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How do you size the branch circuits for sign or outline lighting use?
First, you must characterize the load. For calculation purposes, these circuits are continuous [600.5(B). That means you apply the 125% rule. If you don’t know what this rule means, how can you quickly find it in the NEC? This is a branch circuit, and the branch circuit requirements are in Article 210. Part II is titled “Branch-Circuit Ratings” and that’s where you’ll find this rule.
This rule requires you to multiply the load by 125% and then select a conductor with enough ampacity to carry the resulting value.
In reality, this exercise imposes fairly strict limits on how many lights you can have on that circuit. The limit is 20A [600.5(B)(2)], unless the circuit supplies neon lights. In the latter case, the limit is 30A [600.5(B)(1)].
If you need a convenience receptacle near the same location, you’ll have to run that on a different circuit. The signs or outline lighting cannot be plug and cord-connected (though a system may have its own wiring connectors); the wiring method must terminate within a sign, an outline lighting enclosure, suitable box, or conduit body 600.5(C)(1)].
When you run that other circuit, don’t run it through any of the enclosures of signs or outline lighting. You can, however, run sign or outline lighting circuits of one system through enclosures of another sign or lighting system [600.5(C)(2)]. You can also run sign or outline lighting conductors through a support pole [600.5(C)(3)].