Can a 40 amp fuse replace a 30 amp fuse?

Answer 1

Using "Oversized" Fuses is VERY DANGEROUS!
Although it is sometimes possible to fit a 40 Amp fuse into the "holder" for a 30 Amp fuse, this practice is VERY DANGEROUS and can result in SEVERE DAMAGE to the the electrical conductors [wires] in the circuit, and/or to other components of the electrical circuit.

Even worse, it could result in such massive overheating of the electrical conductors [wires] that a fire would result, causing serious, expensive damage to an electrical system, and personal injury or death.

DO NOT "overfuse" ANY CIRCUIT.

See following answer/explanation of the function of fuses and circuit breakers which has been used previously where questioners have inquired about fuses that continuously "blow" every time they have been replaced. Although the entire answer is extremely informative, pay particular attention to the last paragraph.

Fuses That "Blow" Repeatedly

Without being able to "hands on" troubleshoot the circuit served by the repeatedly blowing fuse, no one can identify the specific cause/defect which is causing your problem.

Therefore, the following generic answer can be applied to any electrical circuit, whether in a vehicle, or in a building, or whether alternating current [AC] or direct current [DC].

Fuses [and Circuit Breakers] are safety devices designed and installed in electrical circuits TO PROTECT the conductors [wires] and other components from short circuit conditions and/or overload conditions which cause an extremely large flow of electrical current [measured in Amperes], which causes overheating of the conductors [wires] that results in damage to the insulation and the conductors.

And in a worst case scenario, the probability of a FIRE which could destroy the vehicle, house, or other structure in which the circuit is located.

When a fuse or Circuit Breaker [and replacement fuses, or "tripping" Circuit Breakers] "blow," especially if it happens repeatedly, is an indication of an UNSAFE CONDITION in that circuit, usually a short.

The fuse or circuit breaker is doing what it was designed, intended, and installed to do, protect the conductors and components of the circuit which it serves.

The proper "fix" is for a qualified technician, who knows what he or she is doing, to troubleshoot the involved circuit, find and identify the defect, and make proper repair [s], BEFORE replacing the fuse again [with the properly sized fuse or before resetting a circuit breaker].

Some ignorant few will suggest installing a larger fuse or breaker to solve the problem, BUT that will only increase the hazard, not correct it. Do not follow "bad" advice by installing a larger fuse in a misguided attempt to correct the problem. To install an over-sized fuse would almost guarantee damage to the wiring, and possibly an electrical system fire.