Lessons · Engineering · Sizing protection to the cable, not the load
Protection guards the cable, not the appliance
A fuse or breaker exists to stop the cable overheating. Its rating must be at or below what the cable can carry continuously, and at or above the load's normal current. The appliance protects itself; the cable cannot.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It reverses the common assumption. People uprate a breaker that keeps tripping and leave the cable alone, which removes the only thing standing between an overload and a fire.
How to think about it
Order the three numbers before choosing anything: load current, then protection rating, then cable capacity, each at or below the next. If the breaker trips, the question is which of those three is actually wrong -- and it is usually not the breaker.
Worked example
Load current is what normally flows.The bottom of the order.
Protection sits at or above it.So it does not trip in normal use.
Cable capacity sits at or above the protection.So the cable is always protected.
Uprating the breaker alone breaks the order.And the cable is then unprotected.
Your turn
Load 20 A, breaker 25 A. What is the minimum cable capacity in amps for the order to hold?
Cable must be at least A
Solve one, graded on the server
The trap
Fitting a bigger breaker to stop nuisance tripping. The cable did not get bigger, and it is now the weakest thing on the circuit with nothing watching it.