SettingsSettings for this calculationUS
The conductor's tabulated ampacity before any correction or adjustment factors are applied.
The table value for the conductor's INSULATION temperature rating — the column, not the conductor alone. The highest-rated column is normally used as the starting point for derating rather than as an answer, because the finished result may not exceed what the TERMINATIONS are rated for, which is usually a lower column. Starting at the top of the table and forgetting the termination limit is the commonest way this calculation comes out optimistic.
The multiplier for the ambient temperature the conductor will actually operate in, from the applicable NEC temperature correction table.
Correct for the temperature the cable SEES, not the room it starts in. A run through a plant room, a roof void or a conduit in sunlight sits well above the space below it, and the codes add a further allowance for conduit exposed on a roof. Below the table's base temperature the factor is greater than one, which is a real gain and one that a cold-store or outdoor run is entitled to claim.
The multiplier for the number of current-carrying conductors bundled together in the same raceway or cable.
Count CURRENT-CARRYING conductors, which is not the same as counting wires. A neutral carrying only the unbalance of a three-phase set does not count; the same neutral on a load rich in triplen harmonics does, because it carries current all the time. Equipment grounding conductors never count. Miscounting here is quiet — the cable simply runs hotter than the calculation says and ages faster than anyone measures.
Derated conductor ampacity
91.5 A
Verify both correction factors directly against the current NEC tables for your specific ambient temperature and number of current-carrying conductors — this calculator applies factors you supply, it does not look them up for you.
They open the calculator with your figures already in it
Feeder Conductor Ampacity Derating Calculator: 91.52 A — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- Derated ampacity = base conductor ampacity × ambient temperature correction factor × conductor bundling/count adjustment factor, per NEC 310.15(B) temperature correction tables and NEC Table 310.15(C)(1) adjustment factors for more than 3 current-carrying conductors in a raceway
Inputs used
- Base Conductor Ampacity (A, from NEC Table 310.16)
- 130
- Ambient Temperature Correction Factor (Per NEC 310.15(B))
- 0.88
- Conductor Count Adjustment Factor (Per NEC Table 310.15(C)(1))
- 0.8
Confidence note: Verify both correction factors directly against the current NEC tables for your specific ambient temperature and number of current-carrying conductors — this calculator applies factors you supply, it does not look them up for you.
What this calculation does not cover
- Derating and terminations are two separate ceilings and the lower one governs. NEC 110.14(C) holds the conductor to the 60°C or 75°C column for whatever the breaker and lugs are listed at, so a run derated down from the 90°C column can still not be used above its 75°C table value — the number this page returns may legitimately be higher than the ampacity you are allowed to claim.
- The adjustment factor you type depends on a conductor count this page cannot verify. Equipment grounding conductors and the neutral of a balanced circuit are not current-carrying, while the neutral of a three-phase, four-wire wye feeding significant nonlinear load is — reclassifying one conductor can drop you a whole band in Table 310.15(C)(1). The adjustment also does not apply at all through a nipple of 24 in (610 mm) or less.
- A derated ampacity proves nothing by itself. It still has to cover the load it serves at 125 percent of the continuous portion, sit correctly with the overcurrent device protecting it, and survive a voltage drop check over the length of the run. This page stops one step before all three.
Add the equipment this sizes
This result is a specification — 91.5 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Derated ampacity = base conductor ampacity × ambient temperature correction factor × conductor bundling/count adjustment factor, per NEC 310.15(B) temperature correction tables and NEC Table 310.15(C)(1) adjustment factors for more than 3 current-carrying conductors in a raceway
Which documents these citations point at
- National Electrical Code (NFPA 70) — 310.15(B), Table 310.15(C)(1) (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.