How to Size Wire for a Subpanel: Step by Step
Running a feeder to a subpanel is one of those jobs where getting the wire size right matters a lot more than most people expect. Undersized conductors overheat; oversized ones waste money and create installation headaches. This guide walks through each step of the sizing process, from calculating your load to verifying voltage drop over a long run.
Always confirm your final design against the current edition of the NEC and have a licensed electrician review the work before you pull a permit.
Step 1: Calculate the Expected Load
Before you pick a wire gauge, you need to know how much current the subpanel will actually carry. Add up the wattage of every circuit you plan to feed through it.
A detached garage subpanel, for example, might have:
- Two 20 A general-purpose circuits: 2 × 2,400 W = 4,800 W
- One 240 V, 30 A circuit for a welder: 7,200 W
- One 240 V, 20 A circuit for a compressor: 4,800 W
Total: 16,800 W. At 240 V, that's 70 A of continuous load.
For continuous loads (running more than three hours straight), the NEC requires the conductor to be rated at 125% of that load. So 70 A × 1.25 = 87.5 A. You'd round up to a 100 A feeder.
If your load calculation comes in low, resist the temptation to install only what you need today. A 60 A feeder to a shop that later gains a dust collector, a mini-split, and better lighting will be undersized before long.
Step 2: Choose the Feeder Breaker Size
The feeder breaker at the main panel determines the maximum current the subpanel can draw. Common residential sizes are 60 A, 100 A, and 125 A. Pick a size that covers your calculated load with a reasonable buffer for growth.
The subpanel's bus bar rating also sets a ceiling. A panel rated for 100 A cannot be protected by a 125 A breaker, even temporarily.
For most detached structures, a 100 A feeder is the practical sweet spot. It costs relatively little more than 60 A to install, and the extra headroom often proves useful within a few years.
Step 3: Select the Conductor Material and Gauge
Copper and aluminum conductors both work for feeder runs. Aluminum is significantly cheaper and lighter, which makes it the common choice for long feeder cables. It just requires one gauge bump compared to copper for the same ampacity.
Here is a quick reference table for common subpanel feeder sizes under typical NEC conditions:
| Subpanel Rating | Copper Conductor | Aluminum Conductor |
|---|---|---|
| 60 A | 6 AWG | 4 AWG |
| 100 A | 4 AWG | 2 AWG |
| 125 A | 2 AWG | 1/0 AWG |
| 150 A | 1 AWG | 2/0 AWG |
| 200 A | 2/0 AWG | 4/0 AWG |
These assume 75°C-rated conductors in typical conduit fill conditions. Terminal ratings, conduit type, ambient temperature, and the number of current-carrying conductors in the conduit can all change the numbers. See ampacity tables and how to read them for a more thorough treatment.
For aluminum feeders, use anti-oxidant compound at the terminals and torque the lugs to spec. Aluminum expands and contracts more than copper, and loose connections cause more problems than the material itself.
Step 4: Size the Equipment Grounding Conductor
This is where many DIYers make a mistake. A subpanel fed from a main panel requires a separate equipment grounding conductor (EGC). The neutral and ground are bonded at the main panel only; at the subpanel, they stay separate. If you bond them at the subpanel too, you create parallel neutral paths that can energize metal enclosures.
The EGC is sized based on the rating of the feeder breaker, not on the load. For a 100 A feeder, the minimum copper EGC is 8 AWG. For a 200 A feeder, it bumps to 6 AWG copper.
You can use a smaller EGC gauge if it runs inside the same conduit as the feeder conductors, but it still must meet the NEC minimum for the overcurrent device size. Consult how to size a ground wire for the full sizing table and the rules for aluminum EGCs.
Step 5: Check Voltage Drop Over the Run Length
Voltage drop becomes relevant any time a feeder run exceeds roughly 100 feet. A drop above 3% on a feeder starts to cause problems: motors run hotter, lights dim, and sensitive electronics may misbehave.
The standard formula for a single-phase 240 V run:
Voltage Drop = (2 × K × I × L) / CM
Where:
- K = 12.9 for copper, 21.2 for aluminum
- I = load current in amps
- L = one-way run length in feet
- CM = circular mils of the conductor (a wire size property, found in any ampacity table)
Worked example: 100 A subpanel, 150-foot run, 2 AWG aluminum
2 AWG aluminum has 66,360 CM.
Voltage Drop = (2 × 21.2 × 100 × 150) / 66,360
= 636,000 / 66,360
= 9.58 volts
9.58 / 240 = 4.0% drop. That exceeds the 3% guideline.
To bring it under 3%, you would step up to 1/0 AWG aluminum (105,600 CM):
Voltage Drop = (2 × 21.2 × 100 × 150) / 105,600 = 6.02 V
6.02 / 240 = 2.5% drop. That passes.
Ampacity drove the minimum to 2 AWG aluminum; voltage drop pushes the actual choice to 1/0 AWG aluminum for this run. This is a common outcome for subpanel feeders longer than 100 feet, and it is exactly why you check both criteria. The step-by-step cable sizing method covers both constraints in a unified workflow.
Pulling It Together: 100 A Subpanel Example
Say you are running a 100 A subpanel to a workshop 120 feet away from the main panel. The load calculation comes in at 68 A continuous.
- Feeder breaker: 100 A (covers 68 A × 1.25 = 85 A, rounds to next standard size)
- Conductor material: aluminum (cost savings over 120 feet are meaningful)
- Minimum ampacity gauge: 2 AWG aluminum (100 A column in the table above)
- Voltage drop check at 2 AWG: 4.0% as calculated above, too high
- Revised gauge: 1/0 AWG aluminum, drops to 2.5%
- EGC: 8 AWG copper (minimum for a 100 A overcurrent device)
Final answer: 1/0 AWG aluminum for the two hots and the neutral, plus 8 AWG copper for the equipment ground. Four conductors total, pulled through conduit or ordered as a four-conductor aluminum feeder cable.
Frequently Asked Questions
Can I use the same wire for a 60 A and 100 A subpanel?
No. A 60 A feeder uses 6 AWG copper or 4 AWG aluminum; a 100 A feeder requires 4 AWG copper or 2 AWG aluminum at minimum. Using 60 A wire with a 100 A breaker violates the overcurrent protection rules and creates a fire hazard.
Does the subpanel need a main breaker?
Not always. If the subpanel is in the same building as the main panel and the feeder is protected by a breaker at the main panel, a main breaker in the subpanel is not required by the NEC. For a subpanel in a separate structure, you typically need disconnecting means at or near the structure, which is often provided by a main breaker in the subpanel itself.
Can I use aluminum wire for a subpanel feeder?
Yes, for large gauge feeders (2 AWG and larger), aluminum is a practical and code-accepted choice. Avoid aluminum for branch circuit wiring in the #12 or #10 AWG range, where the risks from incorrect terminations are higher. Large feeder lugs in panels are generally rated for aluminum.
What wire do I need for a 100 A subpanel 200 feet away?
At 200 feet with a full 100 A load, voltage drop becomes significant. Running the calculation for 2 AWG aluminum at 200 feet gives roughly 12.8 V of drop, or 5.3%, well above the guideline. You would likely need 3/0 AWG aluminum to get the drop under 3%. Always run the voltage-drop math for your specific distance and load rather than relying on the ampacity table alone.