Temporary Service for a Construction Site

Purpose

A temporary service is a service. It gets a load calculation, a permit, an inspection, a grounding electrode and a utility account, and the only temporary thing about it is that it has a scheduled death. The word in front of it causes the trouble: crews order "a 100 amp temp" the way they order a dumpster, and then a winter pour arrives with two heaters and a thaw blanket on a service nobody sized.

The second reason this is a standing instruction is that a temp is sized against a load that CHANGES during its life, which no permanent service does. A construction site's peak arrives on the day the general contractor decides to keep concrete warm, and nobody calls the electrician first. Building a re-check into the middle of the job turns that from a trip into a phone call.

Scope

Covers obtaining, sizing, installing and retiring the temporary SERVICE for a construction site, single-phase 120/240 V, 200 A or smaller, residential and light commercial construction.

Not covered: the temporary distribution built downstream of it, the ground-fault protection regime for personnel during construction, and the teardown of that distribution, all of which are owned by electrical-temporary-power-setup-and-teardown. Also not covered: the permanent service cutover schedule (electrical-utility-cutover-scheduling-and-customer-notification).

Roles and handoffs

The first handoff fails, then fails again mid-job. A load list given once at bid time is a snapshot of a site that has not started.

Procedure

1. Compute the construction load and size the service from it, not from the permanent service size. List every load the site will actually run, separate the ones running three hours or more, and size the device at 125 percent of that continuous portion plus the non-continuous portion. Accept when the computed device requirement in amps is written with the load list that produced it and the next standard size named. Wrong looks like a temp sized to match the future house service; stop rule, no load list means no service size is quoted, because the number that matters is the site's peak. Hazard: none, a desk step, but its absence is what puts a heater on an overloaded temp in January.

2. Site the pole or pedestal against the construction sequence, not today's site. Walk where the excavator, the crane swing, the trucks and the driveway will be for the whole job and place the service clear of all of them. Overhead clearances under NEC 230.24 apply to a temporary drop exactly as to a permanent one, in the edition your authority having jurisdiction has adopted. Accept when the location is marked, the drop path is clear of future crane and truck routes, and the clearance over each surface is stated. Wrong looks like a pole at the edge of an excavation not yet dug; stop rule, a location that conflicts with the sequence gets moved on paper now, not by a utility crew later. Hazard: setting a pole means digging, so locate buried utilities before anything goes in the ground.

3. Apply to the utility with the right customer of record, and permit the temp separately. The account holder is normally the builder rather than the future owner, and getting that wrong creates a billing dispute that outlives the job. Accept when the utility application number, the account holder's name and the temp permit number are all on the job. Wrong looks like a temp riding on the permanent permit because "it is the same address"; stop rule, no temp permit means no inspection, and no inspection means no energization. Hazard: none, a desk step.

4. Build the service side to Article 230, and give it a grounding electrode. NEC Article 590 permits temporary installations for the period of construction, and 590.4 sends the service itself back to Article 230, so this is real service work. Size conductors from NEC Table 310.16 at the termination temperature the equipment is marked for, because the dwelling service table does not apply to a construction temp. Accept when conductor size, termination marking, electrode type and GEC size are each written with the table row they came from. Wrong looks like conductors sized off the dwelling table, which runs smaller; stop rule, a size you cannot justify from a row is not installed. Hazard: driving a rod near an active site, so nothing is driven blind and the drop stays unconnected.

5. Get it inspected, then energized, and verify before anyone plugs anything in. After the utility connects, read each leg to the grounded conductor and leg to leg, confirm the disconnect opens both legs, and confirm the grounding electrode conductor is continuous to the rod clamp. Accept when both legs sit within the utility's stated tolerance of nominal, the disconnect kills both legs at the load side, and the electrode path reads low and stable. Wrong looks like a temp handed over before the disconnect was ever operated; stop rule, a disconnect that does not open both legs is not put in service. Hazard: energization on an open site with trades present, so the enclosure is closed, the area cleared and the drop confirmed clear of ladder and scaffold routes first.

6. Re-run the calculation whenever the site adds load, before the load arrives. Set a standing trigger with the general contractor: any heater, trailer, welder, pump or lift gets a call before it is plugged in. Accept when each addition has a recomputed device requirement written beside the original, clearing the installed device or not. Wrong looks like a site that has grown two heaters since energization with no recalculation; stop rule, an addition that pushes the requirement past the installed device does not get connected, and the choice is upsizing the temp with a new utility order and inspection, or fuel-fired heat. Hazard: unvented fuel-fired construction heat makes carbon monoxide inside a closed shell, so where that is the alternative it is named as the general contractor's hazard with monitoring, not an electrical shrug.

7. Retire the temp at permanent energization, and prove it is dead before it is dismantled. The temporary installation comes out when construction is complete under NEC Article 590, and its removal needs its own utility disconnect order rather than a crew with cutters. Prove dead live-dead-live, tester checked on a known live source before and after, per NFPA 70E-2021, 120.5, with practices at 29 CFR 1910.333(b)(2) by a qualified person per 1910.332 and 1910.399, and on a construction site lockout and tagging of circuits is 29 CFR 1926.417. Accept when a utility disconnect is confirmed and every conductor at the temp reads 0 V with the after-check passing. Wrong looks like a crew assuming the temp went dead when the house was energized, which is the single most common way somebody gets hurt on this equipment; stop rule, no confirmed disconnect and no proved-dead reading means nothing is cut. Hazard: the pole conductors can remain live long after the permanent service is on, and the two events are unrelated.

When the site does not match the assumption

The general contractor wants power before the permit is issued. The answer is no, because the utility will not energize without the inspection anyway. Offer a generator instead, with its fuel, siting and carbon monoxide hazards owned by whoever supplies it.

The temp has to stay past the completion date. Normal, and a permit question rather than a judgment call. Extend the temp permit before the original expires and record the new date, because an expired temp permit is what an inspector notices at the final.

The record this produces

• The load list and the computed device requirement in amps, continuous and non-continuous separated. Read at step 6 every time the site grows, and useless if only the answer was kept.
• Pole or pedestal location with the sequence conflicts checked. Read by the utility and by whoever is asked to move it.
• Utility application number and account holder name. Read when the billing dispute arrives.
• Conductor size, termination marking, electrode type, GEC size, each with its table row. Read by the inspector, and by the next shop if the temp gets extended.
• Every step 6 recalculation, dated, including the ones that cleared. Read after any trip, because it shows whether the site outgrew the service or something failed.

Worked pass: single-family new build, winter start, 100 A temporary service

Step 1: the general contractor's list is a table saw at 15 A and a compressor at 15 A on 120 V, both 1,800 VA; temporary lighting at 1,000 VA; a site trailer with a 5,000 VA electric heater and 1,500 VA of outlets; and two 1,500 VA heaters for the pour, 3,000 VA together. Connected total is 1,800 plus 1,800 plus 1,000 plus 6,500 plus 3,000, which is 14,100 VA, and 14,100 divided by 240 is 58.8 A of connected demand.

The device requirement is the number that governs. Continuous loads are the trailer heater at 5,000, the lighting at 1,000 and the pour heaters at 3,000, which is 9,000 VA. Non-continuous is the two tools at 3,600 plus the trailer outlets at 1,500, which is 5,100 VA. So 1.25 times 9,000 is 11,250, plus 5,100 gives 16,350 VA, and 16,350 divided by 240 is 68.1 A. A 70 A device would satisfy it; the shop installs 100 A for headroom and writes down why.

Step 2: pole set street-side, 22 ft clear of the planned excavation edge and outside the crane radius, drop crossing only lawn at a measured 13 ft 4 in. Step 3: utility application filed with the builder as account holder, temp permit pulled separately. Step 4: equipment marked 75 C, so 3 AWG copper covers 100 A on Table 310.16; marked 60 C only, the same 100 A would have needed 1 AWG copper, and the marking was read rather than assumed. One rod, and Table 250.66 at 3 AWG copper service conductors gives 8 AWG copper for the grounding electrode conductor; the rod-only cap in 250.66 is a ceiling of 6 AWG, not a floor, so it does not shrink the 8 AWG the table gave.

Step 5: inspected and energized. Legs read 121.1 V and 120.4 V to the grounded conductor and 241.3 V leg to leg, the disconnect opens both legs at the load side, and continuity to the rod clamp reads low and stable.

Step 6 fails in January. The general contractor adds a second 5,000 VA heater and a 3,000 VA thaw blanket, both continuous, and calls before plugging in, which is the point of the trigger. Continuous is now 9,000 plus 5,000 plus 3,000, which is 17,000 VA, and non-continuous is unchanged at 5,100 VA. So 1.25 times 17,000 is 21,250, plus 5,100 gives 26,350 VA, and 26,350 divided by 240 is 109.8 A against a 100 A installed device. Stop rule taken: neither load gets connected. The general contractor chooses fuel-fired heat over a utility upsize and re-inspection, and that choice is written down with ventilation and carbon monoxide monitoring named as theirs.

Step 7 comes in April. The permanent service is energized on a Tuesday and the temp is NOT dead: a separate utility disconnect order is raised for Thursday, every conductor at the pole is proved dead with the tester checked before and after, and only then does the pole come down.

The step 6 failure earns this SOP its place. The trip it prevented would have landed at 6 am on a pour day, on a service correctly sized for the job as it was described in October.

References

• NEC Article 590, Article 230, Table 310.16 with 110.14(C), Table 250.66 with its electrode caps, and 230.24, in the edition your authority having jurisdiction has adopted.
• 29 CFR 1910.333(b)(2) with qualified person at 1910.332 and 1910.399 for general industry, and 29 CFR 1926.417 for lockout and tagging of circuits on construction work.
• NFPA 70E-2021, 120.5, for the live-dead-live sequence, in the edition your electrical safety program adopts.
• See related: electrical-temporary-power-setup-and-teardown for the distribution, its ground-fault regime and its teardown.