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Cable calculation, step 9: Reg 433.1.1 and I2What is I2, and why 1.45?
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Regulation 433.1.1 is a long regulation that boils down to two conditions:
(Iz is the cable's rating after the factors, see section 11)
I2 is the current that causes the protective device to actually operate (Reg 433.1.1(c)). A circuit-breaker marked 32 A doesn't trip at 32.1 A, 33 A, 35 A, 40 A or even 42 A. For a circuit-breaker, I2 = 1.45 × In.
Working
Iz = 57 × 0.94 × 0.7 = 37.5 A 1.45 × Iz = 1.45 × 37.5 = 54.4 A I2 = 1.45 × In = 1.45 × 32 = 46.4 A 24.5 ≤ 32 ≤ 37.5 ✓ 46.4 A ≤ 54.4 A ✓ → Section 433 met: the cable won't overheat
Cut a rubber band so it's a strip. Pull it until it just stops sagging: that's In. Stretch it to 1.45 times that length and leave it: eventually it snaps. That's I2, the device tripping on a sustained overload after a long time. Yank it hard and it snaps instantly: that's Ia, the instantaneous trip (step 13).
Why the slack? Diversity. If the tenants do all cook at once (three pans of pasta on full and a pizza in the oven), the circuit can draw more than the 24.5 A you designed for. I2 gives the device room to "stretch" over that period without tripping, while the cable (sized for this) stays safe.
1) A common mix-up is that I2 trips "after 10,000 seconds, about two hours". 10,000 s is just the top of the time axis on the Appendix 3 graphs (and it's nearer 2¾ hours). For circuit-breakers up to 63 A, the standard conventional time for I2 is 1 hour.
2) Iz is the cable's rating after the factors (37.5 A), not the 57 A table figure. 1.45 × 57 = 82.65 A would flatter the check; 1.45 × 37.5 = 54.4 A is the real limit, and I2 46.4 A is still inside it. For circuit-breakers and RCBOs, I2 = 1.45 × In, so once In ≤ Iz, the I2 check is automatically met.
BS 7671 · Reg 433.1.1 · overload: Ib ≤ In ≤ Iz and I2 ≤ 1.45 Iz (433.1.1(c)) · p.103
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