A practical guide · NEC 2023 Table 310.16, 310.15, 240.4(D), Chapter 9
How wire sizing works
A wire has two jobs. It has to carry the current without getting too hot, and it has to deliver enough of the source voltage to the far end. Each job sets a minimum size, and the larger minimum wins. The chapters below use the circuit in the calculator, so change it and the figures follow.
How do I calculate what size wire I need?
Start with the load, at 125% if it runs three hours or more. Find the smallest size in Table 310.16 that still carries it after temperature and grouping corrections and the terminal limit. Then check voltage drop over the one-way run: if it is over 3%, go up a size until it isn't. Use the larger of the two answers, and no larger.
The calculator does both checks at once and shows which one set the size. Ampacity uses the 90 °C THHN/THWN-2 column as the basis for correction, capped by the 60 °C or 75 °C terminal column and the small-conductor limits of 240.4(D). Voltage drop uses Chapter 9 Table 9 for conductors in EMT at a 0.9 power factor.
Wire sizes for a 100 A subpanel, 60 A subpanel, 50 A range, 60 A EV charger and 200 A service
Worked answers from the calculator, at 240 V single-phase, 75 °C terminals, 30 °C ambient, no more than three current-carrying conductors, and a 3% voltage drop target for conductors in EMT at a 0.9 power factor. Each cell shows the smallest size that passes both checks and its voltage drop.
| Circuit | Sized for | Run | Copper | Aluminum | Set by |
|---|---|---|---|---|---|
| 100 A subpanel feeder | 100 A | 150 ft | #2, 2.56% | #1/0, 2.55% | Distance |
| 60 A subpanel feeder | 60 A | 100 ft | #6, 2.34% | #4, 2.43% | Ampacity |
| 50 A range circuit | 50 A | 50 ft | #8, 1.52% | #6, 1.58% | Ampacity |
| 60 A EV charger circuit (48 A charger) | 60 A (125%) | 75 ft | #6, 1.41% | #4, 1.46% | Ampacity |
| 200 A service conductors | 200 A | 50 ft | #3/0, 0.78% | 250 kcmil, 0.83% | Ampacity |
Longer runs move these sizes up. Dwelling services and main feeders may be sized under 310.12 instead, which this calculator does not model. Change any input in the calculator above to size your own circuit.
Chapter 1
Two checks, and the bigger one wins
The ampacity check sets a floor from the tables: below it, the wire runs hotter than its insulation and terminals allow. The voltage drop check sets a floor from the length of the run: below it, too much of the source voltage is spent in the wire. The two floors don't move together. Ampacity depends on the current and the conditions around the wire; voltage drop depends on the current and the distance.
- #14too small for the load
- #12too small for the load
- #10carries it, drops over 3%, ampacity floorampacity floor
- #8passes both, distance floor, the size to buydistance floor · buy
- #6passes both
- #4passes both
- #3passes both
- #2passes both
- #1passes both
Short runs are almost always decided by ampacity. Long runs, low voltages and heavy loads hand the decision to voltage drop. Knowing which one decided tells you what would change the answer: a shorter route only helps when distance set the size.
Chapter 2
The NEC Table 310.16 value isn't the answer
Table 310.16 lists what each size carries with no more than three current-carrying conductors together at 30 °C. Real installations differ, and the NEC gives three ways the number comes down.
- Ambient temperature. A hotter space leaves less room for the wire's own heat. Table 310.15(B)(1) gives a factor for each band: 0.91 at 40 °C for 90 °C insulation.
- Grouping. Wires bundled together warm each other. With four to six current-carrying conductors in a raceway or cable, Table 310.15(C)(1) takes the ampacity to 80%; seven to nine, 70%.
- Terminals. The wire can't carry more than the lug it lands on allows. 110.14(C) holds the result to the terminal's column: 60 °C for circuits of 100 A or less unless the equipment is marked for 75 °C, which most current equipment is.
The corrections start from the 90 °C column, because that is the insulation's rating, and the lowest limit is the one that counts. On small branch-circuit sizes, 240.4(D) adds one more: #14, #12 and #10 copper are protected at no more than 15, 20 and 30 A.
What #10 copper can carry here
- Table 310.16 at 90 °C
- 40 A
- × 1.00 for 30 °C ambient
- 40 A
- × 1.00 for grouping
- 40 A
- 75 °C terminal column
- 35 A
- 240.4(D) small-conductor limit
- 30 A, the limit that controls
- You need
- 30 A
Chapter 3
Continuous loads get 125%
A load that runs at its maximum current for three hours or more, like lighting in a store or a water heater, is sized at 125% of its current (210.19(A)(1) for branch circuits, 215.2(A)(1) for feeders). The extra quarter covers the heat that builds up in conductors and terminals over a long, steady load.
The 125% is a sizing basis for ampacity only. The wire still carries the real current, so voltage drop is figured with the actual amps.
Chapter 4
Where distance takes over
Voltage drop grows in a straight line with the run. Every size can carry its load only so far before it crosses the target, so as the run gets longer, the size it needs steps up. Below that distance ampacity decides; past it, distance does.
The size 30 A at 240 V needs at every length
Informational notes in 210.19 and 215.2 recommend no more than 3% on a branch circuit and 5% for the feeder and branch together. They are recommendations, not enforceable limits, but specifications and energy codes adopt them widely. Enter the routed length, the path the wire actually takes, not the straight line on the plan.
Chapter 5
Don’t buy copper you don’t need
A common rule of thumb is to go one size up on every home run, or on everything over 100 feet, just to be safe. It works, but it pays for copper on every run that didn't need it, and a bigger wire also takes more raceway fill and a proportionally bigger ground under 250.122(B). Sizing each run to its own length puts the extra copper only where the distance calls for it.
A job with 40 runs like yours
- Everything one size up (#6)
- $9,408
- Each run sized to its length (#8)
- $5,952
$3,456 of copper you don’t buy
ZapSketch measures every run on your PDF plan and sizes each one for ampacity and voltage drop as you draw, so the bid and the pull match.
Size every run on a planChapter 6
Copper vs aluminum ampacity
Aluminum conducts about 61% as well as copper, so the same current needs a larger aluminum conductor, usually one or two sizes up. It is lighter and costs less per ampere, which is why it is common on feeders and services. On small branch circuits copper is the norm.
| Size | Copper | Aluminum |
|---|---|---|
| #12 | 25 | 20 |
| #10 | 35 | 30 |
| #8 | 50 | 40 |
| #6 | 65 | 50 |
| #4 | 85 | 65 |
| #2 | 115 | 90 |
| #1/0 | 150 | 120 |
| #2/0 | 175 | 135 |
| #3/0 | 200 | 155 |
| #4/0 | 230 | 180 |
Table values before 240.4(D), which limits #12 and #10 copper to 20 A and 30 A protection.
A worked example
Take the starting circuit: a 30 A load at 240 V single-phase, 120 ft from the panel, in copper, with 75 °C terminals, 30 °C ambient and two current-carrying conductors. Table 310.16 lists #10 copper at 40 A in the 90 °C column and 35 A in the 75 °C column, and 240.4(D) limits it to 30 A, so #10 passes the ampacity check at exactly 30 A. Over 120 ft, though, #10 drops 2 × 30 A × 1.107 Ω × 120 ft ÷ 1,000 = 7.97 V, or 3.32% of 240 V. #8 drops 5.26 V, 2.19%, so #8 is the size to use. Going one more size to #6 would add about $86 of copper to the run at example prices, for a drop the circuit doesn't need.
What this calculator leaves out
- Modeled scope. Copper and aluminum from #14 through 600 kcmil (aluminum from #12), THHN/THWN-2 on a 90 °C correction basis, 60 °C or 75 °C terminals. Voltage drop uses Chapter 9 Table 9 for single-phase or balanced three-phase AC in EMT at a 0.9 power factor; use the voltage drop calculator for DC, PVC or another power factor.
- Overcurrent protection. The calculator sizes the conductors for the load. It does not pick the breaker or check that the conductor is protected by it beyond the small-conductor limits of 240.4(D).
- Special rules. Dwelling services and feeders (310.12), motors, air conditioning equipment, welders, transformers, parallel sets, rooftop temperature adders and NM cable's 60 °C ampacity are not modeled.
- Equipment grounding conductors. When conductors are upsized for voltage drop, 250.122(B) increases the ground in proportion. The calculator sizes only the circuit conductors.
- Example prices. Copper costs use example THHN prices per foot for #14 through #1, two conductors per single-phase run and three per three-phase run. They leave out grounds, labor and waste and are not a quote.
- Other conditions. Harmonics, conductors in sunlight or near heat sources, underground ampacities and jurisdiction-specific rules may need additional review.
This page and ZapSketch’s checks use NEC 2023. Follow equipment instructions and all applicable codes and regulations.
Wire size questions
How do I calculate what size wire I need?
Find the smallest conductor whose ampacity covers the load (125% of it for a continuous load) after correcting for ambient temperature and the number of current-carrying conductors, and limiting it to the terminal rating. Then check voltage drop over the one-way run; if that size drops more than your target, usually 3%, step up until it doesn't. The larger of the two sizes is the one to use.
What size wire do I need for 30 amps?
#10 copper, at 75 °C terminals and 30 °C ambient with up to three current-carrying conductors: Table 310.16 lists it at 35 A and 240.4(D) limits it to 30 A protection. On a long run voltage drop can call for more. At 240 V, #10 crosses 3% at about 108 ft, and #8 is the next size up.
What size wire do I need for 20 amps?
#12 copper under the same conditions. At 120 V a 20 A load on #12 crosses 3% at about 49 ft, so longer runs often go to #10.
What size wire for 50 amps?
#8 copper carries 50 A in the 75 °C column of Table 310.16, and #6 aluminum carries 50 A as well. With 60 °C terminals, copper moves up to #6. Check voltage drop for the length of the run.
Does a continuous load need a bigger wire?
Often. A load expected to run at its maximum current for three hours or more is sized at 125% of that current (210.19(A)(1) and 215.2(A)(1)). A 40 A continuous load is sized for 50 A, which needs #8 copper instead of #10's 35 A at 75 °C.
Why use the 90 °C column if my terminals are rated 75 °C?
THHN/THWN-2 insulation is rated 90 °C, so 310.15 lets you start ambient correction and grouping adjustment from the 90 °C value. The result still can't exceed the ampacity in the terminal's column (110.14(C)), so the lower of the two controls.
Should I upsize wire to be safe?
Upsize when the run needs it for voltage drop or derating, not by habit. A size up costs more copper, takes more raceway fill, and makes 250.122(B) increase the equipment grounding conductor in proportion. Sizing each run to its own length keeps the margin where it is needed.
Is aluminum wire the same size as copper?
No. Aluminum carries less current for the same size, so it usually takes one or two sizes larger: #6 aluminum and #8 copper both carry 50 A at 75 °C. Aluminum also has more resistance, which matters for voltage drop on long runs.
What size wire for a 100 amp subpanel 150 feet away?
#2 copper or #1/0 aluminum at 240 V, with 75 °C terminals and a 3% voltage drop target. Distance sets the size: the smallest wire that carries 100 A drops more than 3% over 150 ft. Copper drops 2.56%.
What size wire for a 60 amp subpanel?
#6 copper or #4 aluminum from the 75 °C column of Table 310.16. Over 100 ft at 240 V, copper drops 2.34%, within a 3% target; much longer runs need the next size up.
What size wire for a 60 amp EV charger circuit?
A 48 A charger is a continuous load, so the conductors are sized for 125%, or 60 A: #6 copper or #4 aluminum at 75 °C. Over 75 ft at 240 V, copper drops 1.41%. Follow the charger's installation instructions too.
What size wire for a 200 amp service?
From Table 310.16 at 75 °C, #3/0 copper (200 A) or 250 kcmil aluminum (205 A). For a dwelling's service or main feeder, 310.12 lets the conductors be sized at 83% of the rating, which usually allows a smaller size. This calculator does not apply 310.12.
Sources: NEC 2023 (NFPA 70 (opens in a new tab)) 110.14(C), 210.19, 215.2, 240.4(D), 250.122(B), Tables 310.15(B)(1), 310.15(C)(1) and 310.16, Chapter 9 Table 9. Related: voltage drop calculator and conduit fill calculator.
From one wire to the whole job
Size the wire for every home run on a plan
Trace home runs on the drawing and ZapSketch keeps every measured length connected to ampacity, voltage-drop, raceway-fill and material checks, so each run gets the wire it needs.