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I helped my grandpa install a light fixture in his kitchen(he's in the process of remodeling) and noticed the wiring was a little weird. The wiring is going from the fixture to the switch which I thought, and a google search confirmed, is supposed to go like this:

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However, this is how the existing fixture was wired and how my grandpa insists I wire the new fixture:

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The wiring diagram at the top of this page belongs to do-it-yourself-help.com. The original can be found here: https://www.do-it-yourself-help.com/wiring_switches.html

I'm not good with diagrams, but whats happening is the hot is coming from the breaker box(the neutral too, but it's ignored) going into the fan, and then out of the fan to the switch. The switch is then sent to neutral on the breaker box.

He's going to be due for an inspection once all the wiring is done and I'm concerned about this. The guy who did his wiring said he did all rooms like this. This area is under NEC 2005

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    Doing it the second way has the downside that the fixture is always hot - I.e. you couldn't easily remove a broken lightbulb without turning off the breaker whereas the code compliant way allows you to remove all "hotness" from the fixture by turning off the switch.
    – Aaron
    Jun 4, 2014 at 14:48
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    The way your grandfather describes is a lot like how you would have wired it with knob and tube wiring. This is not how you wire things anymore.
    – Brad Gilbert
    Jun 4, 2014 at 16:00

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What is being created is a "switched neutral," which is not proper and not allowed according to the NEC. It's possible, but highly unusual, to run the hot through the switch instead of the neutral and create a switched hot with a separate neutral path back to the breaker. That would be better than a switched hot, but may still be disallowed by NEC and I would never encourage anyone to wire a fixture that way either. The wiring shown in your first diagram, or wiring that runs the line to the switch first and switches the hot there, are the proper methods to wire a switched light.

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There's a few issues with the second diagram.

First, you're switching the grounded (neutral) conductor. This is not allowed, since it would mean the fixture is always hot.

National Electrical Code 2014

Chapter 4 Equipment for General Use

Article 404 Switches

I. Installation

404.2 Switch Connections.

(B) Grounded Conductors. Switches or circuit breakers shall not disconnect the grounded conductor of a circuit.

Exception: A switch or circuit breaker shall be permitted to disconnect a grounded circuit where all circuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.

Second, you're not running all circuit conductors in the same cable. This is not allowed because it could result in phantom voltages, and undetected faults.

National Electrical Code 2014

Chapter 3 Wiring Methods

Article 300 Wiring Methods and Materials

I. General

300.3 Conductors

(B) Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment-grounding conductors and bonding conductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord, unless otherwise permitted in accordance with 300.3(B)(1) through (B)(4).

See this answer for more detail.

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  • For non-metallic cable (not in any form of metallic conduit), you don't have to run all conductors together. That requirement has more to do with unbalanced currents inductively heating metal raceways and the like. Doesn't mean the second way isn't wrong (it is -- there is no way to de-energize the fixture), but it's not because of phantom voltages.
    – Julie in Austin
    Jun 4, 2014 at 4:44
  • @JulieinAustin The NEC seems fairly clear about this, see 300.3(B).
    – Tester101
    Jun 4, 2014 at 10:07
  • @JulieinAustin Actually it has more to do with stray capacitance and inductance. Try to receive a faint AM station with your radio while near it, and you may be able to hear the difference.
    – Brad Gilbert
    Jun 4, 2014 at 16:06
  • @Tester101 - 300.3(B) states that there are exceptions to that requirement, and says where they can be found. One of them is in 300.3(B)(3). That then takes you to 300.20. Of course, consult with the AHJ because this is a horrible wiring method anyway and the AHJ may not allow it.
    – Julie in Austin
    Jun 16, 2014 at 18:01
  • @JulieinAustin I don't see how 300.20(B) makes any difference.
    – Tester101
    Jun 17, 2014 at 12:00

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