Ampacity: Definition, Derating and Usable Value
Ampacity is the current in amperes that a conductor can carry continuously, under the conditions it is actually installed in, without its insulation exceeding the temperature it is rated for.
Ampacity is quoted as though it were a property of the wire. It is not: the same conductor has a different ampacity in a 70 degree F basement than in a 120 degree F attic, and a different one again when nine conductors share the raceway. This page separates the tabulated figure from the usable figure and names the four rules that move between them.
What the definition rules out
Three phrases in the definition do the work.
Continuously. Ampacity is a steady-state thermal rating, not a surge rating. A conductor tolerates inrush and short-circuit current far above its ampacity for a short time, which is why motor circuits are protected well above the conductor ampacity while still being protected.
Under the conditions it is actually installed in. The same conductor has more than one ampacity. Ambient temperature, how many other current-carrying conductors share the raceway, and whether it is in free air or enclosed all change the figure. The conditions are part of the rating, not a footnote to it.
Without exceeding its temperature rating. The limit is the insulation, not the copper. Current heats the conductor, the heat has to leave through the insulation and the raceway, and the rating is the point at which the insulation degrades. That is why the answer depends on the surroundings rather than on the metal alone.
Tabulated ampacity is not usable ampacity
The tabulated figure is a lookup at fixed conditions: 30 degrees C ambient, no more than three current-carrying conductors in a raceway or cable. Usable ampacity is what remains once the real conditions of use are applied.
The gap is not marginal. Six THHN copper conductors in a 104 degree F attic retain 72.8 percent of the tabulated 90 degree C value before any ceiling is applied. Ten conductors in one raceway at the table's own ambient retain 50 percent. Quoting a tabulated value as a conductor's ampacity is therefore correct only in the specific installation the table describes.
The four rules that reduce it
Two of them multiply and two of them cap, and the order matters.
- Ambient correction. A factor for the actual ambient temperature in the column being read. Higher-rated insulation loses proportionally less.
- Conductor count adjustment. A factor for the current-carrying conductors sharing the raceway: 80 percent for four to six, 70 percent for seven to nine, 50 percent for ten to twenty.
- Termination temperature. A ceiling at the tabulated value in the column matching the lowest-rated termination at either end. Applied after the multipliers, never before them.
- Small conductor overcurrent limits. A ceiling on the protection permitted for 14, 12 and 10 AWG. Because the device sets the practical limit of the circuit, this behaves as a ceiling on usable ampacity too.
The usable figure is whichever of those leaves the smallest number. Naming that rule is more useful than the number itself, because it tells you what to change: a termination-governed result is fixed by better-rated equipment, a count-governed result by splitting the raceway.
Terms it gets confused with
Ampere rating describes a device. A 60 A breaker has an ampere rating; the conductor it protects has an ampacity, and the two are chosen against each other rather than being the same quantity.
Ampere capacity is not a code term. Where it appears in product literature it usually means the tabulated value with none of the conditions of use applied.
Voltage drop is a performance limit, not a thermal one. It scales with run length while ampacity does not, and on a long run it is frequently the constraint that decides the conductor size. A conductor can pass ampacity with room to spare and still be the wrong size.
Minimum circuit ampacity, the figure marked on air-conditioning and refrigeration nameplates, is a required conductor ampacity supplied by the equipment manufacturer under Article 440. It is an input to conductor selection, not a property of the conductor.
Tabulated Against Usable Ampacity
THHN copper on 75 degree C terminations at 104 degrees F with six current-carrying conductors, which describes a great many attic and multi-circuit pulls. The left column is what a chart states; the next is what the conductor is good for.
| Size | Table, 90 °C | Usable | Governing limit | Basis | Max OCPD |
|---|---|---|---|---|---|
| 12 AWG | 30 A | 20 A | Small conductor limit | 240.4(D) | 20 A |
| 10 AWG | 40 A | 29.1 A | Conductor count | Table 310.15(C)(1) | 25 A |
| 8 AWG | 55 A | 40 A | Conductor count | Table 310.15(C)(1) | 40 A |
| 6 AWG | 75 A | 54.6 A | Conductor count | Table 310.15(C)(1) | 50 A |
| 4 AWG | 95 A | 69.2 A | Conductor count | Table 310.15(C)(1) | 60 A |
| 2 AWG | 130 A | 94.6 A | Conductor count | Table 310.15(C)(1) | 90 A |
| 1/0 AWG | 170 A | 123.8 A | Conductor count | Table 310.15(C)(1) | 110 A |
Where the Term Decides Something
Four points in the work where the figure is the answer to a question, not vocabulary.
Sizing a conductor
The usable ampacity of the candidate conductor must equal or exceed the design current: continuous load at 125 percent plus non-continuous load at 100 percent. This is the check that selects the size.
Selecting overcurrent protection
The device protects the conductor, so its rating is bounded by the conductor's usable ampacity. Where the ampacity falls between two standard ratings, the next size up is permitted on circuits with no receptacle outlets at ratings of 800 A or less.
Justifying a derated pull
An inspector questioning ten conductors in one raceway is asking for the ampacity arithmetic: tabulated value, ambient factor, adjustment factor, termination cap, and the resulting figure against the connected load.
Evaluating an existing circuit
Ampacity is what decides whether an existing conductor can carry a new load. A 10 AWG copper conductor rated 40 A in the 90 degree C column is still limited to 30 A of overcurrent protection, so the breaker cannot simply be increased.
Which Figure to Use
Reading the term correctly in practice.
Use the tabulated value only for the lookup
The table assumes 30 degrees C ambient and no more than three current-carrying conductors. Treat it as the starting number, never as the answer, unless the installation genuinely matches both assumptions.
Read the column that matches the insulation
The conductor's insulation rating selects the column, so a THHN conductor is read from the 90 degree C column even where the terminations are listed for 75 degrees C. The termination rating limits the result afterwards.
Apply every reducing rule, then take the lowest result
Ambient correction and the conductor count adjustment multiply. The termination cap and the small conductor overcurrent limits are ceilings applied after them. The usable ampacity is whichever of those leaves the smallest figure.
Do not confuse ampacity with voltage drop
Ampacity is a thermal limit and is independent of run length. Voltage drop is a performance concern that scales with length. A conductor can satisfy ampacity comfortably and still be unusable at 250 feet.
FAQs
Is ampacity the same as the breaker rating?
What is the difference between tabulated and usable ampacity?
Does ampacity depend on the length of the run?
Is ampacity set by the conductor or by the equipment?
Does a conductor in free air have a higher ampacity?
Do parallel conductors each carry their own ampacity?
Calculated against the NEC 2023 edition using 310.16, 310.15(B)(1), 310.15(C)(1), 110.14(C), 240.4(D). Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.
Not covered by this tool
- Conductors in free air, which are tabulated separately.
- Motor and air-conditioning circuits, which are sized under their own articles.
- Parallel conductor sets and taps.
Data last verified