Temporary Wiring and Available Fault Current (NEC 590 and 110): Jobsite Power Rules for Apprentices

Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your employer’s safety program and OSHA rules on the jobsite.

When temporary wiring is allowed

Temporary wiring is tied to a purpose. Construction wiring stays only for the duration of the work, holiday decorative lighting has a 90-day limit, and the wiring is removed when its purpose ends.

Use	How long it may stay
Construction, remodeling, maintenance, repair or demolition	For the length of the work
Holiday and similar decorative lighting	Not more than 90 days
Emergencies, tests, experiments and developmental work	During the emergency or for the duration of the test, experiment or developmental work
Any of the above, after the work or purpose is done	Remove it immediately

GFCI on the jobsite

All 125 V, single-phase, 15, 20 and 30 A receptacles that are not part of the permanent wiring and are used by personnel need GFCI protection for personnel, subject to the NEC exceptions. Other temporary receptacles have their own protection rules. An assured equipment grounding conductor program is not a blanket substitute for GFCI; use it only where the specific receptacle and installation qualify under the rule that governs them. OSHA’s construction rules are a separate requirement on the jobsite, so the job has to meet both.

In practice that means:

• A temporary power pole or spider box can provide required GFCI protection through GFCI receptacles or appropriately installed upstream GFCI devices. Verify protection for each applicable outlet before use.
• A permanent receptacle used for construction power may also need GFCI protection under the NEC or OSHA rules. Check which rule applies before workers plug into it.
• A listed portable GFCI cord set can provide personnel protection in installations where the applicable rules permit its use. It does not automatically make an otherwise noncompliant receptacle installation acceptable. Plug it in at the receptacle end, closest to the power source, so it covers the extension cord and the tool, not just the tool. It protects only what is downstream of it and must be used as its listing and the applicable rules allow. It does not fix other receptacle, grounding or wiring problems.

Other temporary wiring rules apprentices get checked on

• Lamps for general lighting must be protected from accidental contact and breakage by a suitable luminaire or guarded lampholder. Do not hang temporary lights by their cords unless they are designed for it.
• Cords and cables must be protected from damage where they pass through doorways, pinch points and traffic.
• Branch circuits originate at an approved power outlet or panelboard and use a wiring method the NEC permits for temporary work. On construction sites, do not install receptacles on a branch circuit that supplies temporary lighting, so a tripped tool circuit does not leave the crew in the dark. Temporary receptacles must be grounding-type, with their grounding contacts connected to an approved equipment grounding path. That path may be a separate equipment grounding conductor or a qualifying wiring method, such as a permitted metal raceway. Follow the applicable NEC and OSHA requirements.
• Disconnects. Suitable disconnecting switches or plug connectors are required to cut off ungrounded conductors of each temporary circuit.

Available fault current

Available fault current is the prospective short-circuit current at a specified point in an electrical system. For a three-phase bolted fault, its magnitude depends on the available sources and the impedance between those sources and the fault location. In a simple radial system it is generally highest near the transformer and decreases farther along the conductors. Motors and other sources can add to it, so a real study looks at the whole system.

Three sections in Article 110 turn this into a job requirement: 110.9 and 110.10 for equipment ratings, and 110.24 for the marking.

• Equipment ratings. Breakers and fuses intended to interrupt fault current need an interrupting rating at least as high as the fault current available at their line terminals, unless they are part of a series-rated combination the NEC permits (NEC 110.9). Equipment and assemblies such as panelboards also need a short-circuit current rating that is adequate for the available fault current (NEC 110.10). These are two separate checks.
• Marking. Service equipment in other than dwelling units must be field marked with the available fault current and the date it was calculated (NEC 110.24). The marking has to be durable, and the calculation documented and available to those authorized to design, install, inspect, maintain or operate the system. There is a limited exception for some industrial installations where only qualified persons service the equipment, so check the 2026 wording before you rely on it. When a modification affects the available fault current at the service, verify or recalculate that current as necessary to confirm the service equipment ratings remain adequate, and update the required field marking to reflect the revised fault current.

Worked example: fault current at a transformer

A 75 kVA, three-phase, 208 V transformer has an impedance of 5%. This simplified estimate assumes an unlimited source on the primary side and ignores upstream impedance, secondary conductor impedance and motor contributions. It gives the transformer’s three-phase bolted fault current at its secondary terminals, not the total fault current available everywhere in the installation.

1. Full-load current: 75,000 ÷ (208 × 1.732) = 75,000 ÷ 360.3 = 208.2 A.
2. Convert 5% to a decimal, 0.05. Fault current at the secondary: 208.2 ÷ 0.05 = 4,164 A. Lower impedance gives higher fault current.
3. Under these assumptions, the estimated three-phase bolted fault current at the transformer secondary terminals is about 4,164 A. The calculated 4,164 A is below a hypothetical breaker’s 10,000 A interrupting rating. That is a useful first comparison, but it does not prove the installation is suitable. The actual fault current at the breaker’s line terminals and the short-circuit current rating of the connected equipment must also be evaluated. On a real job, determine the available fault current at the breaker’s line terminals, accounting for applicable source and motor contributions. Check the breaker’s interrupting rating against that value, and separately verify that each relevant equipment assembly has an adequate short-circuit current rating at its installation point.

Check it in one step: 75,000 × 100 ÷ (1.732 × 208 × 5) ≈ 4,164 A, rounded to the nearest ampere. Same answer. For the kVA step on its own, use our transformer kVA calculator.

Check yourself

1. A worker plugs a drill into a 20 A, 125 V receptacle on a construction site’s temporary power pole. Is GFCI protection normally required? Yes. A 125 V, single-phase, 20 A temporary receptacle used by personnel generally needs GFCI protection, subject to the exceptions in the rule.
2. When must temporary wiring come out? Immediately when the work or purpose it was installed for is finished.
3. A transformer’s three-phase secondary full-load current is 208.2 A and its impedance is 5%. Assuming an unlimited primary source and ignoring conductor impedance and motor contribution, what is the estimated bolted fault current at the secondary terminals? About 4,164 A (208.2 ÷ 0.05). This is a terminal estimate, not a full fault current study.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Articles 110 and 590