Sorry guys. I'm going to need to back up here a minute.
HS - I'm not sure if you are agreeing or disagreeing with me.
I agree with you in that over current will vaporize the fuse. I have seen, and heard
, fuses pop from an over current draw. I also agree that a fuse can sustain an over current draw for short time periods. However, if you're saying that the over current situation can exist for 5 or 10 seconds, I have to disagree. "Short time periods," in this context, means milliseconds
A starter can, and should be capable of turning for up to 30 second bursts, with a 60 second rest (33%, or less, duty cycle).
Mitempo - the OP started this thread asking about the Blue Sea MRBF Terminal fuse. Sorry, but I got confused when you injected the ANL fuse into the discussion.
Tree - HS is right in that the fuse for the starter circuit is not required (or that is what I think that he means by stating "main battery fusing is more of a philosophical matter..." You'll read below that it is not required.
However, I'll tell you why I agree that this is a good idea. My boat has a 1986 Universal M-25 engine. The original alternator bracket on these engines were infamous for breaking when you least expected a problem.
You can read about that here
. When this happens, the still spinning alternator would either slam into the timing case cover and break it, or touch on the positive post of the alternator. In the later situation, an uninterrupted circuit would be created from the selected battery bank through the switch, to ground. I believe that it could also have run through the ammeter wiring, thus frying the entire electrical instrumentation system. You can read about that here
Mitempo is right in pointing to Blue Sea as a resource for the discussion. Here is what Blue Sea says (at Part 2: Select a Fuse and Fuse Holder For Your DC Product Installation - Resources - Blue Sea Systems
) about selecting the appropriate fuse for any given application;
U.S. Coast Guard regulations, and ABYC E-220.127.116.11.1, require every ungrounded wire except those in the engine starting circuit to have circuit protection. Many DC installations employ fuses rather than circuit breakers. Choosing correct fuse amperage and the best fuse holder or fuse block will help protect your boat and your safety.
Always select a fuse size to protect the wire according to its rating. In some cases, a product manufacturer will specify a fuse value and it will usually be lower than that required for protecting the wire. If this fuse value is too high to protect the wire, use a bigger wire in the circuit. Review Part 1 for more information on choosing wire size for a DC circuit.
Follow the steps to choose fuse amperage and a fuse holder or fuse block.
Choosing the Fuse Amperage
In this step, find appropriate fuse amperage within a range of minimum and maximum
A Find the MAXIMUM FUSE AMPERAGE by following your AWG wire size (from Part 1 of this series) across the chart. Maximum fuse amperage reduces nuisance blows but offers less protection for the wire. Select:
Single Wire or Bundled Wire column, and
Outside Engine Room or Inside Engine Room grey bar
Example: For a 4 AWG single wire outside an engine room, maximum fuse amperage is 150A.
B Calculate MINIMUM FUSE AMPERAGE by multiplying product amperage rating by 125%. Minimum fuse amperage provides more protection for the wire but may result in nuisance blows.
Example: 80A x 125% = 100A.
C Choose FUSE AMPERAGE near the middle of this range. Mid-range values are usually acceptable. Consider the requirements for each individual circuit.
Example: 125A is between 100A minimum and 150A maximum.
D Find AVAILABLE FUSES by using the chart.
Example: Fuses available at or near 125A include MIDI®/AMI®, MRBF, MEGA®/AMG®, and ANL fuses.
- Used without advance permission, but I am clearly referring to, and crediting Blue Sea's website, so I hope that they will agree that I am retaining their copyright.
I hope this helps you select the right fuse for your application.