Circuit Percent Fuse Rating

An example was given to use a 15A fuse to protect a 8A ckt, because 150% of the ckt current will be 12A and the next available fuse rated size to be considered should be 15A. However a 15A fuse on a 8A ckt is 187.5% of the ckt current. As i understand the reason for having fuses not rated low enough to prevent hindrance to ckt operation, is it safe to say that 125% (10A fuse) the ckt current should be considered in the example given and thus the fuse percent rating per ckt should be factored with available fuse amperage sizes in mind?

The main reason to use a fuse, or some other form of circuit breaker is to protect the appliance/device and the wiring in case of an over-current (as can occur with some electric motors when stalled, for example) or short circuit. The fuse rating is normally the maximum current it can pass before breaking the circuit, but there is normally a correction factor of 35-50% given to ensure it doesn't break the circuit during normal operation.

There are a few other things to take into account, such as whether it is a DC or AC circuit because of the wave form (not something we should need to worry about); inrush or other normal, expected peak currents that can be several times higher than the normal operating current (many electric motors, for example, can have start up peak currents 6 or more times higher than normal running levels) and so a delayed or "slow blow" circuit breaker may be required; something that may be a concern is the temperature the circuit breaker is operating in, as a higher temperature will normally make it easier for the circuit breaker to operate, and so it may need a higher rating because of the correction factor; some types of electric motors, if there isn't sufficient voltage to them, may have a higher than expected current draw - but these are primarily AC and not something to worry about here.

Overall, the +35% and round up to the next fuse rating is a good rule of thumb. If there's a problem with the fuse/circuit breaker failing, and you're sure there isn't some other issue and the wiring is up to it, you could try another size up, if you're confident there is no other issue.

Thank you my guy for the detailed explanation. Appreciate it.