Detached Structure Feeder Installation

Purpose

A detached garage, shop or barn is not a subpanel in another room. Once the conductors leave the house, three things change and every inspection turns on them: the structure gets one feeder and not several, a disconnecting means of its own at the structure, and its own grounding electrode system with the grounded conductor left unbonded at that end.

The failure this prevents is the trench that gets backfilled at the wrong depth, which is the one defect on this job that cannot be corrected with a screwdriver. Everything else on the list is a fitting or a conductor; cover depth is a day with a shovel, and the customer is watching.

Scope

Covers a single feeder from an existing dwelling or small commercial building to a detached structure on the same premises: route and locate, wiring method and burial depth, the disconnecting means, the separate grounding electrode system, and the energization checks.

Does not cover feeder conductor sizing and panel selection, owned by the subpanel installation and feeder sizing SOP; the load calculation deciding whether the existing service carries the new structure, owned by the addition circuit planning SOP; branch circuits inside the structure, owned by the garage and workshop SOP; or overhead spans, which have their own clearance rules.

Roles and handoffs

Procedure

1. Confirm the structure gets one feeder, and settle what that feeder has to carry before anything else. NEC 225.30 permits a single feeder or branch circuit to supply a building or structure, subject to the exceptions the article lists, so a second run added later for a shop tool is not a small change. Acceptance: the structure's calculated load written, the feeder sized under the subpanel SOP, and a written statement that this is the only supply. Wrong looks like a feeder installed alongside an existing branch circuit nobody removed. Stop rule: an existing supply found at the structure is removed or the design changes. Hazard clause: none here, a desk decision.

2. Get the utilities located and keep the ticket current on the day you dig. Call the one-call service, wait the required period, and have the marks on the ground before a blade moves. Acceptance: an open locate ticket with its number on the job sheet, marks visible and photographed, and the ticket inside its valid period on the day of excavation. Wrong looks like a ticket called two weeks ago with marks washed off by rain. Stop rule: no marks, no digging, whatever the schedule says. Hazard clause: if a gas line is struck, everyone leaves immediately and moves upwind, nobody starts an engine, uses a phone or operates a switch near the break, and 911 and the gas utility are called from a distance; nobody tries to plug or crimp the line.

3. Choose the wiring method and read the required cover depth off the whole row of the table, with the column named. NEC Table 300.5 gives minimum cover for circuits up to 1000 V by wiring method, and the columns differ by a factor of four: direct-burial cable, rigid or intermediate metal conduit, and listed nonmetallic raceway each carry their own figure. The shallow residential row applies only to a branch circuit rated 120 V or less with ground-fault protection and a maximum 20 A device, so a feeder never qualifies for it. Acceptance: method chosen, the table column named on the job sheet, required cover written as a number. Wrong looks like 12 in of cover over a feeder because somebody remembered a residential row. Stop rule: where the route cannot achieve the depth for the chosen method, the method changes rather than the depth. Hazard clause: none here, a table read at a desk.

4. Trench the route, and treat the excavation as an excavation even though it is shallow. Keep spoil back from the lip and provide a means of egress where required. Under 29 CFR 1926.651(c)(2) a stairway, ladder or ramp is required in a trench 4 ft or more deep, and under 29 CFR 1926.652(a)(1) a protective system is required at 5 ft or more unless the excavation is in stable rock. Acceptance: trench cut to the depth giving the required cover plus the bedding the method needs, spoil set back, and any hand-dig zone around a located utility respected. Wrong looks like a machine bucket taken to within inches of a marked line. Stop rule: inside the tolerance zone the digging is by hand, and a line exposed but undamaged is reported to the utility before it is covered. Hazard clause: a shallow trench can still collapse, so nobody works below grade with spoil piled at the lip.

5. Install the conductors and verify the cover with a tape before any backfill goes in. Where conductors emerge from grade they are protected by an enclosure or raceway from below grade to the height the article requires. Acceptance: cover measured with a tape at a minimum of three points including the shallowest, every reading at or above the step 3 figure, photographed with the tape in frame, plus any warning ribbon the local amendment requires. Wrong looks like a depth reported as one number for a trench that rises over a tree root. Stop rule: any reading below the required cover stops the backfill and the trench is deepened, and a shallow section is never averaged against a deep one. Hazard clause: reaching into an open trench puts a hand and a head below grade, so the section is checked for sloughing first and spoil stays back from the edge.

6. Set the disconnecting means at the structure and the grounding electrode system that goes with it. NEC 225.31 requires a means to disconnect all ungrounded conductors supplying the structure, and 225.32 requires it at a readily accessible location outside the structure or inside nearest the point of entrance. NEC 250.32 requires a grounding electrode system at the separate structure, with an equipment grounding conductor run with the feeder, and the grounded conductor not connected to the equipment grounding conductor or to the electrode at that end. Acceptance: disconnect installed, readily accessible and marked; electrode installed and its conductor landed; the neutral bar isolated with the bonding screw or strap removed and that removal recorded. Wrong looks like a factory bonding screw left in a remote panel, which puts neutral current on the grounding conductor and on anything metallic joining the two buildings. Stop rule: a panel whose neutral cannot be isolated is exchanged, never modified by cutting the bar. Hazard clause: driving a ground rod puts a striking tool at head height and can find an unlocated utility, so the rod goes inside the located area and the driver has eye protection and a clear swing.

7. Energize and prove the three things that make this a separate structure, not a long extension cord. Isolate at the source, lock and tag under 29 CFR 1910.333(b)(2), and prove dead on a known live source before and after per NFPA 70E-2021, 120.5 before landing conductors. Acceptance: line-to-line and line-to-neutral voltages read at the remote panel and matching the source; every ground-fault device tripped and reset on its own button; the structure disconnect operated and confirmed to kill every circuit inside; and neutral-to-ground separation confirmed at the remote panel with the neutral bar isolated. Wrong looks like a remote panel that reads correct voltage and still has a bonded neutral. Stop rule: any reading that does not match, or any circuit surviving the disconnect, holds the job open until it is found. Hazard clause: the first energization is done with the structure clear of people, the crew to the hinge side of the panel rather than square in front, and no hand on a conductor when the source device closes.

The record this produces

One detached structure sheet per permit: the statement that this is the only supply and what was removed; the locate ticket number, date and expiry with the photograph of the marks; the wiring method with the table column named and the required cover in inches; the trench depth readings at every measured point with the open-trench photographs; the disconnect location and marking; the electrode type and location; a line confirming the bonding screw was removed and where it was left; and the energization readings.

The inspector reads the trench photographs, which is why they are taken before backfill rather than after a question. The office keeps the bonding-screw line, because a remote panel later serviced by somebody else with a screw back in it is a fault nobody finds by looking.

Worked pass: 60 A feeder to a detached two-bay garage, 96 ft run

Step 1: calculated structure load comes to 48 A, so a 60 A feeder is sized under the subpanel SOP. The garage has a single branch circuit fed from the house, disconnected and removed at both ends so the new feeder is the only supply, with that removal written on the sheet. Step 2: locate ticket opened, marks placed and photographed, ticket valid on the day of digging, with a gas service marked crossing the route 31 ft from the house.

Step 3: direct-burial cable chosen, and the column for direct-burial cable in the table gives 24 in of cover. The shallow 12 in row is read and rejected in writing, because it applies to a residential branch circuit at 120 V or less with ground-fault protection and a 20 A maximum device, and this is a 60 A feeder.

Step 4: machine trenching stops 24 in either side of the marked gas service and that section is hand-dug; the service is exposed undamaged, reported to the utility, and left uncovered until they clear it. Maximum trench depth is 29 in, well under the 4 ft that would require a means of egress, and spoil is kept back from the lip.

Step 5 fails. Cover is measured at three points: 27 in near the house, 26 in at mid-run, and 19 in where the trench crosses a root shelf. The 19 in reading is short of the required 24 by 5 in. Stop rule taken, so no backfill and no averaging. That 11 ft section is cut a further 8 in deeper, giving 27 in, and re-measured before the sand bedding goes in. All three readings are then photographed with the tape in frame.

Step 6: a 60 A disconnect is set on the garage's exterior wall, readily accessible and marked. A ground rod is driven inside the located area with the grounding electrode conductor landed at the panel. The factory bonding screw is removed from the remote panel's neutral bar and taped inside the enclosure door for the next person to find, and that is written on the sheet.

Step 7: isolated, locked and tagged, proved dead on a known live source before and after. The remote panel reads 240 V line to line and 120 V line to neutral on both legs, matching the source. Both ground-fault devices trip and reset on their buttons. The structure disconnect is opened and every circuit in the garage confirmed dead, then closed. Neutral-to-ground separation is confirmed at the remote panel with the neutral bar isolated.

References

• NFPA 70, National Electrical Code, in the edition your authority having jurisdiction has adopted: 225.30 number of supplies, 225.31 and 225.32 disconnecting means and its location, 250.32 buildings or structures supplied by a feeder, Table 300.5 minimum cover requirements, 300.5(D) protection from physical damage. Read the whole table row and name the column; the figures differ by wiring method by a factor of four.
• 29 CFR 1926.651 and 1926.652 for excavation, including the 4 ft means-of-egress trigger and the 5 ft protective-system trigger; 29 CFR 1910.333(b)(2) for electrical work practices, with proving per NFPA 70E-2021, 120.5.
• The one-call utility locate service for your jurisdiction, and the utility's own procedure for a struck or exposed line.
• See related: subpanel installation and feeder sizing; garage and workshop circuit installation; grounding and bonding verification.