Electrical Calculators
Long wire runs to a detached garage, a shed, a well pump, an EV charger or a string of low-voltage landscape lights lose voltage along the way, and the breaker table alone will not tell you how much. These calculators use the same resistance data the National Electrical Code prints in Chapter 9, so you can check a run before you buy a spool of wire and talk it through with your electrician or inspector.
Voltage drop: why long runs need bigger wire
Copper and aluminum both resist current a little, and over a long run that resistance adds up to volts the load never sees. The National Electrical Code’s Chapter 9, Table 8 lists the resistance of every standard conductor at 75 °C, and electricians turn it into a single constant: about 12.9 ohms per circular-mil-foot for copper and 21.2 for aluminum. Multiply by 2, the current and the one-way length, divide by the wire’s circular mils, and you have the drop in volts. The voltage drop calculator does that for single-phase, three-phase and DC circuits, finds the smallest size that meets your target and shows the longest run a given size can manage.
Quick reference: 3% at 120 V on copper
| Load | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
|---|---|---|---|---|
| 10 A | 91 ft | 145 ft | 230 ft | 366 ft |
| 16 A | 57 ft | 91 ft | 144 ft | 229 ft |
| 20 A | 46 ft | 72 ft | 115 ft | 183 ft |
These are the longest one-way runs that stay within 3.6 V. Double them for the same current at 240 V. Sixteen amps is 80% of a 20 A breaker, the most it should carry for a continuous load once the 125% sizing rule is applied, which is why it appears in the table.
Three checks before you buy wire
- Ampacity. The wire has to carry the breaker’s current without overheating. NEC Table 310.16 sets that by size and insulation temperature, and 240.4(D) caps 14, 12 and 10 AWG copper at 15, 20 and 30 A.
- Voltage drop. The NEC’s informational notes suggest 3% for a branch circuit and 5% for feeder and branch together. They are recommendations, but Mike Holt notes that motors and electronics running below their rated voltage can lose service life or shut down, and heaters and lights simply put out less.
- Grounding. If you upsize the circuit conductors for voltage drop, the equipment grounding conductor has to grow with them under 250.122(B).
When to call an electrician
Rules on who may do electrical work, and which jobs need a permit and an inspection, vary by state and city. Use these numbers to plan the run and price the wire, then confirm the design with a licensed electrician or whoever signs the permit.
Related project calculators
Trenching out to a shed or a detached garage often happens alongside other outdoor work. The concrete calculator sizes a slab for the building, the fence calculator counts posts along the same property line, and the deck calculator handles the boards if the circuit is feeding deck lighting.
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