Finding Common Faults in Residential Wiring

On a new house, faults come from how the work was done. In an old house, they come from age, heat and the last person who touched it. Both are hard for the same reason: the wires are in the wall, so you have to work out the problem from what you can reach. A beginner sees that a circuit is dead. A mid-level electrician works out why, in a few readings, and fixes the cause. All of this is done under a licensed electrician, and live readings are made by or under a qualified person.

What You Will Be Able to Do

• Prove your meter and pick the right test for the fault you suspect.
• Tell an open, a short, a ground fault and a crossed neutral apart.
• Find a miswired three-way without guessing.
• Measure voltage drop under load to locate a bad joint.
• Say what to do when you find knob-and-tube or other old wiring.

Warning: Readings on live 120- and 240-volt circuits are for qualified people, or for an apprentice under direct supervision, with a meter rated at least CAT III 600 V and the protective equipment your employer requires. Do everything you can with the power off and locked out.

Before You Measure

Prove the Meter

A reading of zero means nothing if the meter is bad. Use the three-step check, also called test-verify-test or live-dead-live:

1. Test the meter on a known live source.
2. Test the circuit you are checking.
3. Test the known live source again.

Skip step 3 and a blown meter fuse can tell you a live wire is dead.

Know What Each Wire Is For

Faults make more sense when you remember what each conductor does.

The neutral carries normal load current back to the source. The EGC carries current only during a fault, so its job is to trip the breaker fast. Current on an EGC during normal use means something is wrong, and that one fact explains many faults below.

Do not trust colors in an old house. A white wire may be a switch leg that nobody marked. Test it.

Name the Fault Type

Decide which type fits before you pick up the meter. Each one has a different test.

Dividing the Circuit

Every fault search works the same way. Split the circuit near the middle, and test which half has the problem. Then split that half. On a circuit with 12 boxes, you need about four splits instead of 12 box openings.

You rarely know the true order of the boxes. Use these to work it out:

• A plug-in circuit tracer finds which breaker feeds which box.
• The home run is usually the box nearest the panel.
• Cable routing often follows the room layout, with a box at each corner.

Always work with the power off when you lift wires. Mark every wire you disconnect.

Faults Common in New Work

AFCI or GFCI Trips With Nothing Plugged In

A new circuit that trips at once usually has one of three problems:

1. A neutral touching a ground, often pinched at the back of a box.
2. A neutral shared with another circuit.
3. A staple or screw through the cable.

Lock out the circuit and lift the wires at the panel. Measure neutral to ground on the cable. With the neutral lifted from the bar and all loads off, you should read open or very high resistance. A near-zero reading means the neutral touches ground somewhere. Divide the circuit to find it.

Crossed Neutrals

Two circuits run through one box, and someone splices the neutrals together. Each breaker sees a mismatch between its hot and its neutral. An AFCI or GFCI trips as soon as either load runs.

To find it:

1. Turn on circuit A only and run a load on it. If it holds, turn on circuit B as well.
2. If it trips only with both on, look at boxes both circuits pass through.
3. Turn off both, separate the neutrals and test each neutral for continuity back to its own breaker.

Tying each circuit's conductors together at rough-in stops this from happening.

A Miswired Three-Way

A three-way pair has a common screw on each switch and two travelers between them. The usual symptom is a light that works from one switch only, or works only with one switch in a certain position.

The common terminal is the one that matters. If a traveler has landed on the common, the circuit acts strangely. With power off and locked:

1. Find the common terminal on each switch, usually a darker screw.
2. Identify the supply cable at one box, and the switch leg at the other.
3. Confirm with a continuity test that the two travelers run between the switches, and land on the traveler screws at both ends.

If the light works in only two of the four switch positions, a traveler and a common have almost always been swapped. Do not start swapping wires at random. Each wrong swap can create another fault.

Multiwire Circuit on One Leg

If lights on a multiwire circuit dim or a neutral runs warm, check the two hots with the circuit live. Across the two hots you should read about 240 volts. If you read 0 volts, both hots are on the same leg, and the neutral is carrying the sum of both loads. With the power off, a qualified person moves one hot to the opposite leg.

Faults Common in Older Homes

A High-Resistance Joint

A loose back-stab or burned splice carries current poorly. At no load it reads a normal 120 volts, because no current means no voltage drop. Under load, the voltage falls and the joint heats up. A voltage reading with no load cannot find this fault.

Worked Example: Voltage Drop Under Load

A customer says a space heater makes the lamp in a bedroom dim. The outlet is on a 12 AWG circuit about 50 feet from the panel. A qualified person takes these readings at the outlet:

1. No load: 121 volts.
2. With a 12-amp heater running: 108 volts.
3. Drop: 121 - 108 = 13 volts, which is 10.7 percent.

Now compare that with what the wire should do. 12 AWG copper has about 1.9 ohms for each 1,000 feet. A 50-foot run has 100 feet of wire, out and back, so about 0.19 ohm.

1. Expected drop: 12 amps x 0.19 ohm = about 2.3 volts.
2. Actual resistance: 13 volts / 12 amps = about 1.1 ohms.

The wire accounts for less than a fifth of that. The rest is a bad joint. Turn off the circuit, open the upstream boxes and look for a discolored or loose connection. Back-stabbed receptacles are the first suspect.

Bootleg Grounds and Old Two-Wire Circuits

A three-prong receptacle on a two-wire cable has no real ground, unless someone added one. A jumper from the neutral to the ground screw, called a bootleg ground, fools a plug-in tester. It is dangerous because an open neutral puts the metal case of anything plugged in at 120 volts.

A two-wire receptacle may be replaced with a GFCI marked "No Equipment Ground." That protects people, but it does not provide a ground. Report what you find and take it to your supervisor.

Knob-and-Tube Wiring

Knob-and-tube wiring uses separate conductors on ceramic knobs and tubes, with no EGC. The insulation is often brittle, and old splices may be soldered in open air. Some of it is still in service.

If you find it during a remodel, or a customer wants to tie a new load into it:

1. Stop. Do not add a load or extend it.
2. Do not bury it in insulation. It relies on open air to shed heat, and most codes restrict this.
3. Tell your supervisor, and show the customer what you found.
4. Price a rewire of the affected circuits.

Adding a ceiling fan to a knob-and-tube circuit is exactly the job to turn down. Run a new circuit instead. Insurance companies may also have their own rules about it.

Aluminum Branch Wiring

Aluminum branch wiring needs devices and connectors listed for it. It expands and creeps under heat, so connections on copper-only devices can loosen and overheat. If you find it, tell your supervisor. Do not treat it like copper.

Symptom Guide

When to Stop and Get Help

Stop and call your supervisor when you see any of these:

• Signs of heat damage inside a wall or panel.
• Voltage on a grounded surface, or a shock from an appliance.
• Lights that get brighter and dimmer together, which points to an open neutral at the service.
• A fault that traces into a panel or service you are not qualified to open.

Do not leave a hazard behind you. Tell the customer and your supervisor what you found, in plain words, and write it down.

Common Mistakes

Check Your Understanding

1. You lift a circuit at the panel with the neutral off the bar, and read 0.3 ohm from neutral to ground on the cable. What does that tell you?

The neutral touches ground somewhere on the circuit. Divide the circuit at a middle box and test each half.

2. Two 120-volt hots on a multiwire circuit read 0 volts between them. Why is that a problem?

They are on the same leg. The shared neutral now carries the sum of both loads, and no breaker protects it. With the power off, a qualified person moves one hot to the opposite leg.

3. An outlet reads 120 volts at no load and 103 volts with a 10-amp load, on a short run. What is the resistance in the circuit, and what do you do?

17 volts / 10 amps = 1.7 ohms, far more than a short run of copper should have. Look for a loose or burned connection upstream.

4. A homeowner asks you to splice a new bathroom fan into nearby knob-and-tube wiring to save money. What do you tell them?

No. You do not add load to it. Tell your supervisor, and offer a new circuit, or a rewire of the affected section.

5. A new AFCI holds until the circuit next to it turns on. Where do you look?

Boxes where both circuits run. A neutral from one has likely been spliced to the other.

Key Takeaways

• Prove your meter before and after, every time you rely on a zero.
• The neutral carries normal current. The EGC carries current only during a fault.
• Name the fault type first, then pick the test that proves it.
• Divide a circuit near the middle to cut the number of boxes you open.
• A bad joint shows up only under load, so measure voltage with the load running.
• Do not add loads to knob-and-tube. Stop, report it and price a rewire.