Subpanel Installation and Feeder Sizing

Purpose

A subpanel is not a smaller panel. The grounded conductor connects to the equipment grounding system exactly once in a premises wiring system, at the service, and a subpanel sits downstream of that. Break the separation and normal load current divides between the feeder neutral and the feeder equipment grounding conductor, every metal box and raceway on that feeder joins the return path, and nothing trips to say so.

The other half is the feeder, and shops miss it in one direction: they size it off the panel's bus rating, because that is what they bought. A 100 A load center gets a 100 A feeder whether the calculated load is 42 A or 96 A. A bus rating is a ceiling on how the panel may be protected, not a load and not a conductor specification.

Scope

Covers a subpanel in the SAME structure as the service, fed from an existing panel, single-phase 120/240 V, feeder device 200 A or smaller, residential and small commercial.

Not covered: the whole-dwelling load calculation that sets service size (electrical-load-calculation-before-a-service-upgrade owns the Article 220 method); replacing the service panel; the fault-return-path proof-out (electrical-grounding-and-bonding-verification); and a feeder to a SEPARATE STRUCTURE, which adds a grounding electrode and a disconnecting means at the second building.

Roles and handoffs

The first handoff is the one that fails: hand over a panel size instead of a load and the installer has nothing to size a conductor from, so they default to the bus rating.

Procedure

1. Confirm a subpanel is the right answer and write the demand in amps. It solves "no spaces left," not an undersized service, which is a service upgrade wearing a cheaper hat. Accept when the ticket carries calculated demand in amps for the circuits moving here plus known future load. Wrong looks like "100 A sub in the basement" with no number; stop rule, no demand figure means no material order. Hazard: none, a desk decision, but a bad number here is the one defect this SOP cannot catch later.

2. Verify the supplying panel can host the feeder breaker before buying anything. Read its label for an available space of the right type and a breaker family that label lists; equipment is installed per its listing under NEC 110.3(B). Accept when both exist. Wrong looks like a tandem in every slot of a panel whose label allows none; stop rule, no listed space makes this a panel replacement or a tap, re-quoted first. Hazard: reading a label means standing at an energized panel, so nothing comes off the deadfront until step 3.

3. De-energize the supplying panel and prove it dead before a cover moves. This is electrical utilization work, so the practice is 29 CFR 1910.333(b)(2), by a qualified person per 1910.332 and 1910.399, locked and tagged. Prove dead live-dead-live, tester proved on a known live source before and after, per NFPA 70E-2021, 120.5. Accept when every ungrounded conductor reads under 1 volt on that proved instrument and the after-check passes, since induced voltage on a de-energized conductor routinely reads a fraction of a volt. Wrong looks like a tester reading zero on the live source too, which is a blown fuse; stop rule, a failed after-check voids the verdict. Hazard: the line side of the service disconnect stays live from the utility regardless, so that lug compartment is energized all day.

4. Size the feeder from the demand, at the termination temperature the equipment allows. Terminations govern: NEC 110.14(C) holds you to the 60 C column unless both ends are marked 75 C, so read the marking. Accept when the conductor's ampacity from the applicable NEC 310.16 column, after corrections, sits at or above both the demand and the feeder device. Wrong looks like picking the 90 C column because the wire is THHN, which rates insulation and not the lug; stop rule, if corrected ampacity lands under the device you wanted, drop the device. Hazard: none, this is arithmetic done off the truck.

5. Size the EGC from the overcurrent device, then confirm the assembly fits. The equipment grounding conductor comes off NEC Table 250.122 sized to the feeder device rating, not to the load and not to the phase conductor. On a 120/240 V single-phase feeder the neutral carries only unbalanced current and is not counted for adjustment. Accept when the EGC size is written beside its table row and raceway fill clears 40 percent. Wrong looks like an EGC matched to the neutral because the cable came that way; stop rule, undersize changes the conductor, never the breaker. Hazard: none here, but a line-to-case fault years out clears through whatever you installed.

6. Mount where the working space exists, and prove the space with a tape. NEC 110.26 sets clearance in front of equipment likely to be examined energized, and the dimensions people lose are depth and headroom. Accept when depth, width and headroom are each measured, written, and meet the figure for that voltage and condition in the edition your authority having jurisdiction has adopted. Wrong looks like a panel behind a water heater; stop rule, space that cannot be made will not be inspected into existence, so relocate before drilling. Hazard: overhead drilling puts debris in eyes and finds concealed circuits, so eye protection on and scan the cavity first.

7. Land conductors with the neutral bar isolated and the ground bar bonded, and remove the main bonding jumper. The screw, strap or factory bond tying the neutral bar to the can comes OUT. Neutrals to the isolated bar, equipment grounding conductors to a bar bonded to the enclosure, one per terminal unless listed for more.

   Service panel                 Subpanel
   -------------                 --------
   L1 ---------------------------- L1 lug
   L2 ---------------------------- L2 lug
   neutral bar ------------------- neutral bar, isolated
   ground bar -------------------- ground bar, bonded to can
        |
   main bonding jumper stays HERE,
   and nowhere else in the building

Accept when the bonding screw is in the job envelope and continuity from the neutral bar to the can reads open with the feeder dead. Wrong looks like a neutral and a ground sharing a terminal, a bond you built by hand; stop rule, continuity means something is still bonded, and after energizing it reads as normal operation. Hazard: the feeder is dead but its neighbours are not, so keep it locked out and work one-handed.

1. Install the feeder device and torque every termination with a calibrated tool. The breaker goes in the SUPPLYING panel, protecting the conductor at its supply end. Set the driver to the value printed on the equipment label for that lug and conductor size. Accept when every lug in both panels is torqued to its labelled value and the values are on the ticket. Wrong looks like a lug tightened by feel, where the loose connection a thermal scan finds two winters later begins; stop rule, a lug that will not hold torque gets replaced, not another turn. Hazard: a driver slipping off a lug throws steel toward a bus, so keep the far bus covered and the feeder locked out.

2. Re-energize, then prove the separation you built actually holds. Deadfront on, stand to the hinge side rather than square in front, announce it, close the feeder. Then clamp a meter with 0.01 A resolution around the feeder EGC alone and switch a known 120 V load on and off. Accept when it reads at or below 0.5 A and does not track the switching; a small neutral-to-ground voltage that RISES with load corroborates it. Wrong looks like EGC current moving with the load; stop rule, any tracking current stops close-out and the feeder goes back off under 1910.333(b)(2) until the bond is found. Hazard: energy goes back in beside a customer, so clear the space, deadfront stays on, jaws around one conductor, other hand out.

When the site does not match the assumption

No listed space and no budget for a service change. Two honest options, and a tandem the label forbids is not one: a tap ahead of the panel under the tap rules, or the job stops. Write which was offered and what the customer chose.

The existing feeder is three-wire. It has no separate EGC, so that panel cannot be separated. Do not pull the bonding screw and call it corrected; you have removed the only fault return path it had. The fix is a fourth conductor or a new feeder, quoted.

The feeder has to be fished. That changes conductor type, not arithmetic, so re-run steps 4 and 5 for the cable you can pull.

The record this produces

• Calculated demand in amps, with its source worksheet. Read by whoever adds the next load, and it is what stops a second tech filling the panel to its bus rating.
• Conductor size, insulation, ampacity column used, corrected ampacity. Read by the inspector at rough and by anyone later tempted to raise the feeder breaker.
• EGC size and the table row it came from. Read by whoever troubleshoots a fault that did not clear.
• Torque value read off each label, plus confirmation of a calibrated tool. Read when a thermal scan finds heat at that lug.
• Step 9 clamp reading and the load that was switched. The office pass or fail, stored with the panel directory as the next visit's baseline.

Worked pass: 100 A subpanel in a 1978 ranch, kitchen remodel

Step 1: the load SOP returned 68 A of demand. Step 2: the panel label lists the breaker family and shows two adjacent full-size spaces. Step 3: main opened, locked, tagged, both ungrounded conductors read 0.2 V, under the 1 V acceptance, tester proved on a known live receptacle before and after.

Step 4: both panels are marked 75 C. Copper 3 AWG at 75 C is rated 100 A, clearing the 68 A demand and equalling the 100 A device. Indoors at 30 C with two current-carrying conductors, no correction or adjustment applies. Voltage drop over the 85 ft run at 68 A comes to about 2.8 V, and 2.8 V on 240 V is 1.2 percent, inside the shop's 3 percent design target. That 3 percent lives in the informational notes to NEC Articles 210 and 215, which are advisory, so it is our standard and not the inspector's.

Step 5: Table 250.122 at 100 A gives 8 AWG copper, and three 3 AWG plus one 8 AWG THHN come to roughly 0.33 sq in, inside the 0.346 sq in that 40 percent of 1 in EMT allows, with about 5 percent of margin - tight enough that a later added conductor forces the upsize. Step 6: depth 40 in, width 33 in, headroom 6 ft 9 in, taped and written. Step 7: bonding screw in the envelope, neutral bar to can open, sixteen neutrals and sixteen grounds each on their own bar. Step 8: this panel's label reads 180 lb-in at the feeder lugs and 35 lb-in at the ground-bar screws, which is what THIS label said and not a general figure.

Step 9 fails. Deadfront on, breaker closed from the hinge side, clamp around the feeder EGC alone: 6.2 A, dropping to 0.1 A when the counter circuit is switched off. It tracks the load, so it is a second path and not noise. Stop rule taken: feeder off, locked, tagged, panel reopened. Two neutrals from the old junction box had been landed on the ground bar at rough-in. Both moved. Re-clamped after re-energizing: 0.1 A, no movement, inside the 0.5 A acceptance, with neutral-to-ground voltage at the bus reading 0.4 V unloaded and 1.1 V under that load, which is drop on the feeder neutral and what a separated panel should show.

The ticket keeps the failure rather than erasing it: that line tells the next tech this panel had grounds and neutrals mixed once.

References

• NEC Articles 215, 240 and 250 Parts VI and VII, plus 110.3(B), 110.14(C), 110.26 and Tables 250.122 and 310.16, in the edition your authority having jurisdiction has adopted.
• 29 CFR 1910.333(b)(2), with qualified person at 1910.332 and 1910.399; 29 CFR 1926.417 is the construction counterpart for lockout and tagging of circuits.
• NFPA 70E-2021, 120.5, for the live-dead-live sequence, in the edition your electrical safety program adopts.
• See related: electrical-load-calculation-before-a-service-upgrade, electrical-grounding-and-bonding-verification, electrical-infrared-scan-of-a-panel.