What the result means
The result checks two independent limits: ampacity for the load and voltage drop along the one-way run. It selects the larger conductor size so the wire meets both checks within the supplied tables.
The ampacity table assumes 30°C (86°F) ambient and no more than three current-carrying conductors. Installation temperature, bundling and dwelling service or feeder rules can change the applicable value; those adjustments are not calculated here.
How it's calculated
- Required ampacity = load × 1.25 for a continuous load; otherwise load
- Single-phase voltage drop = 2 × K × I × L ÷ CM
- Three-phase voltage drop = 1.732 × K × I × L ÷ CM
- Selected size = the larger of the ampacity size and voltage-drop size
K = 12.9 for copper and 21.2 for aluminum; L is one-way length in feet; CM is conductor area in circular mils.
Small-conductor limits: copper 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A; aluminum 12 AWG = 15 A, 10 AWG = 25 A.
Watts mode assumes unity power factor. Confirm actual equipment nameplate current when available.
Worked example
20 A on 120 V single-phase, 50 ft one way, copper, 60°C terminals, noncontinuous load, with a 3% maximum voltage drop.
- Required ampacity is 20 A; 12 AWG meets the supplied ampacity table and small-conductor limit.
- 12 AWG drops 3.94 V, or 3.29%; it exceeds the 3% voltage-drop limit.
- 10 AWG drops 2.49 V, or 2.07%; it meets the voltage-drop limit.
Select 10 AWG copper. Voltage drop, not ampacity, determines the final size.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Conductor size | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 60°C | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | — | — | — |
| 12 AWG | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 AWG | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 AWG | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 AWG | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 AWG | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 AWG | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 AWG | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 AWG | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 AWG | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 AWG | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 AWG | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 AWG | 195 | 230 | 260 | 150 | 180 | 205 |
| 250 kcmil | 215 | 255 | 290 | 170 | 205 | 230 |
| 300 kcmil | 240 | 285 | 320 | 195 | 230 | 260 |
| 350 kcmil | 260 | 310 | 350 | 210 | 250 | 280 |
| 400 kcmil | 280 | 335 | 380 | 225 | 270 | 305 |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 |
| 600 kcmil | 355 | 420 | 475 | 285 | 340 | 385 |
| 750 kcmil | 400 | 475 | 535 | 320 | 385 | 435 |
Values are base ampacity in amperes. Small-conductor limits apply separately: copper 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A; aluminum 12 AWG 15 A and 10 AWG 25 A. Ambient correction, conductor-count adjustment and NEC 310.12 are not applied.
Sources and assumptions
K = 12.9 for copper and 21.2 for aluminum; L is one-way length in feet; CM is conductor area in circular mils. Small-conductor limits: copper 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A; aluminum 12 AWG = 15 A, 10 AWG = 25 A. Watts mode assumes unity power factor. Confirm actual equipment nameplate current when available. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box. Where a code table, manufacturer specification or field measurement is required, use the applicable local source. Copper voltage-drop tables use K = 12.9 ohm-cmil/ft at 75°C.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
Why can the ampacity size differ from the selected size?
The selected size is the larger of the ampacity size and the voltage-drop size. A long run can need a larger conductor even when the smaller one carries the load.
How is a continuous load handled?
The required ampacity is multiplied by 1.25 when the continuous-load checkbox is selected.
Does this include temperature and conductor-count corrections?
No. It uses the supplied base table for 30°C ambient and not more than three current-carrying conductors. Apply any required code adjustments separately.
Does it size dwelling services and feeders?
No. The tool does not apply the alternate dwelling service and feeder rules in NEC 310.12.
Which NEC edition does the table use?
The ampacity values are the supplied Table 310.16 data. Check the edition adopted by your jurisdiction and confirm all installation conditions.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Where a field asks for a length, confirm whether it is one-way or total; where it asks for current, confirm whether the load is continuous or intermittent. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. A different conductor material, temperature, power factor or load type can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
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