The Circuit Detective    -  Wiring Basics

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Basic House Wiring
Issues, Parts, and Code

Here I intend to give clear information on a number of basic house wiring issues that may be unfamiliar or confusing to the do-it-yourselfer. These certainly don't cover everything you might want to know. In what I say I am careful to qualify my statements if they have exceptions, using words like "usually" or "typically." If a practice is specified by the National Electrical Code I will tend to indicate this by "is to be," "should," or "must," and I won't talk that way if the matter is just common practice.

For problems with existing wiring, circuits, and connections, be sure to see my Main page. For issues about some particular appliances see this. To understand electrical terms see the Glossary. For wire ampacity and conduit capacity see SparkyJohn.

Cables
Boxes
Connections
Breakers
15-amp versus 20-amp
Main panel

Romex cables. Your wiring could be done with a conduit system or metal-sheathed cables, but in most places these plastic jacketed cables are the norm for houses and non-highrise apartments. On them it says "NM-B," which means non-metallic-sheathed, with wires that can stand 90 degrees of heat (Celcius); before "B" (1985) they were only good for 60 degrees. It also says the size and number of insulated wires inside: 14-2, 14-3, 12-2, 12-3, 10-2, 10-3, etc. (The listing I have just given is in increasing size order.) It also says "With ground," which means a bare (or even green insulated) wire is in the cable but is not included in the "-2" or "-3" count.

These cables are to be secured to the home's framing every 4.5 feet at most. This support can consist of holes in the framing or approved staples or wire-ties. In addition, such support is to be provided within 12 inches of where the cables enter boxes (but within 8 inches of one-gang boxes which have no built-in clamps).

Inside the box a cable's sheath is to be removed no closer than .25 inches from where it enters the box's interior. In practice, leaving more than an inch of sheath visible starts to interfere with the flexibility of the wires, and space for devices and wire connectors. The wires themselves are not to be cut back any shorter than 6 inches from the resulting end of the sheath and must initially be able to extend at least 3 inches outside the front of the box.

Electrical boxes. An electrical box is almost always required for mounting devices (switch, receptacle) and light fixtures, and also wherever circuit connections or splices are made -- which is usually at the same places. Boxes are not only a mounting place but minimize shock hazard and the possible effect of sparks or heat. They may be made of metal or plastic. According to the number of devices that can be mounted in it, a switch/receptacle box will be called one-gang, two-gang, etc.

The number and gauge of wires that may be installed in a box will be related to how many devices will be mounted in it and the volume of the box, which is stated inside it (in cubic inches). The formula for calculating this "box fill" is somewhat involved. When it is a romex cable that brings wires into a box (other than a one-gang plastic box), the box will or should have a clamp to hold the cable securely to it.

Connecting wires. The main ways you will make electrical connections of wires are by wire connectors ("wirenuts") [at light fixtures and within boxes] and at terminals [of devices]. For which wires to connect to what, and where, see the Connections tutorial.

For combining (splicing) wires in wirenuts, you should strip insulation off the ends of the wires enough that the metal spiral inside the nut does not bite into any insulation, but don't strip off so much that any copper ends up showing outside the nut (when viewed from the side). The fine print on the packaging of the wirenuts usually tells you how many wires of what sizes the nut is designed to hold effectively; follow this, even if it means going after a different size at the store. Twist the wirenut onto the wires clockwise. It is practically impossible to overtighten a wirenut; if the insulated parts of the wires are twisting around each other, you are getting tight enough. The biggest trick in making a good wirenut connection is keeping the ends of the wires relatively lined up with each other, so that none of them is liable to slip away from being held in contact, metal-to-metal. Some electricians advocate pre-twisting the wire ends together with pliers, but this doesn't always keep them in line either. Finally, another tradition often recommended online and in manuals is to tape around the finished nut and wires, supposedly to retard sparks that might develop or to keep the nut from untwisting. I don't find this notion at all convincing.

At the terminals of switches and receptacles wires may connect in a variety of ways. The means provided may include screws to loop wire (only one) around (clockwise), holes to push wires into (a.k.a. "backstab" or "quickwire"; only for 14 gauge wire anymore), and/or wire clamps to tighten down on the wire using a screw. In all these methods, it is important to strip enough insulation off the end of the wire that no insulation will be caught under the pressure of a screw or clamp or prevent a wire from inserting far enough in a hole. Video. Stripping too much insulation off the wire could make accidental shorting or shocks a bit more possible. The practice of finally running tape around a device to cover its side-screws makes some sense when the box is metal or live parts of other devices are nearby, but not otherwise.

Circuit breakers. Breakers come in different brands, sizes, and number of poles. A 2-pole or double-pole breaker is for 240-volt circuits or for two 120-volt circuits that share a neutral. When a breaker is available in skinnier sizes, it is often a unit with two breakers together; some names used to refer to these are: peanut, duplex, tandem; physically, these might only fit in a certain section of the panel.

If possible, a circuit breaker should be of the same brand as the panel. If it is not, it may not work or may not function reliably, but in any case the warranty of the breaker and panel will be voided. Still, if the fine print for the breaker or panel allows certain other brands, then this is fine, and the warranty would hold. And there is a degree of practical interchangeability among some brands, which may sometimes even be acceptable to inspectors. Speaking of brands, two that have achieved negative reputations are Federal Pacific and Zinsco; many breakers of these brands are still in service and doing well, however.

When mounted properly in the panel, a breaker may have some side-to-side play (across its short width), but if its depth is odd (not flush) with nearby breakers, it may not be fully seated in the panel's retaining bracket or down onto its live busbar; this might even raise the question of whether the breaker is compatible with the panel or that section of the panel.

Most commonly, only one wire will be held under a breaker's screw terminal. Unless the fine print of the breaker allows two, installing more than one wire per terminal (called "double-tapping") is not allowed and will prove unreliable. If two parts of an intended circuit each go out from the panel, their black wires can be pigtailed to the breaker.

15-amp vs. 20-amp. Most circuits and devices in a home are rated as 15-amp or 20-amp. What this means varies a bit from one item to another. In a way, it all starts with the size of the wire. 14-gauge wire is the smallest allowed for the permanent wiring of a circuit. It is rated as able to carry up to 15 amps of current. 12-gauge wire is the next size larger and is allowed to carry up to 20 amps.

As a result, the amp rating of a circuit breaker has the following relation to the wire size that has been chosen. A 20-amp breaker is never* allowed to run any circuit whose wires (anywhere on the circuit) are 14-gauge. But a 15-amp breaker's wires out on the circuit may be 14-gauge, 12-gauge, or even a mixture. Yes, even larger wires than these are possible (10-gauge), but become difficult to install properly and give little benefit. The rating of a circuit itself is that of the breaker. Any general purpose circuit in a house might be 20-amp, but some circuits are required to be; for example, for the outlets in kitchen, dining, laundry, and bathrooms. I'm not going into the ins and outs of Code on that.

When Code allows you to design a circuit to be either 15- or 20-amp (e.g., bedrooms, living room, family room), you might want to consider the area to be covered (600 sq. ft. max. for a 15-amp circuit, 800 sq. ft. for 20-amp), the wire-size capacity of your outlet and switch boxes, the stiffness of heavier wire in the process of making connections and installing devices, the heaviest likely appliance that might be used (vacuum?), and the effect of voltage drop (greater for smaller wire and for greater distance) on lights, TVs, or computers.

The receptacles in a home also have an amp rating. Whether it is a standard receptacle or a GFCI receptacle, 15-amp are the most commonly installed. The only cases where a 20-amp rated receptacle is required is if it is for a specific appliance known to use over 15 amps of current; such an appliance will come with one of its plug prongs turned at a different angle, which a 20-amp receptacle accommodates. Otherwise, where the breaker is 20-amp, you have the choice of installing 15-amp or 20-amp receptacles (or a mixture). But if the breaker is 15-amp, you may not have any 20-amp receptacles installed on that circuit, mostly because an appliance using over 15 amps would be fooled into thinking it would not trip if plugged in there. (A 2008 Code requirement for all of the 120-volt receptacles newly wired or replaced in homes is that they be of a new type -- "tamper-resistant".)

Light switches tend to be rated for 15 amps (20 is less common) and may control up to their amp rating worth of lights. So a switch on a circuit with a 20-amp breaker could be rated 15 amps, as long as it was not controlling over 15 amps of lights. A switch controlling a motor, such as a garbage disposal, must be rated at least 125% of the amps of the motor.

* An exception would be a circuit dedicated to a one-third horsepower or one-half horsepower motor.

Electrical panel. A circuit breaker panel has two kinds of capacities -- its amperage and its space for breakers. When a homeowner wonders if the main electrical service is adequate or else "overloaded," he/she should not confuse these two capacities.

Don't judge a panel by its individual circuit breakers. Even a 200-amp panel is not "overloaded" just because the sum of its circuits' ratings is, say, 500. In such a case, if you could purposely load down every branch circuit to its full rating, the main breaker would trip. If there is no main, the power company's fuse could blow. According to Code, technical overloading can only be determined by a complicated formula, but even when technically over, a panel rarely does run enough amps of current to trip its main breaker.

As long as the load of a service, as calculated above, remains proper, any number of circuits can be run out of that service. Not all of them will fit physically in the main panel, but the main panel can feed subpanels and sub-subpanels, etc. Even before considering adding a subpanel, see if a lack of space in the main panel is only apparent. If it says somewhere in it that it can accept physically half-size breakers (see breakers above), at least in some section, you might be able to replace some full size ones, gaining space for new circuits that way.

Above all, don't think that the main service is too small or too old just because an individual breaker keeps tripping for certain heavy loads. That is most likely a problem of usage on that particular circuit. Upgrading the main panel will not change that.

"I've done some electrical work following the directions of an electrician or with the use of a book. But until I discovered your site I NEVER had a clear grasp on how home wiring is set up." -David, CA

"I've looked here and there for a website to direct answer-seekers and do-it-yourselfers to in the past, and I find that yours is very thorough and easy to understand. I'm a master electrician in Maryland, and have been posed many of the questions which you explain quite well on your site." -Andy, MD

© 2009-2013 Larry Dimock

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