Power distribution
Cable Derating & Iz Calculator
Combine the ambient, grouping, and thermal-insulation correction factors with a cable’s tabulated current-carrying capacity to find the real installed capacity Iz, and check it against the breaker rating on the In ≤ Iz rule. Read the base capacity and factors from the wiring tables adopted for the project.
Why tabulated capacity is not the installed capacity
Cable tables give a base current-carrying capacity It for a reference condition — a stated ambient, one circuit, a defined installation method. Real installations are hotter, grouped, or wrapped in insulation, all of which reduce how much current the conductor can carry before it overheats. The installed capacity is the base value scaled by every applicable factor:
Protection is only valid when the breaker rating sits below the corrected capacity, not the table value:
To work backwards, the minimum base capacity a cable must have for a chosen breaker is It ≥ In / (Ca × Cg × Ci) — the figure shown on the right.
Where each factor comes from
- Ca — ambient: below 1.0 when the ambient exceeds the table reference (usually 30 °C in air, 20 °C in ground); above 1.0 for cooler surroundings.
- Cg — grouping: below 1.0 when several loaded circuits share a route, tray, or conduit and heat each other.
- Ci — thermal insulation: below 1.0 where a cable is in contact with or surrounded by thermal insulation that traps heat.
Frequently asked questions
Which current do I compare against Iz?
The breaker rating In. The design current Ib must be ≤ In, and In must be ≤ the corrected Iz. If In exceeds Iz, upsize the cable — never upsize the breaker to stop tripping.
Can a factor be greater than 1.0?
Yes — the ambient factor exceeds 1.0 when the surroundings are cooler than the table reference, which slightly raises the capacity. Grouping and insulation factors are 1.0 or below.
Do the factors multiply or add?
They multiply. Three factors of 0.9 give 0.9 × 0.9 × 0.9 ≈ 0.73, so combined effects reduce capacity faster than any single factor suggests.
Match the protective device
With the installed capacity known, confirm the breaker rating and curve stay within In ≤ Iz.
Estimate for guidance only. Base capacities, correction factors, and the final cable and breaker selection must be taken from the adopted wiring tables and verified by a qualified person for the actual installation.