NEC 590: Temporary Wiring, GFCI on the Job Site, and the 90-Day Rule
Every candidate has worked under temporary wiring, which is exactly why the exam asks about it: everyone has seen it done, and about half have seen it done wrong. Four rules cover most of what gets asked, and one number, 90 days, gets asked more than the other three combined.
Last reviewed September 2026
The 90-day rule
Section 590.3(B) carves out one category with its own clock: temporary installations for holiday decorative lighting and similar purposes may stay for a period not exceeding 90 days. The exam offers 30, 60 and 120 as the wrong answers, and it also likes to describe a retail store’s seasonal display and wait for you to apply the “remove immediately” rule instead. Decorative lighting gets 90 days. Everything else gets removed when its job is done.
GFCI on everything workers plug into
Under 590.6(A), every 125-volt, single-phase, 15-, 20- and 30-ampere receptacle that is not part of the permanent wiring and is in use by personnel gets ground-fault protection. A GFCI breaker, a GFCI receptacle, or a listed cord set with built-in protection all count. The exam will offer you an exception for receptacles on a generator or a two-wire circuit. Do not take it; the rule is written to cover the plugs people use.
Lights and plugs on separate circuits
Section 590.4(D) keeps receptacles off the circuits that feed temporary lighting. The picture that makes it stick: a saw trips the breaker, and forty people are standing in the dark holding spinning blades. Separate circuits, always, and the string lights stay on when the tool trips.
Cords and lamps
- Flexible cords are hard-usage or extra-hard-usage types, and where they run through doorways or across traffic they are protected from damage (590.4).
- Lamps for general lighting are guarded against contact and breakage, by a guard or by the fixture's construction (590.4(F)).
- Splices and boxes: temporary wiring may be run with fewer boxes than permanent work in some cases, but exposed conductors and unprotected splices are never permitted.
Where candidates lose the point
- Answering 30 or 60 days for holiday lighting. It is 90.
- Applying the 90 days to a construction site. Construction wiring comes down when the work is finished, not on a calendar.
- Accepting an exception to job-site GFCI. Every 125-volt 15, 20 and 30 amp receptacle workers use is protected.
- Putting temporary lights on the receptacle circuit. Tripped tool, dark floor.
- Allowing a light-duty cord because the run is short. Hard usage or tougher, protected at the pinch points.
Special occupancies are 42% missed in our data
Temporary wiring, hazardous locations and pools sit in the section candidates study least. The free diagnostic shows whether it is yours.