# Homework Help: Current Rating for Fuse

1. Dec 30, 2017

### Richie Smash

1. The problem statement, all variables and given/known data
Hello, the question is number 35 in this picture here: http://uploads.im/uEWAf.jpg

It states, ''A suitable fuse rating for the mains supply has current rating - (A)10A (B)15A (C)30A (D)60A

2. Relevant equations
V = IR
P = IV
P = I^2 R
P = V^2/R

3. The attempt at a solution
I tried using all formulas above, but I only got the answer for questions 32 and 33 which are in the picture, I'm not sure how to find the current rating of the fuse, but the answer in my book is C which is 30A. Am I missing something here?

Last edited by a moderator: Dec 30, 2017
2. Dec 30, 2017

### Tom.G

The general rule for sizing a fuse is that it will carry the maximum load current but not more current than the supply or wires can safely carry.

You state that you have found the answers to questions 32 and 33. Assume that the suppy and the wiring can support at least the stated load.

What is the minimum fuse rating, among those listed, that will meet the requirements in my first sentence?

3. Dec 30, 2017

### Richie Smash

AH I see what you're saying Tom, yes the answer for 32 is 20A so the maximum load current, and among those listed 30 is the minimum I guess? but how would I know if the wires can safely carry 30A? I'm just choosing that because it seems like 60A is far too much and 30 is the minimum here, but I was thinking there was some formula perhaps?

4. Dec 30, 2017

### Tom.G

Excellent question!
In the real world, the wire size would be specified or you would have to measure it; then look up the maximum current rating.

In school, such 'complications' are usually ignored in the earlier courses in order to get basic concepts across. The problems are presented as 'simplified' or 'idealized'; i.e. "These are the parts we want you to think about. Assume all other aspect are perfect." As you found, it sure can be confusing for those that have a better handle on things.

5. Dec 31, 2017

### CWatters

The fuse needs to be greater than 20A so that obviously rules out 10 and 15A fuses. Both 30A and 60A could be ok but 30A is safer because not all faults are ideal short circuits, you might have a fault that draws say 40A.