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why do cables joining the wall sockets in a house need to be thicker than the cables joining the light fittings? I don't understand and I need to know before I buy it.

  • intuitive: your ice maker waterline is smaller than the house's water main too. – dandavis Oct 30 '17 at 21:49
  • @dandavis not intuitive: in most houses in which I have looked at the plumbing, the water lines gradually reduce in diameter as waterlines branch off to rooms/fixtures. – user4302 Oct 31 '17 at 2:25
  • @dandavis I'm pretty sure this logic doesn't work at all here - it's about max current, which works differently from max flow, no? – David Doria Feb 27 at 14:23
  • @DavidDoria the diameter of household pipes and copper wire determines capacity, right? – dandavis Feb 27 at 16:32
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The wires to the light fixture will only carry the current drawn by that particular light fixture (on the order of 1A or less, for simple fixtures). The wires in the wall between fixtures carry current for all fixtures on the circuit, and are protected by a fuse or circuit breaker in the main panel. The wires in the wall must thus be sized to carry the maximum current allowed by that circuit breaker. For 15A circuits, this is #14 wire; for 20A circuits, it is #12 wire.

Note that the wires to the fixture must also be sized to carry the maximum expected current, but that is (largely) determined by the manufacturer of the fixture. Light fixtures have a sticker somewhere stating the maximum wattage of bulb that can be installed. This is partially due to heat (not really a concern anymore with CFL and LED bulbs) and partially due to the size of the wiring.

Some (better quality) fixtures / equipment will also have internal fuses or breakers that are sized to protect the wiring in the cord.

  • 3
    If a light fixture would short out, the smaller in-fixture wiring from the house #14 or #12 wiring would get momentarily overheated until the breaker tripped, but the fixture wiring is not inside an NM cable inside a wall and so would present a negligible fire risk. – Jim Stewart Oct 30 '17 at 20:52
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    Fixture wire also has higher tempature rated insulation and the rating of the insulation is what the over current protective device is protecting. #12 & 14 wire can handle 5x the normal breakers but the insulation will melt if a continuous load at this value was used. – Ed Beal Oct 30 '17 at 22:08
  • Hmmm what about lightbulb to plug adapters? We only use them in attics due to convince problems otherwise but ... – Joshua Oct 31 '17 at 2:04
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    @Joshua what about them? If the fixture is rated for 75W, then anything plugged into that socket better draw 75W or less, whether it's a bulb or a bulb plus adapter plus plugged-in thing – mmathis Oct 31 '17 at 2:24
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Because house wiring must be capable of carrying the load that the breaker allows (typically 15 or 20 amps), whereas a light fixture has a specific load limit related to the bulbs it is rated for (maybe just an amp or two).

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    With the underlying assumption that people will plug random stuff in electric outlet (up to breaker tripping) but will not use bulbs rated above significantly above fixture spec – Jeffrey Oct 30 '17 at 19:13
  • Which is probably a bad assumption, being that there are outlet adapters you can screw into a light fixture's bulb socket... – R.. Oct 31 '17 at 1:07
2

They don't. Wiring to a light fixture must be the same gage as the circuit proper (12AWG for 20A).

Wiring inside a fixture can be a smaller gage, but those follow inside-appliance rules, which come from UL, not Code.

There's also a set of deep-arcana buried in Code called the "tap rules", but good luck finding a home inspector who will let you use tap rules in a residence in his jurisdiction.

  • Actually, tap rules do come into resi work, usually when you have several builtin cooking appliances that are taken together as a single range load and put on a range circuit (40/50A) – ThreePhaseEel Oct 31 '17 at 1:20
  • Correct the only way around this would be to put a fuse in the plug to protect the lighter appliance wire.... the UK is the only country that I am aware of that does this and you pay for it in the size of the plug. – dougal 5.0.0 Oct 31 '17 at 15:28
  • @ThreePhaseEel Wait, so past the 6AWG junction box, you can run 12AWG to each of three burners/ovens if they're <4kW each? Whoa... – Harper Oct 31 '17 at 15:29
  • @dougal4.0.0 UK has a very interesting and unique view on the tap rules... Those fuses in the plug are the key to it. I wonder, since many US plugs also have GFCI protection in the plug (well 6" up the cord)... Put overcurrent protection there too. – Harper Oct 31 '17 at 15:32
  • @Harper -- yep on the 12AWG -- this is 210.19(A)(3) Ex 1 in the Code – ThreePhaseEel Oct 31 '17 at 22:28
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The primary factor limiting the amount of current wires can carry is the amount of heat that wires can safely dissipate without getting too hot. Wires which are exposed to ambient air can dissipate a lot more heat than wires that are buried within a mass of insulation. Rather than requiring different sizes of wiring based upon the level of exposure to ambient air or flammable objects that could ignite if a wire gets too hot, the Standard generally mandates the same size of wiring regardless.

Within a luminary, however, it's practical to define requirements in more functional terms. The wiring within a luminary should be large enough to avoid overheating even if the filament of a dying bulb falls in such a way as to draw any excessive amount of current that doesn't trip the breaker. The size of wire necessary to accomplish that within a luminary, however, would be much smaller than would be required to safely carry current through an insulated wall.

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