Building a wattmeter for a portable generator

Discussion in 'The Projects Forum' started by mscott, Dec 9, 2007.

  1. mscott

    Thread Starter New Member

    Dec 9, 2007
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    0
    Hello,
    I've just bought a portable 220V/5500W portable generator, and had it wired into my house panel as per the directions. It all works great, BUT I now wish I had a pair of wattmeters, to indicate the current load on each leg of the supply. (Ironically the generator manual recommends keeping both legs somewhat balanced with regard to load, but doesn't provide any gauges to help).

    The expensive (and wasteful) way of doing this would be to buy a pair of clamp-on meters and hang them off the conductors.

    What I would like to do is install a pair of wattmeters in an enclosure right at the panel. Getting 110V wattmeters with a range of 0-3000 shouldn't be that big a deal (right?). As for the sensors, a bit tricker.... An hour's worth of surfing indicates that I need things called donut-style Current Transformers (or CTs). These appear to come in an extensive variety of specs. I would have no idea which one to buy, how to wire it to the wattmeter, and what other components I would need?

    Any suggestions or advice is welcome.
    -many thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    How long are the connecting wires from the generator to the panel?
    What gauge are the wires?
    Are they stranded, or solid?

    Just for example, 4 AWG stranded wire has a resistance of 0.2533 Ohms per 1000 feet, 2 AWG stranded is 0.1594 Ohms per 1000 feet.

    Let's say your wiring is 50' long from the generator to your panel, and the electrician used 4 AWG stranded wire to connect L1 and L2 (the two 115V lines)
    0.2533 x 50/1000 = 0.012665 Ohms
    Measuring the voltage drop across the 50' of wire (each leg) will give you the current:
    E = IR (Voltage = Current x Resistance)
    For power:
    P = EI
    current:
    I = E/R

    So, let's say you measured a 0.33 volt drop across your 50' of 4 AWG stranded wire.
    I = 0.33/0.012665
    I = 26.056 Amperes
    P = 115 x 26.056
    P = 2996.44 Watts
     
  3. mscott

    Thread Starter New Member

    Dec 9, 2007
    3
    0
    Thank you for your detailed response - much appreciated. I haven't thought about E=IR since high-school (a few decades ago!). In answer to your question, 50' of 10/4 stranded.

    What I am hoping to accomplish with this project is two simple meters at my panel that show me (or the next owner of this house) the real-time load on L1 and L2 in watts (or Kwatts, or even amps). Any direction on an appropriate current transformer and meter would be great. Or, is there is another type of device to measure current that would be more applicable? What about an in-line ammeter rated to 30A for each leg? Are these even available?
    Thanks all.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Do you mean, AWG 10, 4 conductor, stranded? L1, L2, N, GND?

    If so, that's 1.018 Ohms per 1,000 feet stranded; in your case resistance for 1 wire will be 0.0509 Ohms. 25 Amperes across 0.0509 Ohms should measure 1.2725 Volts.

    How accessible is the wiring? Is it running through a conduit, buried,?

    Installing a current meter would mean adding more resistance to the existing connection, since they would go in series in the circuit. Basically, a current meter consists of a conductive bar or rod of known resistance, with either a meter or electronics measuring the voltage drop across the bar.

    In your case, you already know what the resistance of your wiring is (I just informed you ;) ) so if you can measure the voltage drop across the wire from the generator to the panel, you know what current you're pulling by using Ohm's Law.

    I don't know what's on the market offhand for such measurements. But if you could run a couple of small-gauge stranded wires run from L1 and L2 on the generator to your panel (NEMA approved for such installations, of course) then you'd have the voltage drop available right at the panel. Tweaking a circuit to drive an inexpensive milliameter wouldn't be very difficult at that point.

    But, is that approach even viable? Depends on how the wiring was run.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Oh, Mouser Electronics has quite a few panel meters in their online catalog; they stock well over 600 types.

    Here's a page out of their catalog:
    http://www.mouser.com/catalog/632/1872.pdf
    An analog ammeter rated 0-25 Amps would probably work for you if you wanted to wire one in series per hot lead - they're around $75/each. Simpsons are pricey, but they're fine equipment.
     
  6. JamCarJohn

    New Member

    Dec 16, 2007
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    0
    Since the two legs are grounded through neutral, and the neutral will carry any imbalance back to ground, you can put a voltmeter from the neutral to ground and measure the difference on the two lines. You will need to experiment with the loads to see the effect each one has.
     
  7. JamCarJohn

    New Member

    Dec 16, 2007
    3
    0
    At any rate, two cheap clamp-on amp meters are a quick and easy fix.
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Why not just buy one single clamp-on ammeter, and sequentially check both legs? Pencils run pennies apiece and scraps of paper come in the mail for free. The ammeter could also be used for other tasks.
     
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