Searching for tool to predict voltage loss at GFI

Discussion in 'General Electronics Chat' started by steveparrott, May 11, 2006.

  1. steveparrott

    Thread Starter Active Member

    Feb 14, 2006
    For low voltage lighting, electricians typically install one or more GFI outlets at outdoor locations near the 12V transformers. The lighting installer will design and install the system under the assumption that the GFI's will deliver the expected 120 volts.

    It's sometimes the case, however, that the 120v side drops as much as 10v under the lighting load. Is there a tool or piece of equipment that can be used to predict voltage loss at the GFI under a set amperage load?

    For example, an installer would know that his system will apply about 8 amps. He would take this tool, plug it into the GFI, dial it up to 8 amps, then measure the voltage. This would also help predict potential overloads at the breaker box.

    Side question, if you see a big voltage drop at the GFI under reasonable loads, does that mean that the GFI was installed with under-rated wire (or daisy-chained wirring instead of pigtails with GFI's in series)? If so, any way to calculate that from load/voltage drop?

    Thanks (and note, I'm not suggesting that non-electricians do any work on the 120v side, it would just be helpful for them to know when a GFI is suspect and to predict what voltage taps to use on the low voltage side).
  2. Erin G.

    Senior Member

    Mar 3, 2005
    If you're experiencing a voltage drop in a lighting circuit, then probably something is wrong with the circuit. Loose wires, (severely) undersized conductors, bad connections, and things like that. A GFI receptacle does not cause volatge drops. It merely has a CT (current transformer) in it that monitors load balance between hot and neutral, and ensures that ground is not drawing current, then trips if somethings goes wrong. Other than that, it's just one more receptacle in the branch circuit.

    Usually the lighting that you're refering to works within a voltage range, such as 110- to 130VAC, so even if you're on the low end of that, you're probably not in danger of hurting the fixtures' x-formers.

    There is an old tool called a "decade box" where you can dial up different resistances and "plug" it into a circuit, but I've only seen a decade box used on electronics troubleshooting. You would have keep increasing the resistance until a clamp-on amp meter told you that you were drawing your 8 amps, then measure the voltage to see if there is a noticeable drop.

    Two things:
    One, it would be pretty impracticle to do this on a receptabcle.
    Two, if there is a voltage drop, then there is probably something else wrong in the circuit, as mentioned above.

    hope this helps :)
  3. Gadget

    Distinguished Member

    Jan 10, 2006
    I'm fairly sure your GFI's are what we call RCDs or ELB's (residual current devices/Earth leakage breakers) The only resistance between the input and output would be in the Relay contacts, which in a good unit should be zero... so you shouldn't be getting any voltage drop.
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    You get this drop on every outlet on the circuit? (Have electrician check for bad connection at breaker box or incorrect wiring on circuit.) Or just at the second receptacle of the GFI? (Have elecrician check that GFI is correctly wired - no loose or corroded connections.) Or just at the transformer primary? (Check for loose or corroded connections.)

    You're certain you've got 120Vrms and not 110Vrms at the building in question?
  5. imported_admin

    New Member

    May 14, 2006
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  6. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    I agree totally. It would be near impossible for a small item like a low voltage lighting circuit to cause the 120VAC to drop to 110VAC. If it is then there must be a high resistance somewhere in the circuit. Try measuring the voltage at another outlet or all outlets in the house and see if it affecting all points. If it is then you have a problem close to the power/switch board. If it is only happening at the point you are plugged into then I would be looking for a bad connecting on that line.