as 230V x 16.63RLA = 3,825 watts? I've had calculations of up to 11,800 watts from electricians. The installer says 6600 watts. What am I missing? (Besides a basic knowledge of electricity? ;-) )

    And is there info on the nameplate that would give me the starting amperage? (versus the running or steady state?)

    Thanks again!
     
  5. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
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    Does your heat pump system have auxiliary electric heat? most do, but it's an option. That could be where your various figures are coming from.
    And , as stated before, you must clamp on the amprobe around just one conductor (not the ground) at a time.
    Also when buying a generator, look at the rating for "continous duty" not just "surge". Usually the figures for surge are written in large print and continous is in much smaller print.
     
  6. greenhorn1

    Thread Starter New Member

    Dec 22, 2011
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    The system is 3 stage - 2 stages at the compressor (outdoors) and a 3rd 'auxillary' stage, which is a heating coil, atop the air handler in the basement. I know code requires everything be calculated for 'full load' or whatever (which makes zero sense to me, as it seems to assume consumers are morons and in the event of a power failure will run around their homes cranking everything to the max) (and in the alternative, even if I were an idiot and did that, what's the downside? Doesn't the breaker on the genny just trip? And what about allowing for load shedding modules to prevent just such a situation, or prevent motor burnout on select devices, like a well pump? That makes as much sense to me as a building code that would require that every house be 4500sf because, hey, who knows, you might have 6 kids someday!).

    The way I run the system is down at night (to 60f) and then in the AM, turn it up in 2 degree increments so the system never comes off stage 1. Sure, if I cranked it to 70f 1st thing, the system would interpret that as a huge call for heat and light off all 3 stages, but who can afford that electric bill? :)
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,993
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    The RLA (Rated Load Amps) is the maximum current the device draws so your power calculation is correct. Under normal operating conditions it should take less than that.
     
  8. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    I'm speaking from the CEC, which is close to the NEC. Hermetic motors have circuit calculations based on the Locked rotor current, which is often included in nameplate. Alternatively, code calculates it at 600% of FLA, or full load amperage. Fusing is a maximum of 50% of that value. Given that your circuit is fused to 300% of FLA, you could expect a draw that could calculate to the 11kw region. The problem you'll be faced in supply power from a generator is in voltage dip. You need to maintain sufficient voltage at current draw to prevent stall.
     
  9. greenhorn1

    Thread Starter New Member

    Dec 22, 2011
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    So the LRA is listed as 82 on the nameplate, so 82LRA x 230v = 18,860 watts!

    But what does this really have to do with anything?, If I understand LRA correctly, this will be the max draw if the compressor is broken and tries to start. 1) If it's broken, it can't start anyway 2) It's on a 40 amp breaker, so I assume at a 41amp draw, it's moot, i.e., the breaker trips.

    So in my tiny little brain, the *worst* case is 40amp x 230v = 9,200 watts before the breaker trips. So why would I need a generator any bigger than 10,000 watts, for the sake of discussion?
     
  10. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    it really only applies to code.

    for sake of discussion;
    what you want to consider is that if your supply dips during startup, you'll have an extended start condition, which may trip your breaker. Will a 10kw gen start it?, your equipment supplier/installer would be your best reference.
     
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