Wiring the subpanel neutral and ground bars means physically and electrically isolating the grounded (neutral) conductor from the equipment grounding conductor in any panel downstream of the main service disconnect. This isolation changes the fault current path by preventing normal neutral return current from traveling on appliance chassis and grounding wires, and it is most commonly confused with main panel bonding, where the two bars must be intentionally connected.
The Physics of the Return Path
To understand why we separate these bars, you have to look at what the wires actually do. The neutral is a current-carrying conductor; it completes the circuit by carrying unbalanced current back to the source. The ground (equipment grounding conductor, or EGC) is a non-current-carrying safety path designed solely to clear faults.
Think of the neutral as the designated return lane for traffic (current), and the ground as the emergency shoulder. If you connect the neutral and ground bars together at a subpanel, you create a parallel path. Everyday return traffic spills onto the shoulder, meaning the metal casing of your table saw, washer, or HVAC unit becomes an active current-carrying conductor. If the neutral wire upstream ever breaks or develops high resistance, the full 120V return current will seek a path through your grounding system, potentially energizing every metal appliance in the building and creating a severe shock hazard.
By NEC Article 250.24(A)(5), the neutral and ground can only be bonded at the first point of disconnect (the main service panel). Everywhere downstream, they must remain strictly separated.
Worked Numeric Example: Sizing a 60A Garage Feeder
Let's walk through the exact math and part selection for wiring the neutral and ground bars on a 60A subpanel feeding a detached garage workshop.
Breaker: 60A 2-pole (e.g., Eaton BR260, ~$15)
Feeder Wire: 4-wire setup. 6 AWG Copper THHN for the two hots and the neutral.
Equipment Grounding Conductor (EGC): 10 AWG Copper THHN.
Many DIYers mistakenly size the ground wire to match the hots (6 AWG). While oversized ground is safe, it is a waste of copper. NEC Table 250.122 dictates the minimum EGC size based strictly on the rating of the upstream overcurrent device (the breaker), not the ungrounded conductor size. For a 60A breaker, the minimum copper EGC is 10 AWG.
When wiring the bars in the subpanel:
- The two 6 AWG hot wires land on the main breaker or main lugs.
- The 6 AWG white neutral wire lands exclusively on the isolated neutral bar.
- The 10 AWG bare or green ground wire lands exclusively on the separate ground bar, which is bolted directly to the steel panel enclosure.
Where You Meet This in Practice
You will encounter the need to isolate neutral and ground bars in three primary residential scenarios:
- Detached Buildings: Garages, barns, or detached workshops fed by an underground or overhead 4-wire feeder.
- Same-Building Additions: A subpanel installed in a finished basement, a large kitchen island appliance circuit, or a solar power distribution panel.
- Generator Interlocks: When wiring a manual transfer switch or generator subpanel downstream of the main meter-base disconnect.
In all these cases, the physical panel you buy might look identical to a main panel. The difference lies entirely in how you configure the internal bonding hardware before wiring the first circuit.
Decision Tree: Bonding Screw In or Out?
The most critical decision when wiring the subpanel is handling the bonding jumper (usually a green screw or a metal strap). Use this table to make your final hardware pick.
| Installation Scenario | Bonding Hardware State | Ground Bar Requirement | Concrete Hardware Pick |
|---|---|---|---|
| Main Service Disconnect (First point of power entry) | Bonding screw/strap INSTALLED | Neutral and Ground share the same factory bar | Square D HOM2040M100PC (Main breaker, factory bonded) |
| Subpanel in the same building (e.g., basement addition) | Bonding screw/strap REMOVED | Must install a separate, isolated ground bar | Square D HOM1224L125PGC + PK15GTA ground bar kit (~$75 total) |
| Subpanel in a detached building (e.g., garage) | Bonding screw/strap REMOVED | Separate ground bar + local grounding electrode (two ground rods) | Square D HOM1224L125PGC + PK15GTA + two 5/8" x 8' copper-clad ground rods |
Physical Wiring Steps and Torque Specs
Modern electrical inspections heavily scrutinize termination torque. NEC 110.14(D) requires that connections be tightened to the manufacturer's specified torque values. Guessing by 'feel' leads to loose connections that arc and melt over time, or over-tightened screws that snap the wire strands.
Required Tools: Wire strippers, torque screwdriver (dial or digital), #2 Phillips or square drive bit, and a flashlight.
- Prep the Wires: Strip exactly 1/2 inch to 5/8 inch of insulation from the THHN conductors. Do not nick the copper.
- Land the Neutral: Insert the white 6 AWG neutral into the neutral bar. Ensure no bare copper is visible outside the lug, and no insulation is pushed inside the lug.
- Land the Ground: Insert the 10 AWG EGC into the separate ground bar. Only one ground wire is allowed per lug hole (unlike neutrals, which sometimes allow two, but stick to one-per-hole for best practice).
- Apply Torque: Set your torque screwdriver to the manufacturer spec. For most standard Square D and Eaton residential panels:
- 14 to 10 AWG wires: 20 in-lbs
- 8 to 2 AWG wires: 40 in-lbs
- The Tug Test: Give each wire a firm, sharp pull. It should not move.
Subpanel Wiring FAQ
Can I just use the neutral bar for grounds if I run out of space?
No. Even if the neutral bar is isolated from the panel can, NEC 408.41 explicitly requires a separate equipment grounding bar. Furthermore, you are generally only allowed to land one ground wire per lug hole, whereas some neutral lugs allow two. Mixing them creates a code violation and a troubleshooting nightmare.
Do I need to drive ground rods for a detached garage subpanel?
Yes. While the 10 AWG EGC wire runs all the way back to the main panel to clear faults, NEC 250.32 requires a local grounding electrode system at the detached building to protect against lightning and voltage surges. Drive two 5/8-inch copper-clad steel rods at least 6 feet apart, and bond them to the subpanel's isolated ground bar using a minimum 8 AWG bare copper wire.
What happens if I leave the bonding screw in a subpanel?
You create a parallel neutral path. Under normal operation, return current will flow on the grounding wires. If the neutral feeder wire breaks between the main panel and the subpanel, 120V will backfeed through the ground wire, energizing the subpanel enclosure and every metal appliance plugged into it. It is a lethal hazard that standard breakers will not trip to clear.
