Fuse are normally
sized so that they carry up to 70-80% of full load current.
Hence say 30A/.7 = 42.7A => a 40A fuse.
The surge current at start up can probably be handled by the fuse.
IE - a 40A ATC fuse takes approximately 10 seconds to open at ~75A (based on the "Time-Current Characteristic Curves - Average Melt" graph in Cooper-Bussman's b05e23f5-5c01-4464-bfe8-802312799173.pdf aka "Automotive Blade Fuses ATC® Series Fast-Acting"; publication BU-SB09723 (Data Sheet 2009 (0809 = August 2009?)).
I have another document for MAXIs from Littelfuse (AutoFuseology), but its graph is useless - the curves are misaligned and don't match with the text - ie, "
From the curve in Figure 1, one can see that the 20A fuse opens in about 0.5 seconds at 100 amps, whereas the 60A fuse does not open until about 50 seconds.". (The curves are in error - it shows fuses blowing with LESS than their rated current.)
Otherwise that document is quite good - it explains the
Time-Current Characteristics (current versus time to blow) and
Selective Coordination (what I call discrimination) which relates to making sure downstream fuses blow before the upstream fuse.
[ FYI - Guru izu was involved in solving why a faulty PC monitor tripped or blew a 200kVA UPS's main breaker or fuse (probably with ~300A rating) instead of the local 16A breaker or fuse, and another intermediate breaker/fuse. It's not fun when a 0.1% load takes out the total other 60% of "critical" load - the system is designed to NEVER drop supply to such loads 0 that;s why people spend $millions in UPS systems! Obviously poor protection (fuse/breaker)
coordination - the upstream protection must give downstream protection enough time to operate. ]
Anyhow, the point is that most fuses will carry typically 110% of rated current all day, maybe even 120%.
A 150% overload may take tens of seconds to blow.
You should not have to worry about the equipment type for
distribution fusing which is what you are after - ie, the source (battery) fuse to protect the cable.
Using similar design rules for the fan power cable, you'd select for (say) 30A/0.7 = 40A cable, though that depends on cables ratings - some cables that are 30A rated may be 40A as they provide some margin. That all gets tricky at that level... is it 30A in a bundle or free air or next to metal? Is it rated for automotive use hence 30A at 80°C or 120°C?
Alas I never worry about that because I design for max voltage drop and that always seems to be way conservative - my "acceptable" cable thickness is invariably thicker than automotive current tables.
So -
not a 30A fuse as a 30A fuse should not operate at 30A. Hence 40A is the "smallest" in the ATC range.
And according to the Narva catalog, 50A is the largest blade fuse (which they call a ATS fuse) so 40A should be ok - not that I'd use those (IMO) flimsy plastic blade fuseholder arrays with side spade connectors. (They might be limited to 30A fuses anyhow...)
The Narva pdf fuse catalog is great (it's their "5.pdf" titled "Fuses and Fuse Accessories"; pages 120-146 of their master catalog circa 2008). It shows all(?) the automotive fuse types and current ranges as well as circuit breakers.
PS - a 40A ATC/ATS allows upgrade to 50A if needed.
And using a 50A or 100A cable through a 60A or 75A etc relay with a "mere" 40A or 50A fuse for a 30A load is quite acceptable.
Remember - this is distribution protection, not equipment/load protection. But there is no need to protect a fan - just prevent them burning!