For a standard 30A 120V subpanel feeder under 50 feet, use 10 AWG copper THHN/THWN-2 wire protected by a 30A single-pole breaker. This applies to typical small detached structures like greenhouses, pump houses, or garden offices that only require 120V power.

Baseline Assumptions for This Guide

  • Conductor Material: Copper (unless explicitly noted as aluminum).
  • Temperature Rating: 75°C column (standard for modern residential breakers and panel lugs).
  • Ambient Temperature: 30°C (86°F).
  • Installation Method: Individual THHN/THWN-2 conductors in Schedule 40 PVC conduit.
  • Current-Carrying Conductors (CCCs): 2 (Line and Neutral). The Equipment Grounding Conductor (EGC) does not count as a CCC.
  • System Type: 120V single-phase, single-pole feeder (Line, Neutral, Ground).

While most residential subpanels are 120/240V split-phase, a dedicated 120V-only subpanel is a specific, code-compliant solution for outbuildings with no 240V loads. Because the neutral carries the full return current of the ungrounded line conductor, both the line and neutral are full-sized current-carrying conductors. Sizing this feeder correctly requires balancing National Electrical Code (NEC) ampacity tables with real-world voltage drop physics.

120V Subpanel Feeder Sizing Chart

The table below provides the minimum wire sizes for common 120V subpanel main breaker ratings. These values are derived from the 75°C column of NEC Table 310.16. The maximum distance is calculated to maintain a voltage drop of 3% or less at 120V under full continuous load.

Main Breaker Size Min Copper AWG (75°C) Min Aluminum AWG (75°C) Max Distance for <3% VD (Copper)
20 Amp 12 AWG 10 AWG 48 feet
30 Amp 10 AWG 8 AWG 64 feet
50 Amp 6 AWG 4 AWG 76 feet
60 Amp 6 AWG 4 AWG 91 feet

Note: If your conduit run exceeds the 'Max Distance' listed above, you must upsize the wire by one or two AWG steps to compensate for voltage drop, even if the ampacity is technically sufficient for the breaker size.

Why 10 AWG and Not 12 AWG for a 30A Feeder?

A common mistake among DIYers is assuming that because a 12 AWG copper wire might not immediately melt at 30 amps, it is safe to use. The NEC strictly forbids this under NEC 240.4(D), which governs small conductors. Under this article, the overcurrent protection for 12 AWG copper is hard-capped at 20 amps, and 10 AWG is capped at 30 amps, regardless of the insulation's 90°C thermal rating.

This limitation exists for two reasons:

  1. Terminal Ratings: Most residential breakers and subpanel lugs are rated for 75°C. A 12 AWG wire in the 90°C column is rated for 30A, but we are legally required to use the 75°C column for termination sizing, where 12 AWG drops to 25A. You cannot protect a 25A-rated termination with a 30A breaker.
  2. Thermal Runaway at Terminals: The breaker's internal bimetallic trip strip is calibrated to the mass and heat-dissipation profile of a specific wire size. Undersized wire acts as a fuse, potentially degrading the breaker's internal connections long before the breaker trips.
Safety Warning: Never install a 120V subpanel without a dedicated Equipment Grounding Conductor (EGC). Even if the subpanel is mounted to a metal building frame, NEC 250.32 requires an explicit wire-type EGC (bare or green) pulled through the conduit alongside your line and neutral. The subpanel's neutral bar and ground bar must remain isolated (unbonded) from each other.

Variables That Change Your Wire Size

The chart above assumes a perfect, short, single-circuit run. Real-world jobsite conditions frequently force you to upsize your conductors. Here is how the three main variables alter your 120V subpanel wiring plan.

1. Voltage Drop Over Distance

The NEC recommends a maximum 3% voltage drop on feeders (NEC 310.15(B) Informational Note). At 120V, a 3% drop means your voltage at the subpanel cannot fall below 116.4V under full load. Let's run the math for a 30A load on 10 AWG copper at 100 feet:

  • Formula: VD = (2 × K × I × D) / CM
  • K (Copper): 12.9 ohms
  • I (Current): 30A
  • D (Distance): 100 feet
  • CM (Circular Mils for 10 AWG): 10,380
  • Result: (2 × 12.9 × 30 × 100) / 10380 = 7.45 Volts (6.2% drop)

A 6.2% drop will cause lights to dim and motors to overheat. For a 100-foot run at 30A, you must upsize to 6 AWG copper to bring the drop back under 3%. You can verify these calculations using the Southwire Voltage Drop Calculator, an industry-standard tool for verifying feeder sizing.

2. Conduit Bundling and Derating

If you are pulling multiple circuits through the same conduit to reach your outbuilding, you must apply NEC derating factors. The heat generated by adjacent wires cannot dissipate, effectively lowering the wire's ampacity.

Suppose you pull two separate 120V circuits (4 current-carrying conductors total) in one PVC pipe to the subpanel. According to NEC Table 310.15(C)(1), 4 to 6 CCCs require an 80% derating factor.

  • 10 AWG THHN (90°C column base) = 35A.
  • 35A × 0.80 = 28A.

Because 28A is less than your 30A breaker, 10 AWG is no longer legal. You must upsize to 8 AWG THHN (50A × 0.80 = 40A) to maintain compliance. This is a frequent failure point in DIY trenching projects where homeowners try to save money by pulling multiple small-gauge wires in a single sweep.

3. Aluminum vs. Copper

Aluminum is significantly cheaper than copper, but it has lower conductivity and different physical properties. You cannot swap them 1:1. If you choose aluminum for a 30A 120V subpanel feeder, you must use 8 AWG aluminum (or larger, as 10 AWG aluminum building wire is not standard). Furthermore, aluminum requires anti-oxidant paste (like Noalox) at the terminations and specific torque settings on the panel lugs to prevent thermal expansion/contraction from loosening the connection over time, which causes arcing.

When to Call an Engineer or the AHJ

While standard 120V subpanel wiring for a backyard shed is well within the scope of NEC prescriptive rules, certain scenarios require professional intervention:

  • Continuous Loads Exceeding 80%: If your 120V subpanel will power continuous loads (defined as running for 3 hours or more, like greenhouse heaters or server racks), NEC 210.20(A) requires the breaker and wire to be sized at 125% of the continuous load. A 24A continuous load requires a 30A breaker and wire sized for 30A, not 24A.
  • Extreme Distances (>200 feet): When voltage drop requires upsizing wire by more than two AWG steps, the physical size of the wire may no longer fit into the standard lugs of a 30A or 60A breaker. An engineer may need to specify a terminal block or a larger panel with aftermarket lugs to accommodate 2 AWG or 1/0 wire.
  • Local AHJ Amendments: Some municipalities have strict local amendments regarding detached structures. For example, certain jurisdictions require a local disconnecting means at the outbuilding (NEC 225.32) or mandate that all outdoor feeders be installed as direct burial UF-B cable rather than THHN in conduit. Always check with your local Authority Having Jurisdiction (AHJ) before digging your trench.

Properly sizing your 120V subpanel feeder ensures your outbuilding receives clean, stable power without risking thermal damage to your panel or tripping the main breaker. Stick to the 75°C column, respect the voltage drop math, and always pull a dedicated ground.