1. Feeder Sizing & 4-Wire Isolation Rules for Residential Subpanels
When a main 200-amp load center runs out of physical breaker slots—or when powering a detached garage, workshop, basement suite, or Accessory Dwelling Unit (ADU)—installing a 60A, 100A, or 125A feeder subpanel is the primary engineering solution. However, subpanel installations must comply with strict feeder ampacity (NEC Article 215/225) and neutral-ground isolation mandates (NEC 250.24(A)(5) and 250.32).
Unlike a main service panel where the neutral bus and equipment grounding bus are bonded together, a subpanel must maintain 100% electrical isolation between the neutral bar (floating) and the grounded metal enclosure. Mixing neutrals and grounds in a subpanel energizes metal conduits and equipment frames with return current.
Subpanel Feeder Amperage, Conductor Gauge & Voltage Drop Reference (240V)
| Subpanel Rating | Copper Feeder (75°C) | Aluminum Feeder (75°C) | Equipment Ground Wire | Max Run (<3% Voltage Drop) |
|---|---|---|---|---|
| 60-Amp Subpanel | 6 AWG Copper (4-Wire) |
4 AWG Aluminum SER | 10 AWG Copper | Up to 110 Feet |
| 90-Amp Subpanel | 3 AWG Copper (4-Wire) |
2 AWG Aluminum MHF/SER | 8 AWG Copper | Up to 135 Feet |
| 100-Amp Subpanel | 3 AWG Copper (4-Wire) |
1 AWG Aluminum SER | 8 AWG Copper | Up to 150 Feet |
| 125-Amp Subpanel | 1 AWG Copper (4-Wire) |
1/0 AWG Aluminum SER | 6 AWG Copper | Up to 175 Feet |
| 150-Amp Subpanel | 1/0 AWG Copper (4-Wire) |
2/0 AWG Aluminum SER | 6 AWG Copper | Up to 200 Feet |
2. Detached Structure Grounding Rods (NEC 250.32(A))
Whenever a feeder supplies a separate detached building (such as a detached garage, barn, or backyard ADU), NEC 250.32(A) requires establishing a local grounding electrode system at the detached building—typically two 8-foot driven ground rods spaced 6 feet apart—connected to the subpanel's isolated grounding bar, while keeping the neutral bar unbonded.
3. Does Adding a Subpanel Increase Total Home Amperage?
Adding a 100A subpanel to an existing 100A main panel adds physical breaker spaces but does not increase your home's total service capacity. If your NEC 220.82 total connected demand exceeds 100 amps, you must upgrade the main service entrance to 200 amps first.
Technical Questions & Code Clarifications
Why do detached garage subpanels require 4 wires plus ground rods?
The 4-wire feeder (Hot L1, Hot L2, Neutral, Equipment Ground) provides a low-impedance fault current path back to the main panel breaker so faults trip immediately, while the local ground rods dissipate lightning and atmospheric voltage gradients at the outbuilding.
Can I use 2-2-2-4 Aluminum Mobile Home Feeder (MHF) for a 100-amp subpanel?
No. Under NEC Table 310.16, 2 AWG aluminum is capped at 90 amps at the 75°C terminal column (unless the entire dwelling service is carried under 310.12, which does not apply to subpanels). A 2-2-2-4 Al feeder must be protected by a 90A breaker maximum.
