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Ampacity Derating Calculator — Temperature & Conductor Adjustment

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Starting ampacity (A) Use the applicable table/listing/manufacturer value.Insulation correction rating Ambient temperature (°C) Use actual design ambient; modeled range 0–85°C.Current-carrying conductors Whole-number count after applying applicable counting rules.Terminal / equipment ceiling (A, optional) Enter 0 when not applying a separate known ceiling.Design load (A, optional) Used only for headroom comparison.

Calculated result

25.48 A adjusted ampacity

Starting ampacity: 40 A

Ambient factor: 0.91 ×; conductor-count factor: 0.7 ×

Correction result before optional terminal ceiling: 25.48 A

No terminal/equipment ceiling entered.

Reduction from entered starting ampacity: 36.3%

Entered load 20 A → 5.48 A headroom

Show the working

1. Temperature correction: 40 A × 0.91 = 36.4 A.
2. Conductor-count adjustment: 36.4 A × 0.7 = 25.48 A.
3. Usable planning ampacity = 25.48 A because no separate ceiling was entered.

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