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I am doing some wiring from a subpanel. The main panel is sending power via 3-way wiring (3 plus a ground) to the subpanel. This subpanel is intended for both 220 and 110 volt add-ons. I have a 110 line from the subpanel via a 15-amp breaker switch. It is a very short wire from the subpanel--maybe 3 feet away to an electrical outlet.

I have thus far installed (1) a small radio to test the circuit; it ran fine for 10 minutes, then burned out and started smoking. Then, (2) I tried a lamp to test the circuit; the bulb lit up and immediately blew.

Is there too much voltage/amperage going through this circuit? How? Shouldnt the 15-amp breaker switch keep this from happening?

Roger Roots
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5 Answers5

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Is there too much voltage/amperage going through this circuit?

There is most likely too much voltage, as ArchonOSX commented.

How?

You miswired it and got 240V. You can check this using a voltage tester.

Shouldnt the 15-amp breaker switch keep this from happening?

No, the job of the breaker is not to block too much voltage but to disconnect the circuit when too much current is flowing. This is mostly to protect the wiring in the wall from overheating and setting fire to your home. It won't protect a 100W incandescent bulb or anything similar. Those normally draw less than 1 Amp - far below the 15A breaker rating.

In the case of overvoltage, a higher current may briefly flow through your appliance/lamp but the appliance dies before the current reaches the breaker's limit.

I am doing some wiring from a subpanel

Sorry to be a killjoy but I suggest you don't. At least until you have gained a higher level of understanding of household electricity. Since the higher voltage can fairly easily kill you, a family member or a visitor, you might want to pay an electrician to fix this for you properly and then consider taking some evening classes in the subject (if available in your part of the world).

Useful links

RedGrittyBrick
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You may also have a problem with your neutral wire. In any sort of split-phase 120/240V wiring, if the neutral is loose or poorly connected, it can cause the two 120V sides to have wild voltages between 0 and 240V. The dead giveaway is that they will still add up to 240V, so one will be higher than 120 and the other lower.

So if your careful rechecking reveals that you did not mix up any wires, take a hard look at the neutral wire connectioms.

Harper - Reinstate Monica
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Only expanding on the breaker issue.

The breaker/fuse is connected in series and is designed to break when current through it reaches the specified value. Period.

Breakers use the relationship between current in the coil and intensity of the magnetic field it generates; fuses use the relationship between current and power dissipation.

If we know what the mains voltage is, we can estimate what power must be consumed in the circuit to cause the breaker/fuse to open. What power is needed to burn the fuse is a completely different story.

If you have a 15A fuse protecting 110V mains the faulty device will consume 1,65 kW and I think the fuse itself will draw around 10 W to heat and burn.

If you use the very same fuse to protect 12V mains in your car the faulty device must consume 180 W to break the fuse.

If you have accidentally used 220V mains instead of 110V the device must consume 3,3kW to burn the fuse!

In your case, say you have used 100W/110V lamp. In normal opperation it draws 0,9 A. If you connect it to 220V mains it dissipates 400 W. If I use a 50% safety margin in the lamp design it will withstand a 150 W load which is way too small to sustain the accidental load. The current of 1.8 A is way too small to trigger the 15A fuse...

To your question: I will repeat what has been already suggested. Get this work done by qualified electrician. Do not do it yourself, unless you are damn sure what you are doing. And get yourself a multimeter.

psmears
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Crowley
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In an electrical circuit, voltage is "pushed" through the lines and current is "pulled" by any device drawing power from the lines. This means that at your wall outlet you are providing a constant 120 volts (push) and no amps (pull), assuming nothing is connected. If you connected a 120-watt bulb the break-down is that you are "pushing" 120 volts (constant) and you are "pulling" 1 amp. The pull is determined by the resistance of the device drawing the power. The equation to determine amps drawn is:

v=I*r (where v=volts, I=amps, r=resistance (ohms))

therefore,

I = v / r

Just to add, the maybe not so obvious, is that you cannot force more amps into the connected devices. You cannot control the amps at all, actually (from a wall outlet). The amps "pulled" are purely determined by the devices internal resistance.

To answer your question it doesn't sound like your breaker is the issue. I think the other responders got it right by suggesting your voltage is high.

twegner
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As others have noted, breaker is for amps rather than volts, and volts is your problem.

You should have a multi meter set to ac voltage 220v+ setting in order to test. You should know what you are doing or get someone to help you who does before playing with electrical stuff because it is dangerous.

David Kay
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