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Q&A About a Treadmill Tripping an AFCI Breaker, Upgrading a Panel and Knob and Tube Wiring

December 18, 2009

Question Bob asks:
I have a new treadmill that trips the AFCI breaker when first turned on but is fine when breaker is reset.

How do you turn this on? Do you turn this on and off by plugging it in? If so, then this is the problem. When you plug the treadmill in, you will get an initial spark. The AFCI breaker is designed to trip immediately when it senses an arc-fault. If this is not the problem, then I need more information.


Steve asks:
I am installing a new load center to replace my Zenco. Problem is that it does not have enough space for the cks I have. The new panel is a Siemens but does not indicate that I can use qt breakers. If I can use 4 20 amp qt breakers it will work. Can I do this. The breakers will fit. I cannot seem to find a tech support on the Siemens web site.

Your breaker box must indicate that it is Class CTL rated. You will typically find this information on the door of the breaker box. If the breaker box is not Class CTL rated, then you may not use the QT breakers. Using the half-sized breakers will overload the breaker box and possibly start a fire. If you are installing a new breaker box, then go get one with enough capacity and leave some space for future circuits.


Dan Rath asks:
I would like to replace a light fixture that is Knob and tube wired with, as far as I can tell, 2 black wires. The Fixture is Powered First with wires then running to the switch. Using a Volt detector I notice that one wire changes (hot/not hot) with the switch. I assume this is the (black lead) and the other would be the White. The thing I don’t understand is when the Light is off, both leads to the fixture are hot and when the light is on only the (white). I would think that it should be the other way around, that with the switch off, there should only be one hot lead. Could you advise what to do.

My first recommendation is to replace the knob and tube wiring. This type of wiring is unsafe because of the cloth insulation. All of the heating and cooling of the wires over the years will cause this insulation to break down and become brittle. You typically find this in lighting fixtures first.

Voltage detectors are OK to check if something is hot quickly, but you should not rely upon them. I recommend using a solenoid tester or multi-meter for a more accurate test. You do not have 2 hot wires in this box. I’m guessing that your voltage detector is picking up some voltage which is being induced onto the other wire. A solenoid tester or multi-meter will show you there is no voltage there.

Another thing to watch out for with knob and tube wiring is switched neutrals. This will hurt you more than a hot wire. Do not rely on the switch killing the power. Go turn it off at your breaker or fuse box and verify that it is off at the light.


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One Response to “Q&A About a Treadmill Tripping an AFCI Breaker, Upgrading a Panel and Knob and Tube Wiring”

  1. Ralph Phillips on December 19th, 2009 8:29 am

    For the “I measured voltage on both black wires” guy – with a voltage detector, it IS measuring voltage on both lines when the switch is open. Remember that the incandescent filament is a length of wire, and since the voltage detector doesn’t load down the line any, you’re measuring full voltage all the way to the switch (infinite resistance yields infinite voltage by ohm’s law). As soon as you close the switch, current is pulled, voltage is dropped over the lamp filament, and the voltage on the neutral side is now down to zero. Any high enough impedance tester will lie about the voltage present on that side. Plus, a high enough wattage bulb and grabbing the return side of the lamp will let you know there’s voltage present, as you complete the circuit.