Assessing existing wiring?

Discussion in 'Electrical Forum discussion & Blog' started by Nate R, Oct 27, 2007.

  1. Nate R

    Nate R New Member

    Messages:
    472
    Location:
    Milwaukee, WI
    http://www.psihq.com/iread/faqvolt.htm

    Found this page today. Is there a known way to apply a 15 amp load?

    Better yet, what should I do to assess the existing wiring in my home? As I redo the kitchen and bath I'll be putting in a subpanel with many new circuits, but what about the old ones that will remain for elsewhere in the house? What kind of tests can be done, what should be looked at?

    The house was built in the early 20s, and in the early 70s went to breakers and several circuits were added by running conduit around the exterior to distribute the power to receptacles. I'm not too worried about the 70s stuff, but the 20s stuff that remains scares me. Especially with what i found stuffed into a couple boxes that I removed already.
  2. leejosepho

    leejosepho DIY scratch-pad engineer

    Same here, and I am replacing it all. Re-wiring can be difficult, but re-building after a fire would take much longer and cost a lot more.
  3. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    A quick way to apply a 12.5 amp load is to get a small portable room heater rated at 1500 watts. This would give you some idea.


    We live in a small condo, and rather than run the gas forced air unit, are content to heat our place with 1 or 2 portable oil filled radiators. Usually it is one, but on a really fierce San Diego winter night ( temp down to 35ºF) we might have 2. Last year, I noted the the plugs were warm when running. I cut off the factory plug and installed a heavy duty replacement, and also replaced the builder-grade receptacle with a commercial grade. Now, the cord, the plug, and the receptacle box, stay cool as a cucumber.

    Naturally, we have to be selective where to plug these in. I first found that a convenient receptacle in the living room was also tied to the kitchen cofee pot! That doen't work. There is a dedicated wall A/C outlet, so I use that one. In the bed/bath area, just can't use the hair dryer at the same time!
  4. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,566
    Location:
    North Carolina
    Ideal makes a tester called the Sure Shot that will give you the voltage drop on a circuit but the voltage drop is a very overrated scenario.

    The Code does not address voltage drop except as it concerns a fire pump.
    To have each receptacle in a dwelling with less that a 5% voltage drop would cost into the 10s of thousand dollars.

    If the circuit works now then why would you think there is a problem.
  5. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    Five percent of 120 Volts = 6 Volts.

    Resistance of 1000 ft of #14 copper is 3.1 Ohms, so the 2-way resistance is 6.2 Ohms, which is 0.0062 Ohms per foot.

    At the code limit of 15 Amps (more than the load on most circuits) the voltage drop will be 15 x 0.0062 = 0.093 Volts per foot.

    Cable length for 5% voltage drop = 6 Volts/0.093 = 64.5 ft.

    The same calculation for #12:
    2.0 Ohms per 1000 x 0.002 = 0.004 Ohm per foot of cable, which gives 0.08 Volt per foot at 20 Amps; so the cable length for 5% voltage drop is 6 Volts/0.08 Volts per foot = 75 ft.
  6. BrianJohn

    BrianJohn DIY Senior Member

    Messages:
    151
    Location:
    VA
    Existing wiring is tested with a High potential tester refered to as a megger (megger is actually a brand name). Ths instrument utilizes a 1000 Volts DC and gives a reading in megohms (one megohm = 1,000,000 ohns).

    Meggers are available with out put voltages from 10 VDC-10,000VDC.

    Placing current on the circuit proves nothing (unless it shorts).
  7. BrianJohn

    BrianJohn DIY Senior Member

    Messages:
    151
    Location:
    VA
  8. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

    Messages:
    2,736
    Location:
    Central Florida
    Is there a standard or recommendation for max VD?

    I may have missed where the 5% figure came from. What's noticeable and/or objectionable for various appliances and fixtures? Are steady-state and transient drops evaluated differently?
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