14.2 Electrical Systems & Temporary Power
Key Takeaways
- Electrical Systems is 3 of 115 items on the NASCLA Commercial General Building exam and does not appear on the AZ ROC trade outline
- Grounding connects a system to earth; bonding connects metal parts together so a fault current can return and open the overcurrent device
- OSHA 29 CFR 1926.404(b)(1) requires either GFCI protection on all 125-volt, single-phase, 15-, 20-, and 30-ampere receptacles used for construction, or a written assured equipment grounding conductor program
- Temporary lighting, cord protection, and the prohibition on using flexible cord in place of fixed wiring are recurring OSHA citations
- Coordinating the service, panel locations, conduit rough-in, and utility timeline is a general contractor duty that drives the project schedule
Enough Electrical to Run a Project
Quick Answer: Electrical Systems is 3 of 115 items on the NASCLA Commercial General Building exam. More importantly, temporary power on the jobsite is governed by OSHA 29 CFR 1926 subpart K, which applies to every project regardless of which exam you took.
Like mechanical and plumbing, electrical work must be performed by the appropriate specialty licensee — C-11, CR-11, or R-11 in Arizona, with a NASCLA electrical exam option available for those classifications at $116. The general contractor's role is coordination, sequencing, and jobsite electrical safety.
Service and Distribution Basics
Power arrives from the utility, passes through the service point, and is distributed:
| Component | Function |
|---|---|
| Service entrance conductors | Utility supply into the building |
| Service disconnect | The main means of disconnecting all power |
| Main distribution panel / switchgear | Divides the service into feeders |
| Feeders | Supply downstream panelboards |
| Branch circuits | Supply the individual outlets and equipment |
| Overcurrent protective devices | Breakers and fuses protecting conductors from overload and short circuit |
Common commercial voltages: 120/208V three-phase wye for general lighting and receptacles in smaller buildings, and 277/480V three-phase wye for larger buildings, with 277V lighting and 480V motor loads stepped down through transformers for 120V receptacle circuits.
Working clearance is the coordination item a general contractor most often gets wrong. Electrical equipment requires clear working space in front of it — a defined depth depending on voltage and the conditions on the opposite side, plus a defined width and height — and that space must be clear and unobstructed. Storing material in front of a panel, or letting an architect place a panel in a closet without the required clearance, produces a failed inspection at the worst possible time.
Grounding Versus Bonding
These two words get used interchangeably in the field and are distinct on the exam.
| Grounding | Bonding | |
|---|---|---|
| What it does | Connects the electrical system or equipment to earth | Connects metallic parts to each other to establish an effective fault-current path |
| Purpose | Stabilizes voltage; provides a path for lightning and line surges | Ensures fault current returns to the source so the breaker opens quickly |
| Typical elements | Grounding electrode (rod, concrete-encased electrode, water pipe), grounding electrode conductor | Equipment grounding conductors, bonding jumpers, metallic raceway continuity |
The insight the exam wants: the earth is not a reliable fault-current path. A ground rod does not trip a breaker. What clears a ground fault is the low-impedance bonded path back to the source, which lets enough current flow to open the overcurrent device. That is why a properly sized equipment grounding conductor matters more to safety than the resistance of the ground rod.
Temporary Power and OSHA Subpart K
Every project has temporary power, and it is a reliable source of OSHA citations.
The GFCI Requirement — 29 CFR 1926.404(b)(1)
The employer must use one of two approaches for 125-volt, single-phase, 15-, 20-, and 30-ampere receptacle outlets used by employees for construction work and not part of the permanent wiring:
| Option | What it requires |
|---|---|
| Ground-fault circuit interrupters | GFCI protection on all such receptacles |
| Assured equipment grounding conductor program | A written program, a competent person, and a documented schedule of continuity and terminal tests on cord sets, receptacles not part of the permanent wiring, and cord-and-plug-connected equipment |
The assured grounding program is genuinely burdensome — tests before first use, after any repair, after suspected damage, and at least every three months — which is why nearly every contractor chooses GFCI protection. The exam item is that there are exactly two compliant options and that the alternative to GFCI must be written and documented.
Other Recurring Requirements
| Rule | Detail |
|---|---|
| Cords not a substitute for fixed wiring | Flexible cord may not be run through holes in walls or ceilings, through doorways or windows, or be attached to building surfaces as permanent wiring |
| Cord condition | Damaged, spliced, or repaired cords with missing ground pins must be removed from service |
| Guarding of live parts | Panels and boxes require covers; openings must be closed |
| Lockout/tagout | Circuits de-energized before work, with controls locked and tagged |
| Temporary lighting | Bulbs guarded against contact; lamps not suspended by their cords unless designed for it |
| Overhead power lines | Minimum approach distances observed for equipment and materials |
| Wet locations | Equipment listed for the location; monsoon rain turns a whole site into a wet location |
Warning: the most common serious jobsite electrical incident is not a wiring error — it is contact between a boom, a ladder, or a length of pipe and an overhead power line. Line clearance planning belongs in the site logistics plan, not in the electrician's scope.
What the General Contractor Coordinates
| Item | Why it drives the schedule |
|---|---|
| Utility service application and timeline | Utility lead times routinely exceed all other long-lead items and must be started early |
| Transformer pad and service routing | Requires civil, structural, and utility coordination before slab work |
| Panel and equipment locations with working clearances | A late relocation cascades through framing, drywall, and finishes |
| Underslab and in-wall conduit rough-in | Must precede the pour and the drywall hold points |
| Temporary power and lighting | Needed before most interior trades can work productively |
| Fire alarm, low voltage, and controls | Integrates with the mechanical, fire-protection, and life-safety systems |
The utility timeline is the one that sinks schedules. A building can be complete and unable to receive a certificate of occupancy because permanent power was applied for two months late — and no amount of crew overtime recovers it.
Exam tip: for the three NASCLA electrical items, focus on the general contractor's perspective: coordination, working clearances, temporary power safety, and grounding versus bonding. Conductor sizing and calculation questions belong to the electrical exam, not this one.
What are an employer's two compliant options under 29 CFR 1926.404(b)(1) for 125-volt, 15-, 20-, and 30-ampere construction receptacles?
What actually causes a breaker to open during a ground fault?
A superintendent runs a flexible extension cord through a hole drilled in a corridor wall to power a temporary office for the remainder of the job. Why is this a violation?
Which coordination item most often delays a certificate of occupancy on a commercial project?
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