The ampacity of a wire is the maximum continuous current it can carry without overheating — and it is the number that decides whether your fuse or breaker actually protects the cable. Get it wrong and a breaker sized too large lets an overloaded wire cook quietly inside the conduit; get it right and every component in the chain is protected.
The rule: base table value × correction factors
NEC Table 310.16 gives the base ampacity for each conductor size and insulation rating, assuming 30°C ambient and three or fewer current-carrying conductors. Your real installation rarely matches those conditions, so the code applies correction factors on top:
Adjusted ampacity = base ampacity × temperature factor × bundling factor
Then the breaker or fuse must be rated at or below that adjusted value. For example, 10 AWG THWN-2 has a base ampacity of 40 A at 90°C; in a 45°C rooftop conduit with 4 bundled conductors that becomes 40 × 0.87 × 0.80 = 27.8 A, so the maximum breaker is 25 A, not the 40 A you might guess from the table alone.
Choosing the insulation column
The column you use is set by the wire's insulation, not its size. USE-2 and PV wire common on rooftops are rated 90°C, as is THHN/THWN-2 in dry locations. NM-B (Romex) is a special case: even though its insulation is rated 90°C, the code limits it to the 60°C column for ampacity. When terminations are rated 75°C, use the 75°C column for the final run — a detail covered by NEC 110.14(C).
Ambient temperature and bundling
Every degree above 30°C reduces what the wire can safely carry — the correction factors come from NEC Table 310.15(B)(1). A conduit on a sun-baked roof in Amman or Arizona can sit at 45-50°C in summer, cutting ampacity by a quarter. Bundling adds up fast too: 4-6 conductors in one conduit derate to 80%, 7-9 to 70%, and 10-20 to 50%. The equipment ground doesn't count as a current-carrying conductor, so a typical PV pair plus ground stays in the "no derating" zone.
Picking the breaker
Round the adjusted ampacity down to the next standard breaker size — 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A and so on. NEC 240.4(B) allows the next standard size up when the adjusted value falls between sizes and the load fits, which is how many solar installs land on a 30 A breaker for a 26 A adjusted value. On the load side, keep continuous solar circuits at or under 80% of the breaker rating.
Field note from a Jordanian installer
On the Ma'an projects the rooftop conduit runs were the real test: 40°C+ summer ambient, dark conduits on bare roofs, and string home-runs bundled with monitoring cables. The 10 AWG THHN runs that looked fine on paper needed re-checking against the derated value — several needed upsizing to 8 AWG to keep the 30 A breaker legit. The lesson: always calculate with worst-case summer temperature, not the comfortable spring day you install on.
Work it with the other wiring tools
Ampacity is only half the story — the other half is voltage drop. Once this calculator confirms the wire is safe, check the run length with the voltage drop calculator, and if you're starting from a load, the cable size calculator does both checks in one. For the breaker side, the fuse & breaker sizing calculator applies the 1.25 continuous-load rule, and the watts to amps calculator turns your power figure into the current this tool needs.
Related guides
For a full walk-through of conductor sizing on the DC side, see the guide on what size cable you need for solar panels and inverters, and for choosing overcurrent protection on the PV array side, read the solar inverter buying guide protection sections.
Frequently asked questions
What is wire ampacity?
It is the maximum continuous current a conductor can carry without exceeding its insulation temperature. It depends on size, insulation rating, ambient temperature, and bundling — NEC Table 310.16 is the standard reference.
Which insulation rating should I use for solar wire?
Most PV wire and USE-2 is 90°C; THHN/THWN-2 is 90°C dry and 75°C wet; NM-B indoor cable is limited to the 60°C column. Use the rating printed on the cable sheath, limited by termination ratings.
Why does ambient temperature reduce ampacity?
Tables assume 30°C. Hotter surroundings slow heat shedding, so the code applies correction factors — a 50°C rooftop run can cut ampacity by up to 25% versus the table value.
How does bundling wires together reduce ampacity?
Bundled conductors heat each other. NEC derating: 1-3 conductors = 100%, 4-6 = 80%, 7-9 = 70%, 10-20 = 50% of base ampacity. The equipment ground doesn't count.
How do I pick the breaker size from the ampacity?
Choose the next standard breaker size at or below the adjusted ampacity. NEC 240.4(B) allows the next size up when the adjusted value falls between sizes and the continuous load fits.
Does this calculator work for both DC and AC wiring?
Yes — ampacity is a property of the conductor. Use array Isc × 1.25 for PV string currents on the DC side and the continuous load current on AC circuits; the table and factors are the same.