If the 30A fuse is between the battery and the fuse block, then that fuse will blow when there is a total fuse block load of 30A or more, whether or not that load is due to one load or a combination of loads. The individual circuits fed from the fuse block should be protected by their own (usually lower rated) fuses.
You probably know that the rating of the fuse does not determine the carrying capacity of the wire, the size of the wire does. Just be careful because it's not uncommon for circuits to incorrectly have larger fuses installed than safe for the wire size used. No one needs to have overheated wires, melted insulation, short circuits, or a fire

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Good luck, have fun

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