When learning how to connect electrical panel subfeeds to expand your workshop or add a detached garage circuit, the physical wiring is only half the battle. The real challenge lies in understanding the exact terminal mapping, maintaining the correct polarity, and strictly isolating the neutral and ground paths at the subpanel. A 100-amp subpanel fed from a main service panel requires a 4-wire feeder (two hots, one neutral, one ground) and precise torque on every lug.
Main Panel to Subpanel: The Node-by-Node Trace
To wire a subpanel correctly, you must trace the current path from the source bus bars to the load lugs, paying strict attention to where the neutral and ground paths diverge. Here is the exact node-by-node trace for a standard 100A, 240V/120V split-phase feeder using copper THHN in conduit.
- Source Bus Bars (Main Panel): The feeder originates at the main panel's L1 and L2 bus stabs. These are 180 degrees out of phase, providing 240V between them and 120V from either leg to neutral.
- Feeder Breaker (Source): A 100A double-pole breaker (e.g., Square D HOM2100) clips onto the L1 and L2 stabs. The breaker's internal trip mechanism monitors both legs simultaneously.
- The Feeder Cable (Path): Four individual THHN wires (Black, Red, White, Green) are pulled through rigid metal or PVC conduit. The white neutral carries unbalanced return current. The green equipment grounding conductor (EGC) carries fault current only.
- Subpanel Main Lugs (Load): The black and red hots land on the subpanel's top main lugs, distributing power down the subpanel's internal bus stabs.
- Subpanel Neutral Bar (Load): The white neutral lands on the subpanel's neutral bar. Critical Node: This bar must be physically isolated from the metal enclosure. The green bonding screw or strap must be removed.
- Subpanel Ground Bar (Load & Fault Path): The green EGC lands on the ground bar, which is directly bolted to the metal enclosure. From here, the fault path travels back to the main panel's ground bar, and ultimately to the grounding electrode system (ground rods or ufer ground), ensuring the breaker trips instantly during a short circuit.
Terminal Mapping & Wire Sizing Data
The most common cause of subpanel failures and melted lugs is under-torqued connections or using the wrong wire gauge for the termination temperature rating. The table below maps every physical terminal in this 100A feeder run, assuming 75°C rated terminations (standard for modern breakers and panels) and copper conductors.
| Physical Terminal / Lug | Wire Color (NEC) | Wire Size (Copper 75°C) | Torque Spec | Function |
|---|---|---|---|---|
| Main Panel Feeder Breaker L1 | Black | #3 AWG THHN | 50 in-lbs | 120V Hot Leg 1 Source |
| Main Panel Feeder Breaker L2 | Red | #3 AWG THHN | 50 in-lbs | 120V Hot Leg 2 Source |
| Main Panel Neutral Bar | White | #3 AWG THHN | 45 in-lbs | Unbalanced Return Path |
| Main Panel Ground Bar | Green | #8 AWG THHN | 35 in-lbs | Equipment Fault Path |
| Subpanel Main Lug L1 | Black | #3 AWG THHN | 50 in-lbs | 120V Hot Leg 1 Load |
| Subpanel Main Lug L2 | Red | #3 AWG THHN | 50 in-lbs | 120V Hot Leg 2 Load |
| Subpanel Neutral Bar (Isolated) | White | #3 AWG THHN | 45 in-lbs | Unbalanced Return Path |
| Subpanel Ground Bar (Bonded) | Green | #8 AWG THHN | 35 in-lbs | Equipment Fault Path |
Decoding the Wiring Diagram Symbols
Electrical single-line diagrams use standardized ANSI/IEEE symbols to represent physical hardware. When reading the schematic for a subpanel feed, you will encounter specific symbols that dictate how the panel must be configured.
- The Diagonal Slash (Breaker): A straight line representing the conductor, crossed by a diagonal line with a small hook at the end. This represents the thermal-magnetic trip mechanism of the 100A feeder breaker. If you see two of these linked by a dashed line, it indicates a common-trip double-pole breaker.
- Parallel Lines with Hash Marks (Cable/Conduit): A single thick line with three or four hash marks (e.g.,
|||) crossing it represents the 4-wire feeder cable. The number of hash marks usually denotes the number of current-carrying conductors plus the ground. - Circle with 'G' or Cross (Grounding Electrode): A circle containing a 'G' or a cross symbol at the bottom of the diagram represents the grounding electrode system. In a subpanel diagram, you will not see a connection to this symbol, because the subpanel relies on the main panel's grounding electrode.
- The Dashed Bonding Jumper: This is the most critical symbol to understand. On a main panel diagram, a dashed line connects the neutral bar to the ground bar (representing the green bonding screw). On a subpanel diagram, this dashed line will have a large 'X' through it or be entirely absent, indicating that the neutral and ground must remain isolated per NFPA 70 (NEC) Article 250.32.
Verification: Testing Every Connection with a Meter
Before you energize the feeder breaker, you must verify the integrity of your wiring. Relying on a visual inspection is not enough; you need empirical data from a digital multimeter (DMM). Follow this exact testing sequence to ensure your subpanel is safe to energize.
Phase 1: Dead Testing (Power OFF, Locked Out)
With the main feeder breaker OFF and locked out, set your DMM to the Continuity or Ohms (Ω) setting.
- Ground to Enclosure Test: Place one probe on the green ground bar and the other on the bare metal subpanel enclosure. You must read < 1 ohm. This confirms the ground bar is properly bonded to the chassis.
- Neutral to Ground Isolation Test: Place one probe on the white neutral bar and the other on the green ground bar. You must read OL (Open Loop) or infinite resistance. If you read continuity here, the bonding screw or strap is still installed in the subpanel. Remove it immediately before proceeding.
- Feeder Short Test: Check Black to White, Red to White, and Black to Red. All must read OL. Any low resistance reading indicates a nicked wire insulation or a short in the conduit pull.
Phase 2: Live Testing (Power ON)
Remove LOTO, close the panel covers (never test live with the cover off unless wearing appropriate arc-flash PPE), and turn ON the 100A feeder breaker. Set your DMM to AC Voltage (V~).
- Line-to-Line Voltage: Measure across the two subpanel main lugs (or a 240V breaker terminal). Expected reading: 238V - 242V. This confirms both legs are present and 180° out of phase.
- Line-to-Neutral Voltage: Measure from L1 to the Neutral bar, then L2 to the Neutral bar. Expected reading: 118V - 122V for both. If one reads 140V and the other reads 90V, you have a floating or disconnected neutral upstream.
- Line-to-Ground Voltage: Measure from L1 to Ground, and L2 to Ground. Expected reading: 118V - 122V.
- Neutral-to-Ground Voltage (The True Test): Measure between the isolated neutral bar and the bonded ground bar. Expected reading: < 2.0V. If you read 5V or higher, your neutral feeder wire is undersized, damaged, or loosely torqued at the main panel, causing voltage drop on the return path.
By strictly following the node-by-node trace, torquing to manufacturer specs, and verifying isolation with a meter, your subpanel will provide a safe, code-compliant foundation for your branch circuits. For further details on branch circuit sizing and breaker selection, refer to Schneider Electric's electrical distribution support resources and your local AHJ guidelines.
