What is the most important sentence to know in the entire National Electrical Code (NEC)?
Thursday, November 28, 2019
It’s near the beginning of the code, at 110.3(B) Installation and Use, and says:
"Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling."
Anything electrical installed as part of the construction of a house must be listed or labeled, which are two levels of approval by an agency such as Underwriter’s Laboratory (UL). Part of the approval process is verifying the correct and safe way to install it, so this sentence says that the manufacturer’s instructions trump the code for correct installation. So always be sure to read the folded up paper in the box.
Unfortunately, we have found that the local building inspector’s interpretation of what should be done may turn out to be the final mandate. As an example, some manufacturers of electric tankless water heaters specify that a pressure relief valve is not necessary, but the local inspector may require it anyway.
Also, see our blog post What does "listed and labeled" mean for an electrical component?
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