DC Wiring Question - Fuse Placement

[camel2012]

Originally Posted by ramblinrod

Commercially available marine bus bars have a max current rating.

Reference: Blue Sea

https://www.bluesea.com/products/category/18/BusBars

A properly designed and wired marine electrical distribution system will always have a feed current rated greater than the highest branch current.

ABYC E11 AC & DC Electrical Systems on Vessels, Section 11.8 identifies how to calculate the load represented by a panel board (DC distribution panel) to size the feeder wire and 11.9 for overcurrent protection.

Basically, the distribution panel and feeder wire must be capable of handling the sum of the continuous loads (irrespective of branch circuit wiring size) plus 10% of the sum of the intermittent loads (or the highest intermittent load, whichever is greater).

Only if all loads were miniscule, could the feeder wire the same size as the branch wires be valid.

Therefore, it is possible for the feeder wire to be correctly sized the same as the branch circuits, but in practice, extremely rare, and in most cases, would likely be a wiring error.

An exception that comes to mind is a 4 or 6 circuit fuse block to low power instruments from an "instrument" breaker. However, in that case, the fuse block fuses all have to be rated lower than the "instrument" breaker feeding the fuse block.

Anything else is a wiring error.

Two overcurrent protection devices of the same rating in a single series circuit, (or a load fuse larger than a branch fuse) is a wiring error.

Individual overcurrent protection devices, rated to the component they are attached, being less than the branch circuit over-protection, is valid, and should be done to prevent other parallel circuits of the branch from being disabled in the event of a single device fault.

With LEDs everywhere and increasing use of multiplex distribution it's not as uncommon as that given the 16 awg recommended min awg size.