Installation design
Cable rating correction factors
- where
- required tabulated current-carrying capacity (A)
- protective-device rated current (A)
- ambient temperature correction factor
- grouping correction factor
Explain this equation
Works out the minimum tabulated current rating a cable needs once ambient temperature and grouping are allowed for. Use it before picking a size from the current-rating tables.
I_n is the rated current of the protective device. C_a is the correction factor for ambient temperature and C_g the factor for grouping with other loaded circuits; both come from the tables and are usually below 1. I_t is the minimum tabulated rating the chosen cable must have.
Tabulated ratings assume reference conditions. Hotter surroundings or bunched circuits stop the cable shedding heat as easily, so it can carry less. Dividing by the factors scales the requirement up, so that after derating the cable still covers I_n.
A 32 A device with C_a 0.94 and C_g 0.8 needs at least about 42.6 A tabulated. Factors well below 1 can push you up one or two cable sizes.
Watch out: divide, don't multiply. Only ambient and grouping are covered here; other factors may apply, and voltage drop and fault protection still need checking.
A tabulated cable rating applies to stated reference conditions. Correction factors reduce the usable rating in the actual installation.
Here only ambient and grouping apply. Divide the protective-device rating by their product to obtain a minimum tabulated rating, then check the other design constraints.
Enter your values to see the result.
Common mistake
Multiplying In by the factors when finding the required tabulated rating.
Limits
Ambient and grouping factors only. Enter applicable verified factors; other factors and fuse provisions may apply. Confirm the complete method, Ib ≤ In ≤ Iz, voltage drop and protective-device requirements. This does not select a cable size.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.