How to Size a Subpanel Feeder (Field Walkthrough)
Sizing the feeder to a subpanel comes down to a load calculation, the right conductor and breaker, a properly sized grounding/bonding scheme, and the neutral rule people forget. Here's the working electrician's walkthrough — verify against the NEC.
⚠️ Before you start
- Feeder sizing must follow a proper NEC load calculation and the current ampacity tables and local amendments — this walkthrough is a conceptual reference, not a substitute for the code.
- The critical subpanel rule: neutrals and grounds must be separated (isolated) at the subpanel, with the main bonding jumper removed and a separate equipment grounding conductor run with the feeder.
- Feeder installation is licensed-electrician work, usually permitted and inspected.
Sizing a subpanel feeder trips up a lot of people because it's really four decisions in one: how much load, what conductor, what breaker, and how to handle the grounding and neutral. Here's the working walkthrough. Treat it as a conceptual reference — the NEC tables, derating, and your local amendments have the final word.
Step 1: Run the load calculation
Don't eyeball it. Do a proper NEC load calculation for everything the subpanel will feed — general lighting/receptacle load, appliances, motors, HVAC, EV charging — applying the code's demand factors. That calculated load, plus headroom for growth, tells you the subpanel and feeder-breaker amperage. Common residential subpanels are 60, 100, or 125 A. Calculate, then round up sensibly; don't default to "it's a garage, 60's fine."
Step 2: Size the feeder breaker
The overcurrent device protecting the feeder (in the main panel) is sized to your calculated load and the conductor ampacity. This breaker defines the subpanel's rating.
Step 3: Size the conductors
Common copper starting points:
- 60 A feeder → often 6 AWG copper
- 100 A feeder → often 3 AWG copper (or 4 AWG at the 75°C column, depending on conditions)
Aluminum runs larger (100 A frequently uses 1 or 2 AWG aluminum). But the exact size hinges on terminal temperature rating, derating, and distance/voltage drop — verify against the ampacity tables for your conditions. On long runs (detached garage or shop), upsize for voltage drop, not just ampacity — see voltage drop explained.
Step 4: The rule everyone forgets — 4 wires, floated neutral
This is the single most-violated subpanel detail:
- Run a 4-wire feeder: two hots, a neutral, and a separate equipment grounding conductor.
- At the subpanel, the neutral bar must be isolated (floated) from the enclosure.
- The grounds land on a separate bonding bar bonded to the enclosure.
- The main bonding jumper lives only at the service — remove it at the sub.
The old 3-wire subpanel method (bonding neutral and ground together at the sub) is no longer permitted for new work, and getting it wrong puts objectionable current on the grounding system — a real hazard. See what is a subpanel.
Step 5: Grounding electrode at detached structures
Feeding a detached building? It generally needs its own grounding electrode system (ground rod[s]) bonded at that structure — another detail the code spells out and inspectors check.
Bottom line
Size a subpanel feeder in order: load calc → breaker → conductor (checking terminal rating, derating, and voltage drop) → 4-wire feeder with a floated neutral and separate ground. Common copper picks are 6 AWG for 60 A and 3 AWG for 100 A, but always confirm against the NEC and local amendments for the real conditions — and permit and inspect the work.
📞 When to call a professional
This is pro-level reference material. Actual subpanel and feeder installation should be performed by a licensed electrician to code, with a permit and inspection.
Frequently asked questions
How do I decide what amperage subpanel I need?
Run a load calculation for everything the subpanel will feed (using the NEC methods — general lighting/receptacle loads, appliances, motors, and the applicable demand factors), then choose a subpanel and feeder breaker that comfortably cover that calculated load with room to grow. Common residential subpanels are 60, 100, or 125 amps. Don't just guess by 'it's a garage, 60 should do' — calculate, then round up sensibly.
What conductor size do I run for a 60 or 100 amp subpanel?
As a common copper starting point: a 60A feeder is often 6 AWG copper, and a 100A feeder is often 3 AWG copper (or 4 AWG at the 75°C column depending on conditions), with aluminum sizes running larger (e.g., 100A frequently uses 1 or 2 AWG aluminum). But the exact size depends on the terminal temperature rating, derating, distance/voltage drop, and local rules — always verify against the NEC ampacity tables for your conditions rather than relying on rules of thumb.
Do I run three wires or four to a subpanel?
Four. Modern code requires a 4-wire feeder to a subpanel: two hots, a neutral, AND a separate equipment grounding conductor. At the subpanel the neutral bar must be isolated (floated) from the enclosure and the grounds land on a separate bonding bar — the main bonding jumper is only at the service. The old 3-wire subpanel method (bonding neutral and ground at the sub) is no longer permitted for new work. This is the single most-violated subpanel rule.
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