For a 50-amp load at 100 feet on a 240V circuit, use 6 AWG copper wire on a 50-amp breaker. While 8 AWG copper handles 50A thermally, 6 AWG keeps voltage drop under 3% for 240V, and is strictly required if your circuit operates at 120V.
- Material: Copper (unless explicitly noted as aluminum)
- Temperature Column: 75°C (per NEC Table 310.16 and standard breaker terminal ratings)
- Ambient Temperature: 30°C (86°F)
- Installation Method: Single circuit in a raceway (conduit) or NM-B cable, not bundled with other current-carrying conductors
- Load Type: Non-continuous (operates for less than 3 hours at a time)
The Sizing Logic: Ampacity vs. Voltage Drop
If you look at NEC Table 310.16, you will see that 8 AWG copper wire with THHN/THWN-2 insulation is rated for exactly 50 amps in the 75°C column. So why are we pulling 6 AWG? Because wire sizing is a two-part test: it must pass both the thermal ampacity test and the voltage drop test.
Voltage drop is the electrical pressure lost as heat due to the resistance of the wire over distance. The NEC recommends a maximum 3% voltage drop for branch circuits to ensure equipment operates efficiently and safely. Let us run the math for a 100-foot run (which means 200 feet of total wire length when accounting for the hot and neutral/return paths) using the standard voltage drop formula: VD = (2 × K × I × L) / Circular Mills.
| Wire Size (Copper) | Ampacity (75°C) | Voltage Drop (240V) | VD % (240V) | Voltage Drop (120V) | VD % (120V) |
|---|---|---|---|---|---|
| 8 AWG | 50A | 7.81V | 3.25% (Fails 3%) | 7.81V | 6.50% (Fails) |
| 6 AWG | 65A | 4.91V | 2.04% (Passes) | 4.91V | 4.09% (Fails 3%) |
| 4 AWG | 85A | 3.09V | 1.28% (Passes) | 3.09V | 2.57% (Passes) |
Here is the critical takeaway most generic guides miss: Voltage drop is entirely dependent on your system voltage. If you are wiring a 240V circuit (like a welder, subpanel, or EV charger), 6 AWG copper is your correct answer. The 2.04% drop is well within the 3% limit. However, if you are wiring a 120V 50-amp circuit (rare in residential, but common in some RV pedestal setups or industrial controls), 6 AWG yields a 4.09% drop. For 120V at this distance, you must step up to 4 AWG copper to stay under 3%.
Variables That Change Your Wire Size
The 6 AWG recommendation assumes ideal, baseline conditions. Real-world jobsites and garages rarely cooperate. Here is a decision tree for when you must upsize your wire.
| Condition | Why It Matters | Required Action |
|---|---|---|
| Continuous Load (On for 3+ hours) | NEC 215.2 requires conductors to be sized at 125% of continuous loads (50A × 1.25 = 62.5A). | Upsize to 4 AWG Copper (Rated 85A) and use a 60A or 70A breaker depending on exact load limits. |
| Conduit Bundling (4+ current-carrying wires) | NEC 310.15(C)(1) mandates ampacity derating due to trapped heat. 4-6 wires = 80% derating factor. | Upsize to 4 AWG Copper to maintain 50A capacity after the 80% derating penalty. |
| High Ambient Heat (Attics >104°F / 40°C) | Wire insulation degrades faster; ampacity must be corrected using NEC Table 310.15(B)(1). | Upsize to 4 AWG Copper or route conduit through conditioned space. |
| Using Aluminum Wire | Aluminum has higher resistance (K=21.2 vs 12.9 for copper) and lower thermal ampacity per AWG. | Use 4 AWG Aluminum (Rated 65A at 75°C) for 240V circuits to pass both thermal and VD tests. |
A note on aluminum: Never treat aluminum and copper interchangeably. While 4 AWG aluminum (like XHHW-2 or SER cable) is a cost-effective alternative for long feeder runs, you must ensure your breaker and equipment lugs are explicitly rated for aluminum (AL/CU). If they are not, you must use copper, or your terminations will oxidize, increase in resistance, and eventually start a fire.
When an Engineer or the AHJ Must Confirm
While the NEC provides the baseline rules, your local Authority Having Jurisdiction (AHJ) or the city electrical inspector has the final say on your installation. You should pull a permit and have an engineer or inspector review your plan if:
- You are exceeding standard residential parameters: If this 50-amp circuit is part of a larger microgrid, solar battery inverter setup, or involves utility interconnection, standard branch circuit rules are superseded by complex fault-current calculations.
- Voltage drop exceeds 5% total: The NEC recommends a combined maximum of 5% voltage drop for the feeder and branch circuit combined. If your subpanel feeder already has a 3% drop, adding a 3% branch circuit drop puts you at 6%, which an inspector may flag for equipment safety.
- Specialized Equipment: If the 50-amp load is a medical device, a heavy-duty industrial motor with high inrush currents, or an elevator, the manufacturer's spec sheet and a licensed electrical engineer will dictate the exact wire size, often requiring upsizing to handle momentary voltage sag during motor startup.
For standard residential applications like a detached garage subpanel or a hardwired EV charger, following the 6 AWG copper (240V) or 4 AWG copper (120V) guidelines while adhering to local permitting will keep you on the right side of the inspector. Tools like the Southwire Voltage Drop Calculator are excellent for double-checking your specific conduit fill and exact run length before buying wire.
Frequently Asked Questions
What size wire for a 50 amp EV charger at 100 feet?
Electric Vehicle Supply Equipment (EVSE) is classified as a continuous load under the NEC because charging sessions routinely exceed three hours. This triggers the 125% sizing rule. A 50-amp EV charger actually requires wire rated for 62.5 amps. Therefore, you cannot use 6 AWG copper (rated 65A, which leaves virtually no margin and fails continuous load derating in warm environments). You must use 4 AWG copper wire on a 60-amp breaker (or configure the EV charger's internal DIP switches to draw a maximum of 48 amps, which allows 6 AWG on a 60A breaker). Always check the specific installation manual for your Tesla Wall Connector, ChargePoint, or Emporia unit, as manufacturer instructions supersede general NEC rules per NEC 110.3(B).
Can I use 8 AWG wire if my 50 amp load is only 100 feet away?
Technically, 8 AWG copper is rated for 50 amps in the 75°C column. However, using it at 100 feet results in a 3.25% voltage drop on a 240V circuit. While the NEC's 3% voltage drop recommendation is an 'Informational Note' and not strictly enforceable as a hard violation in all jurisdictions, many inspectors will reject it. More importantly, the equipment you are powering (like a welder or air compressor) will run hotter, less efficiently, and may trip its own internal thermal overloads due to the low voltage at the terminals. Spend the extra money on 6 AWG to do the job right the first time.
Does the equipment grounding conductor count toward voltage drop?
No. When calculating voltage drop, you only count the current-carrying conductors (the ungrounded 'hot' wires and the grounded 'neutral' wire if it carries unbalanced current). The bare copper or green equipment grounding conductor (EGC) only carries current during a fault condition to trip the breaker. Under normal operation, it carries zero current, contributes no resistance to the active circuit, and does not factor into your voltage drop math. However, if you are upsizing your hot wires for voltage drop (e.g., moving from 8 AWG to 6 AWG), NEC 250.122(B) requires you to proportionally upsize your ground wire as well to maintain a low-impedance fault path.
