We had to re-wire our new kitchen and the areas of our house that were gutted for structural repairs. Of all the work that we did, I enjoyed the wiring the most. I did, however, want to make sure that everything was on the up and up, so I purchased the latest version of the National Electrical Code or NEC.

So very often a simple home owner can be flummoxed by technical jargon from contractors. This bla bladdy bla bla bla is not to code. Code? What does that mean? The National Electrical Code is really a minimum standard for electrical wiring and workmanship in the US. It is revised every three years with the next revision due out in 2008. I say it is the minimum standard because cities, counties and states can override the NEC with their own, more strict electrical code. The only way to know what code is used in your jurisdiction is to check with the building department.

Like all professions, electricians have there own phraseology which can be confusing for the non-electricians among us. Things like voltage drop, box fill, ground fault, service entrance, conduit fill, just to name a few, can leave you scratching your head. I will try to explain these things in more every day terms. Any questions regarding this should be referred to the NEC section cited. I also strongly suggest that anyone engaged in a re-wiring project on there own purchase a copy of the lastest NEC and read it throughly. There are several books explaining the residential sections of the NEC available as well. I do not condone or recommend non electrical workers to attempt to re-wire their houses, if you don’t understand what you are doing, it is time to call a professional.

The Basics

The electrical service entrance for a house is where the electric meter is located. This feeds the electrical panel, or service entrance panel. The size of the service entrance panel is determined by the size load, or amount of power, the house is calculated to need. These calculations are done in Volt Amps which is nearly the same thing as Watts.

Load calculation

General load for every 1 square foot of floor space is 3 VA. Example: The total square footage of the house is 1525 SqFt x 3 VA = 4,575 VA. (NEC 220.82). The General load includes things like lamps, radios, televisions, kitchen appliances, etc.

Kitchens require the most circuits of any room in the house. Each kitchen is required to have two 20 amp appliance circuits to feed the counter top receptacles only (NEC 210.52). Those are calculated at 1,500 VA each, therefore 1,500VA x 2 = 3,000 VA. (NEC 220.82)

Other loads in the kitchen are electric range, typical 7,500 VA (each range is different, all ranges will have a name plate with the maximum load specified in Watts which can be as high as 15,000 watts or VA). Dishwasher, typical 1,200 VA, clothes washing machine, typical 1,500 VA, clothes dryer, typical 5,000 VA. (NEC 220.82)

A separate 20 amp circuit is needed for the refrigerator and if you have one of those whole house central vacuums installed, that should be on its own 20 amp circuit as well. (NEC 220.82) Those two loads are 1,500 VA x 2 = 3,000VA. Those circuits are heavy loads. Other appliances that require separate circuits are garbage disposal, built in microwave oven, trash compactor, stand alone freezer, and anything with a 1/2 HP or bigger motor. When they are run on separate circuits it will prevent circuit breaker trips and dimming lights when the appliance turns on.

Other non-kitchen house loads include electric hot water heater, typical 4,000 VA, well pump, typical 1,500 VA. Each bathroom is required to have a 20 amp circuit, so if the house has two bathrooms that would be 1,500 VA x 2 = 3,000VA. (NEC 210.11)

Heating and cooling loads are based on the size of the heating or cooling appliance. Each appliance will have a name plate rating. If the house is using electric base board heat, each base board unit has a rating. Example, there are 4 2,000 watt base board heaters and 1 1,200 watt baseboard heater, so the heating load would be 9,200 VA. Heat pumps and AC units need to be calculated by there name plate rating as well. If the house has an oil fired furnace that will require one 15 amp circuit to run the burner and circulator pumps or air blowers.

Next step, add all of the general loads together:

To convert that load to Amps, we refer to ohms law which states: P = I x E where P is the power in watts (or in this case VA), I is the current in Amps and E is the voltage. Most incoming residential service voltage is 240 volts, so divide 35,475 VA by 240 volts which results 147 Amps. (NEC 220.82C) Therefore the minimum service panel for this house should be 150 amps.

Circuit breakers

Minimum required wire size for circuit breaker (copper wire) (NEC 240.3):

(AWG stands for American Wire Gauge)

Outlets or receptacles

All living areas must have an outlet placed so that no point measured horizontally along the floor line is more than six feet from any receptacle. (NEC 210.52) This is because most lamps, Televisions, radios, etc have six foot cords and extension cords should not be used.

Kitchen counter tops must have receptacles installed so that no point along the wall line is more than 24 inches from any receptacle. (NEC 210.52) This is because all kitchen appliances come with 2 foot cords.

Arc Fault Circuit Interrupter (AFCI) circuits are required for receptacles installed in all bedrooms. (NEC 210.12)

Ground Fault Circuit Interrupter (GFCI) circuits are required for receptacles located in kitchens, bathrooms, unfinished basements, outdoors, garages, unfinished accessory buildings, and within six feet of any sink in laundry rooms or wet bars. (NEC210.8 )

Wiring methods

All wiring including splices must be terminated in enclosures, i.e. junction boxes, breaker panels, etc. Splices shall be made with an approved splice cap or “wire nut.” (NEC 110.14)

Junction boxes must be installed so that access to wiring in the box is possible without removing any part of the house structure, e.g. no burying boxes behind sheet rock in walls or ceilings (NEC 314.19).

All boxes must be securely attached (NEC 314.23).

When cables are installed through bored holes in studs, joints or rafters, the hole must be at least 1 ¼ inches away from the nearest edge of wood member. Where this is not practical, a metal plate not less than 1/16 of an inch thick must be installed over the area of the cable. (NEC 300.4) This is to prevent nails and screws from penetrating the cable.

Cable types are NM (or non-metallic) also known as Romex and MC (metal clad) also known as BX. They are usually specified by their gauge and number of conductors, not counting the ground conductor. For example 12/2 NM wire will have two twelve gauge wires plus a ground wire in some sort of plastic sheath. 14/3 MC will have three 14 gauge wires plus a ground in a spiral metal clad enclosure.

Cables must be secured every four and a half feet and within 1 foot of each box (NEC 334.30).

The outer jacket of NM cable must be secured to the box and extend into the box at least ¼ inch (NEC 314.17)

The minimum length of all conductors, including ground conductors at all boxes is 6 inches with at least 3 inches extending outside of the box (NEC 300.14)

Only one conductor can be secured under each screw terminal (NEC 110.14)

All electrical equipment, metal boxes, cover plates, outlets, switches shall be grounded (NEC 250.134, 314.4, 404.9)

Service entrance panel

Minimum conductor sizes for entrance service (NEC 310-15)

A listed anti-oxidant compound must be used on all aluminum conductor terminations (NEC 110.14)

Service entrance conductors must be rain tight (NEC 230.54)

Electrical panels must be readily accessible and shall not be located in bathrooms or in the vicinity of easily ignitable materials such as clothes closets. (NEC 230.7, 240.24)

All electrical equipment, including service entrance panels must have sufficient working space around them. The depth of that space in the direction of access shall be a minimum of 3 feet, with a minimum width of 30 inches or the width of the equipment, which ever is greater. (NEC 110.26)

All grounding electrodes that are present must be bonded together to form a single grounding system (NEC 250.50) A grounding electrode consisting of one or more of the following must be installed: metal underground water pipe in direct contact with the earth for 10 feet or more; metal frame of the building; concrete encased electrodes; ground ring; ground rod or pipe driven at least 8 feet into soil; plate electrode. (NEC 250.50).

A metal underground water pipe shall be supplemented by an additional grounding electrode (NEC 250.64).

Minimum size for the grounding electrode conductor is based on the service conductor. The conductor sizes are (NEC 250.66):

Underground wiring

Direct buried cable or conduit must have the minimum cover requirements (NEC 300.5):

Underground service laterals must have their location identified by a warning ribbon placed at least 12 inches above the underground installation (NEC 300.5)

These are the minimum guide lines. There is nothing wrong with using 12/2 NM wire on a 15 amp circuit. For longer cable runs with heavy loads such as an electric range or dryer, it is good to go up one wire size to minimize the voltage drop.

update
There is no way that the entire NEC can be covered in a blog post. This is an informational post that explain some of the jargon used in the electrical industry. I did it to help the average non-electrical homeowner when talking with an electrician.

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