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Renovation 4th Edition

Page 64

by Michael Litchfield


  is deteriorated, splices are incorrectly made, the

  not exceed these capacities; otherwise, you risk overheating wires. To avoid over-

  wiring is overloaded, or buried in thermal insula-

  loading, it’s a good idea to reduce the capacity by 20%. for example, 80% of 1,800w

  tion. Typically, splices that were part of the origi-

  is 1,440w for a 15-amp circuit; 80% of 2,400w equals 1,920w for a 20-amp circuit.

  nal installation will not be in a junction box but

  must be wrapped with electrical (friction) tape

  and supported by porcelain knobs on both sides

  safe Circuit Capacities*

  of each splice. Non-original splices must be

  housed in covered boxes. Have knob-and-tube

  amperes × Volts†

  total CapaCItY (watts)

  saFe CapaCItY†(watts)

  wiring assessed or modified by an electrician

  familiar with it; it’s quirky stuff. The NEC does

  15 × 120

  

  1,800

  1,440

  not allow knob-and-tube wiring to be buried in

  insulation, although some local inspectors are

  20 × 120

  

  2,400

  1,920

  OK with the practice.

  25 × 120

  

  3,000

  2,400

  Knob-and-tube wiring lacks an equipment

  ground (a separate grounding wire), so it offers

  30 × 120

  

  3,600

  2,990

  no protection should a faulty appliance get

  plugged into a receptacle. On the other hand, a

  * Safe capacity = 80 percent of total capacity.

  knob-and-tube system is run completely on insu-

  † Amperes multiplied by volts equals watts.

  lators, a plus. The conductors of knob-and-tube

  wiring were copper, coated with a thin layer of

  tin (to protect the copper from sulfur in the rub-

  ber wire insulation). Uninformed inspectors

  often mistake the tinned copper wire for alumi-

  num wire.

  such as electric ranges and hot tubs. If there are

  S a f e t y a l e r t

  only two large wires running from the utility pole

  is The sysTeM adequaTely sized?

  to the house, they deliver only 120v service. A

  don’t add outlets to circuits

  house with two-wire service probably has a

  If receptacles in your house teem with multiplugs

  that have ungrounded cable;

  30-amp or 60-amp main fuse or breaker, which is

  wires with frayed, discolored, or

  and extension cords, you probably need to add

  inadequate for modern usage.

  melted insulation; blackened

  outlets. But there are more subtle clues: If you

  metal boxes or other signs of arc-

  blow fuses or trip breakers regularly, or if the

  rules of thumb. The only sure way to know if

  ing; or aluminum wire (a soft,

  lights brown out when you plug in a toaster or an you’ve got enough capacity to add an outlet or a

  dull silver) rather than copper.

  electric hair dryer, you may need to add new cir-

  circuit is to calculate electrical loads, as shown

  don’t confuse old knob-and-tube

  cuits to relieve the overload on existing circuits.

  on the next page. But for the benefit of those who

  wire, which is usually tinned cop-

  This section will help you figure out whether

  wish that an electrician would just offer an off-

  per, with aluminum wire; the

  your system has the capacity to do so.

  hand opinion of what works most of the time,

  tinned copper has a shiny silver

  These days, three-wire service feeding a

  here are a few rules of thumb.

  coating on the outside, but at

  100-amp service panel is the minimum required

   Fuse box service. If you’ve got a fuse box with

  the cut end, you can see the cop-

  by the NEC, and many electricians install

  per color of the wire.

  a 30-amp or 60-amp main fuse, the best advice

  150-amp or 200-amp panels if the homeowners

  we can give is to upgrade your service. Don’t add

  plan to enlarge the house at some point or

  outlets or circuits until you replace the fuse box

  acquire a lot of heavy energy-using appliances,

  with a breaker panel. Fused mains are often

  electrical Wiring

  271

  example of load Calculation for single family dwelling

  CalCulatIng general lIghtIng loaD

  type of load

  neC reference

  Calculation

  total Va

  lighting load

  Table 220.12

  2,000 sq. ft.  3 va

  6,000 va

  small appliance load

  section 220.52

  2 circuits  1500 va

  3,000 va

  laundry load

  section 220.52

  1 circuit  1500 va

  1,500 va

  total general lighting 

  10,500 Va*

  A CalCulatIng DemanD For general lIghtIng loaD

  type of load

  Calculation

  Demand Factor (DF)

  total Va

  general lighting

  first 3000 va  df

  100%

  3000 va

  general lighting

  7,500 ‡  df

  35%

  2,625 va

  total lighting, small appliances & laundry 

  A 5,625 Va

  B CalCulatIng DemanD For large loaD applIanCes

  type of load

  nameplate rating

  Demand Factor (DF)

  total Va

  electric range

  not over 12kva

  use 8kva

  8,000 va

  Clothes dryer

  6,600 va  df

  100%

  6,600 va

  Water heater

  6,600 va  df

  100%

  6,600 va

  other fixed appliances

  0 va  df

  100%

  0 va

  total load for large appliances 

  B 21,200 Va

  total Va (A + B)

  26,825 Va

  minimum service size

  total Va /240V

  111.77 Va

  The minimum service size is the next standard size above the total va calculated. based upon these calculations

  the minimum service size is 125 amps.

  *use this to calculate A

  ‡ Total general lighting load 10,500 va – first 3000 va = 7,500 va

  usIng thIs taBle

  1. square ft. for general lighting load is for the entire dwelling including habitable basements and attics.

  2. neC requires a minimum of 2 small appliance loads, but it is important to add small fixed kitchen

  appliances such as refrigerator, microwaves, disposals, dishwashers, large range hood, computers, etc.,

  when calculating this category

  3. Minimum of 1 laundry load is required for a single family dwelling.

  4. The demand factor calculation is designed to take actual use into account (e.g., it is unlikely that all lights

  and small appliances will be running at one time).

  5. all large load appliances (high wattage) are added at 100%.

  6. The final load calculation is the minimu
m. often, increasing capacity has little cost impact and is a good practice.

  Some appliances require dedicated

  The grounding conductor of a cable is connected to a steel All devices, including receptacles, switches, and lighting service because they are heavy energy

  box with a special grounding screw, which must thread

  fixtures, also must be bonded. Here, a bare ground lead

  users. This is a 30­amp, 125/250v

  into a tapped hole.

  from spliced ground wires in the box is attached to the

  dryer receptacle. The breaker for this

  green grounding screw of a duplex receptacle.

  circuit must also be rated for 30 amps.

  272 Chapter 11

  abused by people trying to bypass its protection,

  the service panel via an equipment grounding con-

  so insurance companies often charge higher pre-

  ductor (ground wire). NM cable contains a sepa-

  miums on houses with fuse boxes.

  rate ground wire, whereas armored cable sheath-

  ing and metal conduit provide the path to ground.

   adding outlets. If you have a breaker panel

  with space to add an additional breaker, you can

  boxes. All electrical connections must take place

  most likely add a circuit to feed a new outlet or

  in covered boxes. Boxes where connections are

  two or more lights. If, for example, you have

  made must be accessible, i.e., not buried in a wall

  three-wire service and a 100-amp main, there’s

  or ceiling. Based on local code requirements,

  usually a lot of excess capacity.

  boxes may be plastic or metal. If metal, the box

  must also be connected to the equipment ground-

   adding a circuit for general use. If there is an ing conductor (bonded). If NM cable is used, the

  unused space in the panel, have an electrician deter-

  ground wire must be connected to a metal box

  mine whether the panel can handle another circuit.

  with either a ground screw or a ground clip. If

   adding a kitchen or bath circuit. If you

  AC cable or metal conduit is used, it must be

  want to add a bath fan or some new light fix-

  properly attached to the box to ensure effective

  An AfCI breaker can detect current

  tures, and there’s space in the panel, have an elec- bonding. If the box is plastic, it does not need to

  fluctuations associated with arcing.

  trician see if you can add a circuit. Adding a

  It then shuts off power to protect you

  be (and cannot be) grounded; run a ground wire

  20-amp, small-appliance circuit to reduce the

  from house fires. note: Arc­fault

  to the device or fixture only.

  breakers are designed to detect arcing

  load on an existing circuit is smart.

  patterns of current—many amps

   remodeling a kitchen. Kitchens are compli-

  general-use

  (40 or more) for very short time

  cated and often full of big energy users. Use the

  CirCuiT requireMenTs

  intervals.

  chart at left to help you add up the loads. If there General-use circuits are intended primarily for

  aren’t many open spaces for breakers, you may

  lighting, but small loads that are connected via a

  need to upgrade to a larger panel.

  cord and plug, such as televisions, fans, and vac-

   adding dedicated circuits. If you need to

  uums, are allowed—as long as the power they

  add dedicated circuits for heavy-use items such

  draw doesn’t exceed the capacity of the circuit.

  as an electric range (50 amps) or a hot tub

  lighting and small loads. Although 14AWG

  (60 amps), get out the calculator and do the math. wire is sufficient for lighting, electricians often

  run 12AWG wire on general-use circuits to

  general Wiring guidelines

  accommodate future uses. Calculate lighting

  Electricians follow the NEC, as adopted (or

  loads at 3w per square foot, or roughly one

  amended) by local jurisdictions,which was com-

  15-amp circuit for every 500 sq. ft. of floor space.

  piled to promote safe practices and prevent house When laying out the lighting circuits, do not put

  fires. Consider these requirements before you start all the lights on a floor on one circuit. Otherwise,

  drawing plans, but be sure to consult local electri-

  should a breaker trip, the lights on the entire

  cal code—it is the final authority in your area.

  floor would be affected.

  The guidelines given here apply to all circuits

  receptacles. There must be a receptacle within

  in the house, whether general lighting or heavy-

  6 ft. of each doorway, and receptacles should

  use appliance circuits. Local codes rarely require be spaced at least every 12 ft. along a wall. (This is

  you to change existing circuits—as long as they

  also stated as, “No space on a wall should be

  are safe—but new electrical work should reflect

  more than 6 ft. from a receptacle.”) Any wall at

  current electrical code.

  least 2 ft. wide must have a receptacle, and a

  Circuit wiring. Wire gauge must be large enough receptacle is required in hallway walls 10 ft.

  to carry the circuit load and be protected by a

  or longer. Finally, any foyer of more than 60 sq. ft.

  comparably sized breaker or fuse at the panel.

  must have a receptacle on any wall 3 ft. or longer.

  General-use and lighting circuits are typically

  outlets. The NEC does not specify a maximum

  14AWG wire, protected by 15-amp breakers;

  number of lighting and receptacle outlets on a

  kitchen, bath, and workshop circuits usually have residential lighting or appliance circuit, although

  12AWG wire, protected by 20-amp breakers.

  local jurisdictions may. Figure roughly nine out-

  acceptable cable. Most circuits are wired with

  lets per 15-amp circuit and 10 outlets per 20-amp

  NM cable because the cable is protected behind a circuit.

  finished surface such as drywall or plaster. When light switches. There must be at least one wall

  circuit wiring is to be left unprotected and

  switch that controls lighting in each habitable

  exposed, it must be armored cable or in conduit.

  rooms, in the garage (if wired for electricity), and

  Grounding. All receptacles, appliances, and elec-

  in storage areas (including attics and basements).

  trical equipment must be connected (bonded) to

  There must be a switch controlling an outside

  electrical Wiring

  273

  along a kitchen countertop should be more than

  2 ft. from an outlet—in other words, space coun-

  kitchen lIGhtInG basICs tertop receptacles at least every 4 ft. Every coun-

  ter at least 12 in. wide must have a receptacle.

  Kitchen lighting should be designed to utilize natural light during the day and achieve

  Kitchen lighting. Adequate lighting is particu-

  a balance of general light and task lighting at night. Do not be afraid of energy-

  larly important in kitchens so people can work

  efficient lighting such as fluorescent. today’s energy-efficient lighting is instant,

  safely a
nd efficiently. Lay out a good balance of

  dimmable, and available in colors that match incandescent light. Kitchen lighting is

  general and task lighting. Be aware that many

  often highly regulated for energy efficiency. Check with local building officials before

  jurisdictions have energy-efficiency requirements

  you begin your design.

  for lighting in kitchens, so check with your local

  general lighting

  building authority first.

  general lighting is meant to illuminate the space generally and can come from

  bathroom lighting. It is important to illuminate

  recessed cans, surface-mounted fixtures, track lighting, or cove uplighting. Consider

  the face evenly in mirrors. Common practice is to

  cabinetry and appliances when laying out new light fixtures. a general rule is 2w

  place good-quality light sources either above the

  incandescent or 1w fluorescent per sq. ft. of kitchen area, but even illumination is

  vanity mirror or on either side of it. Be careful

  the goal.

  when using recessed cans over the vanity because

  they can leave shadows across the face. Many

  task lighting

  jurisdictions also have energy-efficiency require-

  task lighting is meant to provide a higher level of illumination at work areas (sinks,

  ments for lighting in bathrooms.

  countertops, and islands) and can be achieved with recessed cans, pendants, or

  dedicated circuits. All critical-use and fixed

  undercabinet fixtures. If cabinets are over countertops, undercabinet fixtures

  appliances should, and in most cases must, have

  (t5 fluorescent or leD strips) are by far the best choice and should be spaced for even

  their own dedicated (separate) circuits. These

  illumination of the counter surface. For islands and sinks, choose a recessed can with

  fixed appliances include the water pump, freezer,

  a slightly higher wattage and narrower lamp beam spread, or install pendants with

  refrigerator, oven, cooktop, microwave, furnace

  similar attributes.

  and/or whole-house air-conditioning unit, win-

  dow air conditioners, and water heater. A bath-

  room heater requires a dedicated circuit, whether

  it is a separate unit or part of a light/fan. Laundry

  light near each outdoor entrance. Three-way

  S a f e t y a l e r t

  room receptacles must be on a dedicated circuit;

  switches are required at each end of corridors

  so must an electric clothes dryer.

  and at the top and bottom of stairs with six steps

  all bathrooms and kitchens

  or more. When possible, put switches near the

  must be gfCi protected. gfCi

 

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