Subpanel Feeder Sizing: 60 A Subpanel 100 ft Away, Step by Step
Size a 60 A subpanel feeder the right way: check ampacity and voltage drop separately, watch the NM-B temperature trap, and pick breaker, wire and ground with a worked 100 ft example.
Step-by-Step Procedure
Fix the load, voltage and distance first
Write down the feeder load (here 60 A), the voltage (240 V for a typical 120/240 V subpanel feeder) and the one-way distance from the main panel to the subpanel (here 100 ft). Every later check uses these three numbers, so do not guess the distance.
Check ampacity in the correct temperature column
Ampacity depends on the wiring method, not just the copper size. 6 AWG copper THHN in conduit can use the 75 C column (65 A), while 6 AWG copper NM-B cable is limited to the 60 C column (55 A) and therefore does not support a 60 A feeder. 4 AWG copper is a common robust choice for a 60 A feeder, and 2 AWG aluminium is a common aluminium alternative. Always check the locally adopted table for the exact installation.
Check voltage drop separately, with the round-trip length
Voltage drop uses the round-trip length, not the one-way length: 100 ft one-way is 200 ft of conductor path for a 240 V circuit. For 4 AWG copper at about 0.2485 ohms per 1,000 ft, the drop is 60 A x 0.2485 x 200 / 1,000 = 2.98 V, which is 1.24% of 240 V and passes a 3% branch-circuit guide. For 6 AWG copper the drop is about 4.74 V (1.98%), which also passes voltage drop, so in this example ampacity, not voltage drop, is the deciding check.
Pick the breaker, neutral and ground
Use a 60 A breaker in the main panel for a 60 A feeder. Size the neutral for the same load path in this typical feeder, and size the equipment grounding conductor from the grounding table: for a 60 A overcurrent device the table value is 10 AWG copper. Keep neutral and ground separate in the subpanel, with the ground bonded to the subpanel enclosure and the neutral isolated.
Check panel capacity and local rules before installing
Confirm the main panel has spare capacity for a new 60 A continuous or non-continuous load as applicable, and check the locally adopted NEC edition and your authority having jurisdiction. This guide is a planning estimate only; final design and installation should be verified by a licensed electrician.
Worked Practical Example
60 A subpanel, 240 V, 100 ft one-way
A detached garage subpanel fed at 60 A, 240 V, 100 ft from the main panel, copper THHN in conduit.
- •Ampacity check: 4 AWG copper THHN at 75 C is above 60 A, so it passes ampacity
- •Voltage drop: round-trip length = 2 x 100 ft = 200 ft
- •Voltage drop = 60 A x 0.2485 ohms per 1,000 ft x 200 / 1,000 = 2.98 V
- •Voltage drop percent = 2.98 / 240 x 100 = 1.24%, inside a 3% guide
- •Breaker: 60 A in the main panel; equipment grounding conductor: 10 AWG copper from the grounding table for a 60 A device
Common Mistakes to Avoid
- ✕Using the one-way length instead of the round-trip length in the voltage-drop calculation.
- ✕Using the 75 C ampacity column for NM-B cable, which is limited to the 60 C column.
- ✕Passing ampacity but never checking voltage drop, or passing voltage drop but never checking ampacity.
- ✕Bonding neutral and ground together in a subpanel, which is not how a subpanel should be wired.
Frequently Asked Questions
What size wire do I need for a 60 A subpanel 100 ft away?
As a planning answer, 4 AWG copper is a common robust choice, with 2 AWG aluminium as a common alternative. The exact answer depends on the wiring method, temperature column, conduit fill, derating and local code, so verify it with the tables and a licensed electrician.
Can I use 6 AWG wire for a 60 A subpanel?
It depends on the wiring method. 6 AWG copper THHN in conduit and 6 AWG copper NM-B cable are not interchangeable, because NM-B is limited to the 60 C column. Do not assume a size that works in one method works in the other.