Frequently Asked Questions:
Home Wiring Basics

The categories listed below are all geared to home wiring basics, but not layout or design concerns (for which, see Basic wiring). For an overview of my troubleshooting information and tips, go to The Circuit Detective Home page. For FAQs on other topics go to FAQ - Troubleshooting or FAQ - Switches, bulbs, testing.

Page Menu Basic knowledge about electrical circuits

What is electricity?

Well, tame electricity (not static electricity or lightning) is essentially a force generated onto loops of conductive material, transferred through their electrons, and applied as useful energy at parts of these loops.

What is meant by a circuit?

A circuit is the actual or intended path of current between points of differing voltage. In the case of a household 120 volt circuit, the path is through a "hot" wire from the breaker, an item that is using the electricity, and a "neutral" wire connected to the grounded neutral bar in the panel. In a sense each loop that current makes (through a single light and its switch, for instance) is a circuit but the most common meaning is the "branch circuit", that is, everything fed (or interrupted) by a given breaker or fuse. See Your electrical system.

To avoid getting shocked if I work on my system, do I need to turn off all power to the home?

It's not a bad idea. If you know enough about your system to turn less off at once, you may be safe turning a single breaker off or just a GFI or switch. It is also a good habit to treat all wires as live anyway, even when you have checked and know they are dead. You don't know everything, and you don't have control of other people in the home who might forget about you and turn something back on. See Safety.

Basic house wiring knowledge

Which wires are supposed to attach where on a receptacle (outlet)?

As you look at the face of the receptacle with its slots above its hole, the (shorter) slot on the right gets the hot (black/red) wire(s) by attachment on that side. The longer slot on the left gets the neutral (white) wire(s) by attachment on that side. The hole gets the (bare/green) ground wire by attachment to the green screw. See Connections Diagram and Connections Tutorial.

Which wires of a light fixture are supposed to connect to the black, the white, and the bare (or green or when no-such exists) wire at the light box?

If there is a bare or green wire in the box, attach the fixture's bare or green (if any) to it, otherwise leave the fixture's alone. If the other wires of the fixture are black and white, connect them each to the same color wire in the box (but if a single red is waiting in the box, attaching the fixture's black to it and not to the blacks in the box is probably the right move). If the fixture's two non-ground wires are not black and white but one of them is smooth and the other has a "ribbed" texture, the ribbed is to connect to the white of the box and the smooth to the other. But if the fixture's wires fit none of these descriptions, one of the two should be a solid color (it connects to the hot; except if it is solid white, connect it to the white) and the other should have striping or lettering on it (it goes to the white neutral; except if its mate was solid white, this striped one goes to the black or red). Is everything clear?

Can you help me understand how a circuit in my home operates?

For this I refer you to my Background material and also to my Tour of a Circuit

Is there a way to map out the circuits in my home?

A thorough mapping would mean you know what box every part of the circuit branches out from. But maybe you just need to know all the things running off each separate circuit breaker. It can take some work. I have a page on how to go about Labeling your panel.

What goes wrong with circuits?

You can lose power or part of your power. Your automatic breakers or GFCIs can switch things off. Your lights can flicker or get bright or strangely dim. To understand why such things happen, see this Background material.

How can I turn off all my power so I can be safe checking or working on my wiring?

There may be a single main shut-off breaker in your main panelbox or somewhere close by, indoors or outside. Or it may be you will need to turn off as many as six different breakers or levers. If unsure, turning off all breakers and pulling out all fuses you are aware of -- indoors and out -- should work, but a tester that will confirm voltage is gone will make you sure. See Testers.

What is meant by pigtailing?

Pigtailing is combining two or more wires with an additional wire which will be the one to attach to a terminal. Screw terminals that clamp down on wires you insert in nearby holes often let you connect more than one wire there, so that there is no need to pigtail. But more often the screws only take wires curled clockwise directly under themselves; then only one wire should be put under each screw, and so it may be necessary to pigtail so that all the (say, white) wires of the circuit connecting there have contact with each other. On some devices there are (additional) holes for terminating wires by simply pushing wires in till they catch. If there are enough such holes, pigtailing will not be needed. Such holes on receptacles will not accept large (12 gauge) wires, in which case the wires will need to be pigtailed if there are not enough screws to put them all under. See Pigtailing Diagram.

Can I tell if my home is wired safely?

Honestly, no. Not even an inspector or electrician can. Some things are hidden from view and from testing. An investigation that tried to be as thorough as possible would still fall short. Even when something loose or against Code is discovered, the degree of hazard it presents is disputable. Most "potentially unsafe" conditions will show themselves as outages, flickerings, shorts, or shocks; and when they do, most prove themselves to be minor or contained. I am not saying an inspection and someone's opinion of the results are pointless or of no value. But because people (even "sincere" people) can make money off your anxieties, the truth about your risk and about their thoroughness can get exaggerated.

Terms and Definitions Also see my Lingo article.

What is the difference between hot, neutral, and ground wires?

When connected properly, these wires serve different functions, as follows. Hot ("live") wires provide a circuit's path between the breaker and any lights or appliances. Neutral wires provide the rest of the path, that is, the path between these same lights or appliances and the panel's grounded neutral bar. The ground wires, like neutrals, are connected to the grounding point in the panel, but they are not supposed to carry current under normal conditions. In an abnormal condition like the hot shorting to metal parts nearby, the ground wire is meant (by its attachment to such metal) to carry a great amount of current suddenly, so as to trip the hot's breaker and stop the condition. More.

What is the difference between open, overload, short, ground-fault?

An open is when a circuit's path is disrupted. An overload is when current on a circuit is a bit excessive. A short is when current takes an unintended path to ground (usually with very little resistance). A ground-fault is when such a short does not use the neutral as its path to ground. More.

What does open ground mean? open neutral? reverse polarity, etc.?

Measured at an outlet's receptacle, the path (wire continuity) to the ground point in the panel can be disrupted or missing for the ground wire (open ground) or for the neutral (open neutral). When it is the path between the outlet and its circuit breaker that is disrupted, this is called an open hot. Reverse polarity (hot and neutral reversed) means neutral wires are connected to the side of the receptacle that is supposed to be for the hots, and vice versa. See Outlet testers and Outlet corrections.

What is the difference between Line and Load?

For this, see the Glossary.

What is the difference between current, voltage, resistance, and wattage?

Voltage pushes current through a resistance, using power (wattage). More at Glossary.

What is the difference between amperes, ohms, watts, and volts?

W volts push X amperes through Y ohms, using Z watts. Amps times ohms = volts. Amps times volts = watts. See Glossary.

What is the difference between outlet, receptacle, plug, switch, breaker?

A switch makes or breaks continuity. A breaker is a switch that automatically breaks continuity when current is too high. An outlet is technically where a light or appliance gets its connection to the circuit's wires. A receptacle is the device we often call "outlet" for plugging cords into. More at Glossary.

What is the difference between fixture, device, appliance?

A fixture is a non-portable light. An appliance is anything else than a light that uses (up) wattage. A device (e.g., switch, breaker, receptacle) is for passing or purposely disrupting the continuity of the circuit to fixtures, appliances, or lamps. See Glossary.

Electrical code

Why do some circuits seem to go all over the place without obvious reason?

There are many Code provisions saying where some circuits may not extend, where they must be provided; where switches, fixtures, and receptacles must and may not be. But residential Code and blueprints do not say everything about how circuits should run. Within code, electricians will then choose convenient and material-saving routes for their cables.

How many things can be on a circuit; how many wired off a GFI?

For a general purpose circuit, the limitation is mainly this: a 15-amp circuit should not extend over more than 600 sq.ft. of the home; a 20-amp circuit not over 800 sq.ft. Some GFI manufacturers only stand behind their device's ability to help eight outlets downstream, but this is arbitrary. It is reported that a very lengthy set of GFI loads can set up enough capacitance to trip the GFI. If that is suspected, each such load outlet could be given its own GFI receptacle, connected to protect only itself.

When were different things required by code?

That's a large question. For GFI requirements and for kitchen and bathroom circuit requirements, see GFI locations. For arc-fault interrupters, see AFCIs.


Do you have a diagram of how switches or outlets are usually hooked up?

Yes, several. For instance, see my Tour of a circuit, or Connections tutorial.

Do you have a diagram of how cables are usually run in a home?

Yes, it is called Floorplan. There are common methods of wiring a house or apartment, but not quite a "usual" way.

Do you have a diagram of a main electrical panel box or how a 240-volt circuit is hooked up?

Yes, you'll see a few of these in Your Electrical System.

Replacing outlets and switches

Will it be easy for me to replace switches and outlets in my home?

Physically, maybe. Electrically, no. For most people, the unknowns they will encounter when they get into this simple sounding project will begin to baffle them. If they go forward anyway, they are likely to end up with one or more malfunctions when they turn power back on and try using things. If they are lucky and everything seems to check out fine, their inexperience at making the new connections may still result in problems in the future. Learn the Pitfalls of upgrading.

I replaced several receptacles or switches with new ones; things don't work quite right now; what did I do?

You'll need to go back over the work you did before you noticed, and see if all connections are solid, none missing, and whether perhaps something was connected to the wrong place. See Pitfalls of upgrading or my Replacing article.

I am replacing old switches or outlets but have maybe lost track of how they were connected; is there a standard way or diagram?

Without consulting me or another electrician, your best bet is the examples in my Tour of a Circuit or the Connections diagram.

What is the green screw on my new switch for?

Because people have been replacing plastic switch covers with decorative metal ones, code has now required the switches to be grounded, so that the covers attached to them will never be able to shock if a hot wire or terminal were to get loose and touch either. So to put the new switch in right, you'll dig the bundle of ground wires out from the back of the box and pigtail a wire from them to this green screw.

I have a lot more trouble plugging cords into my new receptacles. Why?

The receptacles may be a new type, Tamper Resistant (required in many places now). They are to keep children from inserting pointy metal objects into the live side of the receptacle and getting shocked that way. For adults, the trick is to bring the cord-end straight on as you plug it in; only when both blades of the cord enter at the same time will the plastic barriers give way. It may take us twenty or thirty years to get used to this.

©2005-2020 Laurence Dimock