NEC Article 590: Temporary Wiring for Construction Sites
Understand the requirements for temporary electrical installations on construction sites. From GFCI protection to cord and cable rules, learn what it takes to provide safe temporary power that passes inspection.
Construction Site Hazards
Construction sites present unique electrical hazards: wet conditions, damaged cords, missing covers, and untrained personnel. OSHA statistics show electrocution is one of the "Fatal Four" causes of construction deaths. Proper temporary wiring saves lives.
In This Guide
NEC 590 Overview
NEC Article 590 covers temporary electrical power and lighting installations during construction, remodeling, maintenance, repair, or demolition of buildings or structures. It also covers similar installations for experimental/developmental work, emergencies, and tests.
Scope of NEC 590
- 590.1: Scope - temporary installations during construction
- 590.2: Time constraints - 90 days for holiday displays, duration of construction
- 590.3: General requirements - must comply with other NEC articles
- 590.4: General requirements for wiring methods
- 590.5: Lamps and lampholders
- 590.6: Ground-fault protection for personnel
Duration Limits
Temporary wiring has specific time limits per NEC 590.3(B):
- Construction: During the period of construction, remodeling, or demolition
- Holiday lighting: Maximum 90 days
- Emergencies: Duration of the emergency
- Tests/experiments: Duration of the testing
What Article 590 Does NOT Cover
Article 590 applies to temporary wiring installations. It does not relax requirements for:
- • Permanent wiring being installed
- • Equipment grounding requirements
- • Overcurrent protection sizing
- • Conductor ampacity requirements
GFCI Protection Requirements
GFCI protection is critical for construction site safety. NEC 590.6 requires GFCI protection for most temporary power receptacles.
NEC 590.6(A) - Required GFCI Protection
GFCI protection is required for all 125V, single-phase, 15A, 20A, and 30A receptacle outlets that are:
- ✓ Not part of the permanent wiring of the building
- ✓ Used for temporary power during construction
- ✓ Including cord sets (extension cords) used on the site
GFCI Protection Methods
| Method | Location | Advantages | Considerations |
|---|---|---|---|
| GFCI Breaker | Panel | Protects entire circuit | Nuisance trips on long runs |
| GFCI Receptacle | First outlet | Protects downstream | Must be accessible for reset |
| Spider Box | Distribution point | Multiple protected outlets | Portable, rugged design |
| Cord-Type GFCI | Cord end | Portable protection | Subject to damage |
Spider Boxes (Temporary Power Distribution)
Spider boxes (also called power distribution boxes or temp power centers) are common on construction sites. They typically include:
- Built-in GFCI protection for 15A and 20A receptacles
- 30A or 50A input from temporary panel
- Multiple 20A duplex outlets (typically 4-6)
- Circuit breaker protection for each outlet group
- Rugged enclosure rated for job site use
Assured Equipment Grounding Conductor Program
NEC 590.6(B) provides an alternative to GFCI protection: the Assured Equipment Grounding Conductor Program (AEGCP). However, this is rarely used because GFCI protection is simpler and more reliable.
AEGCP Requirements (If GFCI Not Used)
If GFCI protection is not provided, all of the following must be implemented:
- 1. Written program: Document describing procedures
- 2. Designated person: Competent individual responsible for program
- 3. Daily visual inspection: Check cords and equipment before use
- 4. Testing intervals: Test equipment grounding before first use, after repair, and quarterly
- 5. Recorded tests: Document all test results
- 6. Remove defective equipment: Tag and remove from service
Why GFCI is Preferred
The AEGCP was common before reliable GFCI devices were available. Today, most contractors use GFCI protection exclusively because:
- • GFCI devices are inexpensive and reliable
- • AEGCP requires significant documentation and testing
- • GFCI provides real-time protection against ground faults
- • OSHA inspectors prefer GFCI protection
Cord and Cable Requirements
Extension cords and flexible cables are extensively used on construction sites. NEC 590.4 establishes requirements for their use.
Permitted Cord Types
| Cord Type | Description | Typical Use |
|---|---|---|
| S, SO, ST, STO | Hard service cord | General construction, power tools |
| SJ, SJO, SJT | Junior hard service | Light-duty tools, appliances |
| W | Extra hard usage | Severe duty, outdoor |
| G, G-GC | Portable power cable | High amperage feeders |
Cord Requirements
- Equipment grounding conductor: Required in all cords for construction
- Strain relief: Proper cord connections with strain relief
- No splices: Cords shall not be spliced (repaired ends are splices)
- Grounding-type: All cord caps and receptacles must be grounding type
- Protection from damage: Route to avoid physical damage
Common Cord Violations
- ✗ Missing ground prong (cut off or broken)
- ✗ Damaged outer jacket exposing conductors
- ✗ Electrical tape "repairs" (constitutes splice)
- ✗ Cords run through doorways or windows where they can be damaged
- ✗ Undersized cords for the load
- ✗ Cords in water or wet conditions without proper rating
Extension Cord Sizing
Extension cords must be sized for the load and length. Minimum wire gauge recommendations:
Up to 50 ft
16 AWG for 10A
14 AWG for 15A
12 AWG for 20A
50-100 ft
14 AWG for 10A
12 AWG for 15A
10 AWG for 20A
100-150 ft
12 AWG for 10A
10 AWG for 15A
8 AWG for 20A
Temporary Services and Feeders
Temporary services must comply with NEC Article 230 (Services) with modifications allowed by Article 590.
Temporary Service Requirements
- Service equipment: Must meet Article 230 requirements
- Grounding: Full grounding electrode system required
- Overcurrent protection: Standard requirements apply
- Disconnect: Service disconnect required per 230.70
- Metering: Typically required by utility
Typical Temporary Service Sizes
Residential Construction
- • 100A or 200A service
- • Single-phase 120/240V
- • Ground rod required
Commercial Construction
- • 200A-400A typical
- • Three-phase 208V or 480V
- • May use permanent service
Feeder and Branch Circuit Protection
Temporary feeders and branch circuits must have overcurrent protection per NEC 240. Cable assemblies used for feeders must be:
- Listed for the purpose (Type G, Type W, or suitable cord)
- Protected from physical damage
- Supported at intervals per manufacturer specifications
- Not laid in water or subject to vehicular traffic without protection
Temporary Lighting
NEC 590.4(F) and 590.5 cover temporary lighting installations on construction sites.
Lamp and Lampholder Requirements
- Guards required: All lamps must have guards unless recessed or more than 7 feet above working surface
- Grounded lampholders: Required for metal shell lampholders
- Strain relief: Connections must have proper strain relief
- Wet location rating: Required if subject to weather or wet conditions
Temporary String Lighting
Temporary string lights are common on construction sites. Requirements include:
- Listed for purpose: Must be listed temporary lighting string
- Guard requirement: Each lamp must have a protective guard
- Support: Must be properly supported, not by cord
- GFCI protection: Required if 125V, 15A or 20A
Receptacle Installation
Temporary receptacles have specific installation requirements per NEC 590.4(D).
Receptacle Requirements
- Grounding-type required: All receptacles must be grounding type
- Box mounting: Receptacles must be installed in boxes
- Weatherproof: In wet locations, must be weatherproof with cover closed
- GFCI protection: Per 590.6 for 125V, 15-30A receptacles
Temporary Panel/Box Installation
Temporary panels and boxes must be:
- ✓ Mounted on stable support (post, structure, or stand)
- ✓ Protected from physical damage and weather
- ✓ Accessible for operation and inspection
- ✓ Properly grounded with equipment grounding conductor
- ✓ Labeled with voltage and current ratings
Inspection Requirements
Temporary electrical installations typically require inspection just like permanent wiring. Requirements vary by jurisdiction.
Temporary Power Inspection Checklist
NEC Reference On Every Job Site
Access NEC 590 temporary wiring requirements and all electrical codes instantly with Ampora. Get code answers while you work.