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Do-It-Yourself (DIY) Question & Answer Forum
Changing the main fuse
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<blockquote data-quote="avinalarf" data-source="post: 15754" data-attributes="member: 631"><p>Thanks, we were wondering how on earth actual usage could be calculated!</p><p></p><p>Ok, this is where I'm getting confused...how is it decided what size the Main Fuse should be then? You obviously don't just add up the breakers that are in the installation, 'cos 9 times out of 10, the total would always exceed the MF size<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite3" alt=":(" title="Frown :(" loading="lazy" data-shortname=":(" /></p><p></p><p>That's why I was allowing for a 50A mcb in the main CU to cover the circuits in the garage CU:</p><p></p><p>32A - ring main</p><p></p><p>10A - rear garden lights</p><p></p><p>6A - front garden lights</p><p></p><p>6A - garage lights</p><p></p><p>Doesn't the mcb for the garage (in main CU) have to be rated higher than the highest mcb in the garage CU? &lt;- hope that makes sense to you!!!</p><p></p><p>Obviously I was WAY off the mark, which I've now realised, but TBH, if I was to do this again (which I hope I will at some point!), I'd still be a bit stuck as to the size of fuses to use.</p><p></p><p>What additional info is required? Sorry to go on about this, but it's doing my head in and I really need to understand [thoroughly] what I'm doing, otherwise I don't see the point...</p><p></p><p>You've all been a great help so far - much appreciated.</p><p></p><p>Nat</p></blockquote><p></p>
[QUOTE="avinalarf, post: 15754, member: 631"] Thanks, we were wondering how on earth actual usage could be calculated! Ok, this is where I'm getting confused...how is it decided what size the Main Fuse should be then? You obviously don't just add up the breakers that are in the installation, 'cos 9 times out of 10, the total would always exceed the MF size:( That's why I was allowing for a 50A mcb in the main CU to cover the circuits in the garage CU: 32A - ring main 10A - rear garden lights 6A - front garden lights 6A - garage lights Doesn't the mcb for the garage (in main CU) have to be rated higher than the highest mcb in the garage CU? <- hope that makes sense to you!!! Obviously I was WAY off the mark, which I've now realised, but TBH, if I was to do this again (which I hope I will at some point!), I'd still be a bit stuck as to the size of fuses to use. What additional info is required? Sorry to go on about this, but it's doing my head in and I really need to understand [thoroughly] what I'm doing, otherwise I don't see the point... You've all been a great help so far - much appreciated. Nat [/QUOTE]
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Changing the main fuse
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