Wire size calculator

Ampacity and voltage drop, checked together

A wire fails in two different ways. It can run too hot for its insulation and its terminals. That limit is ampacity. Or it can deliver too few volts at the far end because of its own resistance. That limit is voltage drop. The conductor you pull has to clear both. Copperline finds the smallest copper size and the smallest aluminum size that do, then names the limit that made the wire larger.

Ampacity starts from the column in NEC Table 310.16 for the insulation you are actually installing: 60°C, 75°C, or 90°C. If the air is not 30°C, the correction factor in Table 310.15(B)(1) is applied. If more than three current-carrying conductors share the raceway or cable, the adjustment in Table 310.15(C)(1) is applied as well. The derated number is then capped at the terminal rating. THHN is a 90°C insulation, and that column is the right place to start a derate, but a breaker marked 75°C still caps the usable ampacity at the 75°C column.

A continuous load, one that runs for three hours or more, is sized at 125% of the running current. Voltage drop does not use that extra quarter. The extra is a heating allowance in the code, not extra amperes flowing down the wire, so the drop is figured at the current the load actually draws.

Voltage drop uses the direct-current resistance in NEC Chapter 9 Table 8 for uncoated stranded conductors at 75°C. Single-phase and DC circuits multiply by two, once for the outgoing conductor and once for the return. Three-phase circuits multiply by √3. You can turn on Chapter 9 Table 9 when the power factor is below 1 and you care about conduit reactance. Leave it off and the result matches the Table 8 method used in the worked examples.

A 20 A circuit at 100 feet

Twelve AWG stranded copper on a 120 V, 20 A, 100 ft branch drops 7.92 V. That is 6.60% of 120 V. Ten AWG still drops 4.96 V, or 4.13%. Eight AWG drops 3.11 V, or 2.59%, and it is the smallest stranded copper size inside a 3% limit. Ampacity at 60°C would have allowed 12 AWG. Voltage drop is what governs.

The same run in aluminum needs 6 AWG. Aluminum’s Table 8 resistance is higher, so the drop catches it one size sooner. The K-factor shortcut with K = 12.9 and 6,530 circular mils gives 7.90 V on the 12 AWG copper run. The 0.02 V gap is the rounding inside K, not a second physics. Open this example in the calculator.

Where each preset is trying to teach something

Household 120 V branches get tight quickly. Three percent of 120 V is only 3.6 V, so a 15 A lighting run of about 50 ft often outgrows 14 AWG even though the breaker is happy. A 48 A electric-vehicle charger is the other shape of problem: it is a continuous load, the conductor is checked at 60 A, and on a short driveway 6 AWG copper is chosen by ampacity while the drop is still quiet. A 12 V solar run reverses it again. Ampacity is easy at 20 A. The round trip at 12 V spends the voltage budget in a few metres, and the wire gets large because of drop.

Questions electricians actually ask

How does this wire size calculator choose a conductor?

It finds the smallest size that meets two limits at the same time. Allowable ampacity, after ambient correction and adjustment for the number of current-carrying conductors, has to cover the load. Continuous loads are sized at 125 percent. Voltage drop over the one-way run also has to stay inside the percent you allow, using the operating current rather than the 125 percent figure. The result names the limit that forced the larger conductor.

Which temperature column should I use?

Use the insulation you are installing as the starting column. THHN and XHHW-2 are 90°C conductors. THWN and XHHW are 75°C. TW, UF, and NM-B cable are 60°C. Then cap the result at the terminal rating. On a circuit of 100 A or less, unmarked terminals are 60°C unless the equipment is marked 75°C, and most modern breakers are marked 75°C. The 90°C column is the right place to begin a derate. It is not the ampacity you land on when the terminals are cooler.

What do ambient temperature and conductor count change?

Table 310.16 assumes 30°C air and no more than three current-carrying conductors. Hotter air multiplies ampacity by the Table 310.15(B)(1) factor for that insulation. Four or more current-carrying conductors in the same raceway or cable apply the Table 310.15(C)(1) adjustment, down to 35 percent at 41 conductors. Equipment grounding conductors are not counted. A neutral counts when it carries current as a normal current-carrying conductor, and it does not count when it carries only the imbalance of a linear single-phase multiwire circuit.

Is a 3 percent voltage drop required by the NEC?

No. The code mentions about 3 percent on the branch circuit or the feeder, and about 5 percent combined, in informational notes to 210.19(A) and 215.2(A). Informational notes are not requirements. Three percent is the default here because it is the figure those notes give, and you can change it.

Can I switch between AWG and square millimetres?

Yes. AWG mode selects from the sizes in Table 310.16 and uses Chapter 9 Table 8 resistance. Square-millimetre mode selects a standard IEC 60228 cross-section. Its voltage drop uses the IEC maximum resistance, corrected from 20°C to 75°C. The NEC does not list metric sizes, so metric ampacity is taken from the largest AWG or kcmil whose area does not exceed that cross-section.

Does it work for solar DC and for EV chargers?

Yes. DC uses two current-carrying conductors and no reactance, which is the right model for a 12, 24, or 48 volt run from a charge controller to a battery. An EV charger marked 48 A is a continuous load, so the conductor is checked at 60 A. Presets for both are above the form.

Why are the copper and aluminum results different?

Aluminum has higher resistance and a lower table ampacity at the same size, so it usually needs a larger conductor for the same load and the same drop. Both metals are calculated on every run. Aluminum terminations have to be listed for aluminum, and the conductor has to be tightened to the device torque. This tool does not check that listing.

Is this a substitute for the National Electrical Code?

No. It is a planning aid that runs entirely in your browser. The adopted code edition, local amendments, rooftop sunlight, conduit fill, motor tables, and the equipment nameplate can all change the conductor. Have a qualified electrician review the circuit before you buy wire or energize it.