Wiring a Sub Panel From Main Panel: 60A, 100A, and 200A Feeder Sizing Guide

For a standard 100-amp subpanel under 50 feet, use 3 AWG copper or 1 AWG aluminum for the hot and neutral wires, plus an 8 AWG copper ground, protected by a 100A 2-pole breaker. For a 60-amp subpanel, step down to 6 AWG copper or 4 AWG aluminum with a 10 AWG ground.

The Baseline Feeder Sizing Matrix (60A, 100A, 200A)

When wiring a sub panel from main panel, the feeder breaker dictates the minimum wire gauge. The table below provides the exact specifications for the most common residential and light-commercial subpanel sizes. Grounding conductor sizes are derived from NEC 250.122.

Note: Never mix copper and aluminum in the same run, and always use anti-oxidant paste (like Noalox) on aluminum terminations to prevent galvanic corrosion and high-resistance heating.

Why These Sizes? Ampacity, Temperature, and the NEC

A common mistake when wiring a sub panel from main panel is looking at the 90°C column on the wire insulation jacket and assuming a 4 AWG copper THHN wire (rated 95A at 90°C) can handle a 100A breaker. It cannot.

Why 3 AWG and not 4 AWG for a 100A breaker? NEC 110.14(C) requires us to size the wire based on the lowest temperature rating of any connected component in the circuit. Standard residential breakers and panel lugs are rated for 75°C. In the 75°C column of NEC Table 310.16, 4 AWG copper is only good for 85 amps. You cannot protect an 85-amp wire with a 100-amp breaker; the breaker would fail to trip before the wire's terminations overheat. Therefore, you must step up to 3 AWG copper, which is rated exactly 100A at 75°C.

What changes the answer? Three main factors force you to upsize your feeder:

• Bundling: Running more than 3 current-carrying conductors in a single conduit triggers NEC 310.15(C)(1) derating. If you run two separate 240V feeders in one pipe (8 current-carrying wires), you must multiply the ampacity by 0.70, forcing a massive wire upsizing.
• High Ambient Heat: If your conduit runs through an attic in the Southwest US where temperatures exceed 113°F (45°C), you must apply Table 310.15(B)(1) correction factors, reducing the wire's effective ampacity by 20% or more.
• Switching to Aluminum: Aluminum is lighter and significantly cheaper, but it has lower conductivity. To achieve the same 100A ampacity, you must use 1 AWG aluminum instead of 3 AWG copper.

Voltage Drop: When Distance Forces a Size Upgrade

Ampacity keeps the wire from melting; voltage drop ensures your tools and appliances actually work. The NEC recommends a maximum 3% voltage drop on feeders. The formula is: VD = (2 × K × I × L) / CM, where K is 12.9 for copper, I is current, L is one-way length, and CM is circular mils.

Let's look at a real-world voltage drop check for a 100A subpanel:

At 250 feet, 3 AWG copper is no longer acceptable despite having the ampacity for the breaker. You must upgrade to 2 AWG or 1 AWG copper to maintain voltage stability for heavy loads like table saws or EV chargers at the end of the line.

When to Call an Engineer or the AHJ

While standard residential subpanels follow the matrix above, specific conditions require professional sign-off or an engineered design. According to Electrical Contractor Magazine and NEC guidelines, you must consult an engineer or your local Authority Having Jurisdiction (AHJ) when:

• Continuous Loads Exceed 80%: If the subpanel will supply a continuous load (operating for 3 hours or more, like a commercial kiln or heavy server rack), the breaker and wire must be sized at 125% of the load.
• Service Entrance Upgrades: If adding the subpanel requires upgrading your main service from 100A to 200A, this involves utility coordination and meter work that strictly requires a licensed electrician.
• Complex Derating: If your conduit runs are exposed to extreme heat AND bundled with other circuits, the intersecting derating factors require precise calculation to prevent nuisance tripping or insulation failure.

FAQ: Wiring a Sub Panel From Main Panel

Do I need a main breaker when wiring a sub panel from main panel in the same building?

No. If the subpanel is in the same structure as the main panel, the feeder breaker protecting the wires back at the main panel serves as the required disconnect. You can use a main lug only (MLO) subpanel. However, if you are wiring a sub panel to a detached building (like a garage or barn), NEC 225.32 requires a local disconnecting means, meaning you should use a subpanel with a main breaker.

Can I share the neutral and ground bar when wiring a sub panel from main panel?

Absolutely not. This is one of the most dangerous DIY mistakes. NEC 250.32 mandates that the neutral (grounded conductor) and the equipment grounding conductor (EGC) must be kept strictly isolated in a subpanel. You must run a 4-wire feeder (two hots, one neutral, one ground) and ensure the subpanel has separate, isolated bus bars for neutral and ground. Bonding them in a subpanel creates a parallel neutral path, which can energize the grounding system and cause lethal shock hazards.

What size conduit is required for wiring a 100-amp sub panel from main panel?

For a 100A feeder using 3 AWG copper hots/neutral and an 8 AWG copper ground, a 1-inch PVC Schedule 80 or EMT conduit is sufficient, keeping you well under the NEC Chapter 9 Table 1 limit of 40% conduit fill. If you opt for 1 AWG aluminum to save money, the thicker wires will require stepping up to a 1.25-inch conduit to avoid damaging the insulation during the pull.

How much does it cost for wiring a sub panel from main panel in 2026?

For a standard 100A subpanel located 50 feet from the main panel, expect to spend $250 to $400 on materials (including a Square D or Eaton 100A panel, 150 feet of 3 AWG THHN, 1-inch PVC, and a 100A breaker). If you hire a licensed electrician, professional labor typically adds $800 to $1,500, depending on whether the run requires drywall patching, attic crawling, or exterior trenching.