Ignore all of this stuff about electronics "asking for" current. The fuse tap is adding a circuit to the fuse box. That's why it contains two fuses. The bottom fuse goes in line with the existing circuit, and the top fuse goes in line with the new circuit that comes off of the fuse tap.

In your diagram, the radio amplifier circuit is protected by a 30-amp fuse before, and a 30-amp fuse after. This is fine.

Your added circuit is protected by a 20-amp fuse. As long as the wire on that circuit, and the fuse tap pigtail itself, are adequate for 20 amps, and as long as the bus feeding relay-switched side of this fuse box is adequate for 20 more amps than it was seeing before, this is also fine.

Ignore all of this stuff about electronics "asking for" current. The fuse tap is adding a circuit to the fuse box. That's why it contains two fuses. The bottom fuse goes in line with the existing circuit, and the top fuse goes in line with the new circuit that comes off of the fuse tap.

In your diagram, the radio amplifier circuit is protected by a 30-amp fuse before, and a 30-amp fuse after. This is fine.

Your added circuit is protected by a 20-amp fuse. As long as the wire on that circuit, and the fuse tap pigtail itself, are adequate for 20 amps, and as long as the bus feeding the relay-switched side of this fuse box is adequate for 20 more amps than it was seeing before, this is also fine.