Wire Length Calculator
Circuit Settings
Wire Runs
AWG Wire Gauge Reference Table
Common wire gauges, their ampacity ratings, and resistance values for copper conductors at 75°C.
| AWG Gauge | Diameter (in) | Max Amps (Cu, 75°C) | Max Amps (Al, 75°C) | Resistance (Ω/1000ft Cu) | Common Use |
|---|---|---|---|---|---|
| 14 | 0.0641 | 15A | -- | 3.14 | Lighting circuits, general outlets (15A breaker) |
| 12 | 0.0808 | 20A | 15A | 1.98 | Kitchen outlets, bathrooms, general circuits (20A breaker) |
| 10 | 0.1019 | 30A | 25A | 1.24 | Dryers, window AC units (30A breaker) |
| 8 | 0.1285 | 40A | 30A | 0.778 | Ranges, large appliances (40A breaker) |
| 6 | 0.1620 | 55A | 40A | 0.491 | Large HVAC, sub-panels (50-60A breaker) |
| 4 | 0.2043 | 70A | 55A | 0.308 | Sub-panels, feeders (70A breaker) |
| 3 | 0.2294 | 85A | 65A | 0.245 | Sub-panels, commercial feeders |
| 2 | 0.2576 | 95A | 75A | 0.194 | Large sub-panels, service entrance |
| 1 | 0.2893 | 110A | 85A | 0.154 | Service entrance, large feeders |
| 1/0 | 0.3249 | 125A | 100A | 0.122 | 100A service entrance |
| 2/0 | 0.3648 | 145A | 115A | 0.0967 | Large service entrance, feeders |
| 4/0 | 0.4600 | 195A | 150A | 0.0608 | 200A service entrance |
Ampacity values per NEC Table 310.16 for conductors in raceway or cable at 75°C ambient. Actual ampacity depends on installation conditions, number of conductors, and ambient temperature. Always verify with local electrical codes.
NEC Code Reference for Common Circuits
Key National Electrical Code requirements for residential and commercial wiring. Always consult the full NEC and local amendments.
General Lighting (NEC 210.11)
14 AWG / 15A or 12 AWG / 20AGeneral-purpose branch circuits for lighting and receptacles. Calculate load at 3 VA per square foot of living space. Maximum 10 outlets per 15A circuit.
Kitchen Small Appliance (NEC 210.11(C)(1))
12 AWG / 20A minimumMinimum two 20A small-appliance circuits required for kitchen countertop receptacles. GFCI protection required for all kitchen outlets.
Bathroom (NEC 210.11(C)(3))
12 AWG / 20ADedicated 20A circuit required for bathroom receptacles. GFCI protection required. May serve one or multiple bathroom receptacle outlets.
Laundry (NEC 210.11(C)(2))
12 AWG / 20AAt least one dedicated 20A circuit for laundry area receptacles. No other outlets permitted on this circuit.
Electric Dryer (NEC 220.54)
10 AWG / 30ADedicated 30A circuit with 4-wire connection (hot-hot-neutral-ground). Standard NEMA 14-30 receptacle.
Electric Range (NEC 220.55)
8 AWG or 6 AWG / 40-50ADedicated 40A or 50A circuit depending on range rating. 8 AWG for 40A, 6 AWG for 50A. NEMA 14-50 receptacle.
HVAC / AC Unit (NEC 440)
Varies by unit ratingWire size per nameplate MCA (Minimum Circuit Ampacity). Breaker size per nameplate MOP (Maximum Overcurrent Protection). Typically 10 AWG / 30A for residential units.
Voltage Drop (NEC 210.19 Note)
Max 3% branch, 5% totalNEC recommends (not requires) maximum 3% voltage drop on branch circuits and 5% total including feeder. Calculated based on wire length, gauge, and load.
How Wire Length Calculation Works
Estimating electrical wire length accurately prevents costly return trips to the store and ensures your project meets code requirements. The calculation involves measuring each wire run, adding extra for connections, and accounting for routing through the building structure.
1. Measure Each Run
Measure the straight-line distance from the electrical panel (or junction point) to each outlet, switch, or fixture. Include vertical distances when wiring goes up walls or between floors.
2. Add Connection Allowance
Add 6-12 inches at each connection point (outlet box, switch box, junction box, panel). Each run has at least 2 connection points. Panel connections need 12-18 inches for reaching different breaker positions.
3. Account for Routing
Wire rarely travels in a straight line. Add 10-20% for routing through studs, along joists, around corners, and through conduit. Complex routes with many bends need a higher percentage.
4. Check Voltage Drop
For long runs, verify voltage drop stays within NEC recommendations. If voltage drop exceeds 3%, consider upsizing the wire gauge. This is especially important for runs over 50 feet.
Formulas Used
Run Total = Distance + (Connections x Extra per Connection) + (Distance x Routing %)
Grand Total:
Total Wire = Sum of all Run Totals
Voltage Drop (Single Phase):
VD = (2 x Length x Current x Resistance per ft) / 1000
Voltage Drop (Three Phase):
VD = (1.732 x Length x Current x Resistance per ft) / 1000
Voltage Drop Percentage:
VD% = (VD / Source Voltage) x 100
Worked Examples
Example 1: Kitchen Circuit (3 Outlet Run)
Scenario: Panel to 3 kitchen outlets using 12 AWG wire, 120V, 20A circuit
Run 2: Outlet 1 to Outlet 2 = 8 ft, 2 connections
Run 3: Outlet 2 to Outlet 3 = 6 ft, 2 connections
Raw distance = 25 + 8 + 6 = 39 ft
Connection extra = 6 connections x 8 in = 48 in = 4 ft
Routing (15%) = 39 x 0.15 = 5.85 ft
Total = 39 + 4 + 5.85 = 48.85 ft
Purchase: 50 ft spool
Example 2: Dedicated Dryer Circuit
Scenario: Panel to laundry room dryer outlet, 10 AWG, 240V, 30A
Connection extra = 2 x 12 in (panel) = 24 in = 2 ft
Routing (20%) = 45 x 0.20 = 9 ft
Total = 45 + 2 + 9 = 56 ft
Voltage drop = (2 x 56 x 30 x 1.24) / 1000 = 4.17V
VD% = 4.17 / 240 = 1.74% (OK - under 3%)
Purchase: 75 ft spool
Example 3: Workshop Sub-Panel Feed
Scenario: Main panel to detached workshop, 4 AWG copper, 240V, 60A, long run
Connection extra = 2 x 18 in = 36 in = 3 ft
Routing (10% - direct burial) = 120 x 0.10 = 12 ft
Total = 120 + 3 + 12 = 135 ft
Voltage drop = (2 x 135 x 60 x 0.308) / 1000 = 4.99V
VD% = 4.99 / 240 = 2.08% (OK - under 3%)
Purchase: 150 ft spool
Privacy and Limitations
This wire length calculator provides estimates for planning purposes. All calculations are performed locally in your browser and no data is stored or transmitted.
- Wire lengths are estimates; always verify by measuring your actual route before cutting
- Voltage drop calculations assume uniform load and standard temperature conditions
- Ampacity ratings are based on NEC Table 310.16 at 75°C and may vary with installation conditions
- Local electrical codes may have requirements that differ from NEC standards
- This tool is for estimation only and does not replace a licensed electrician's judgment
- Always consult your local building department and a qualified electrician for actual installations
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Wire Length Calculator FAQ
How do I calculate how much electrical wire I need?
Measure the distance from the electrical panel to each outlet, switch, or fixture. Add 6-12 inches of extra wire at each connection point for making connections. Then add 10-20% extra for routing through walls, around corners, and through studs. Always round up to the next standard spool length to ensure you have enough wire.
What is voltage drop and why does it matter?
Voltage drop is the reduction in voltage as electricity travels through wire. Longer wire runs and smaller gauge wire cause more voltage drop. The NEC recommends keeping voltage drop under 3% for branch circuits and 5% total for feeder plus branch circuits. Excessive voltage drop can cause lights to dim, motors to overheat, and equipment to malfunction.
What wire gauge do I need for a 20-amp circuit?
Per NEC code, a 20-amp circuit requires a minimum of 12 AWG copper wire. For longer runs over 50 feet, you may need to upsize to 10 AWG to keep voltage drop within acceptable limits. Always check local electrical codes as requirements may vary.
How much extra wire should I add per connection?
Add 6 to 12 inches of extra wire at each connection point (outlet, switch, junction box, or panel). This provides enough slack to strip the wire, make connections, and fold the wire back into the box. For panels, add 12-18 inches since connections may need to reach different breaker positions.
What is the difference between copper and aluminum wire?
Copper wire has better conductivity (lower resistance), is more flexible, and handles more current for a given gauge. Aluminum wire is lighter and cheaper but requires larger gauge sizes for the same ampacity. Aluminum is commonly used for service entrance cables and large feeder circuits, while copper is standard for branch circuits.
How do I account for wire routing through walls?
Add 10-20% to the straight-line distance to account for routing wire through walls, around obstacles, through studs, and up or down between floors. For complex routes with many turns, add 20%. For straightforward runs along joists or through open basement ceilings, 10% is sufficient.