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An interesting question arose elsewhere. Suppose you break out a "New, Old Stock" item. It is UL-approved and has UL-approved instructions.

The instructions say to do X.

However, since the item was UL-approved, NEC has changed. Doing X is now explicitly forbidden, and you must do Y.

Which one controls? Are you obliged to follow the instructions?

Are you obliged to follow current NEC?

Are you obliged to throw the item in the trash and get a modern one whose UL-approved instructions are consistent with NEC?

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    Sounds like the solution may be ask your AHJ. But perhaps this question is itself an XY question: Give us an example (60A heater circuit?) as opposed to generic "UL vs. NEC".
    – manassehkatz-Moving 2 Codidact
    Sep 3, 2020 at 21:22
  • @manassehkatz-Moving2Codidact I don't think it's an XY question; it's asking if there's a rule or general principle as far as which one takes precedence. If there is no such principle, and every AHJ makes these kind of decisions on a case-by-case basis, then that's an answer.
    – Nate S.
    Sep 3, 2020 at 21:27
  • maybe it would help everyone provide a better answer if you provide information on what is "now explicitly forbidden".
    – Programmer66
    Sep 3, 2020 at 22:12
  • @programmer66 I think you would find a hard time finding a forbidden MFG over ride because the word shall in 110.3.B , shall means it is mandatory most apprenticeships at least in my state teach this in the first year.
    – Ed Beal
    Sep 3, 2020 at 23:23
  • Yes- That is the point I am making, What is this forbidden instruction. The reality is NOT to do anything that is unsafe or dangerous even if a code or manufacture's states to do it. But the forbidden instruction is in the question.
    – Programmer66
    Sep 4, 2020 at 0:09

2 Answers 2

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Very simple... 110.3.B states that the manufacturer's instructions shall be followed. That is, of course, for listed equipment.

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  • Even for really old stuff? Are there any time stipulations?
    – isherwood
    Sep 3, 2020 at 21:12
  • I don't think it's always the case that manufacturer instructions override the NEC, or at least not all AHJs interpret it that way. One really common example is older refrigerators that say do not plug into a GFCI in their labeling (because it'll trip), while modern code requires GFCIs everywhere you might put a fridge. My understanding is that the only way to make that work is with an AHJ variance, and not every AHJ will give one.
    – Nate S.
    Sep 3, 2020 at 21:23
  • My two cents - The manufacture did the testing and met the UL requirement for obtaining the UL listing. NEC writes code to states how things should be in a perfect world. The NEC write these codes based on statistics and what is general best practice for doing something, and feedback from manufactures and organizations. They do no testing. That is the reason why a manufacture instructions takes precedence. I see comment about AHJ, which is NOT the NEC
    – Programmer66
    Sep 3, 2020 at 21:27
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    I don't believe NEC code requires a fridge to be on a GFCI, Specific outlet locations in specific areas need to be GFCI protected. Most recommendations I have seen, if possible, the fridge should not be on a GFCI circuit.
    – Programmer66
    Sep 3, 2020 at 21:38
  • NEC does not require GFCI specifically for fridges -- kitchen fridges in dwelling units don't require it since only the countertop receptacles in dwelling unit kitchens require GFCI protection, not receptacles for other things (save for the dishwasher)
    – ThreePhaseEel
    Sep 3, 2020 at 22:58
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We all know the manufacturer's instructions have to be followed. If the items are being reintroduced into the market they's have to be re certified and instructions updated. So I find an new, unopened 48" fluorescent bathroom fixture in my attic that the builder left 55 years ago. It has a two prong outlet built into it. It was UL- approved. I want to install it in my new house being built. The instructions say to hook it up to a hot and neutral and wall mount it. The instructions don't mention anything about a ground or GFCI protection for the outlet. The current code requires both now and I doubt you'd find an inspector who would say the instructions override the current codes.

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  • I agree. You must follow the manufacturer's current instructions for the product, which may have changed since the packaging was printed.
    – A. I. Breveleri
    Sep 3, 2020 at 21:57
  • The manufacturer's instruction does not state NOT to use a ground. Based on the NEC code, you would have the three wires, and connect the ground wire to the fixture frame. You are still following the manufacturer's instruction and supplementing it was additional requirements from the NEC.
    – Programmer66
    Sep 3, 2020 at 22:06
  • @Programmer66 Not really, there's no ground screws on the fixture and no ground on the outlet... it's old, before grounding. diagram shows no grounding.
    – JACK
    Sep 3, 2020 at 22:24
  • @Jack - this is off subject to the original question. In your example, there is no conflict, the fixture has a two wire plug and the outlet is two wire without ground, The ans above provided example of a new house, which would have the ground. All examples and comments so far are not directly related to a "Prohibited" instructions.
    – Programmer66
    Sep 4, 2020 at 0:22
  • @Programmer66 My bad, two prong outlet built into the fixture, not a plug, fixture is hard wired.... if you follow direction, you've installed an outlet that isn't GFCI protected... code forbids that now.
    – JACK
    Sep 4, 2020 at 0:48

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