Basic Fuse Rating Question

Jul 4, 2015
436
Hunter 34 Menominee, MI; Sturgeon Bay WI
Hi;
If a fuse is rated at 40 Amps at 24 Volts, what would it's rating be at 12 volts? Do you simply calculate it's resistance at 24 volts and then plug that into Ohm's law equation for 12 volts (ie is 20A at 12V)? Somehow doesn't make sense.
Thanks.
 
Jul 4, 2015
436
Hunter 34 Menominee, MI; Sturgeon Bay WI
Thank you! I've long wondered about this. Why is it then that the voltage is stamped on the fuse along with the amperage?
 
Feb 10, 2004
3,918
Hunter 40.5 Warwick, RI
I think the voltage rating is specified in accordance with the insulation of the fuse. A given physical fuse could fit into circuits with differing voltages. A 24V fuse would not be a safe choice for a 220V circuit.
 
Mar 14, 2019
6
Catalina 380 Longport, NJ
Hi;
If a fuse is rated at 40 Amps at 24 Volts, what would it's rating be at 12 volts? Do you simply calculate it's resistance at 24 volts and then plug that into Ohm's law equation for 12 volts (ie is 20A at 12V)? Somehow doesn't make sense.
Thanks.
40 amps is 40 amps. Breakers come in two flavors, AC or DC. Just make sure you choose the proper one for the application. Breakers protect the wire. Make sure the breaker regardless of AC or DC fits the wire size And is not too small for the appliance ie. pump, radio, etc.
Bruce
 
Feb 6, 1998
11,667
Canadian Sailcraft 36T Casco Bay, ME
Thank you! I've long wondered about this. Why is it then that the voltage is stamped on the fuse along with the amperage?
The voltage rating, in regards to a marine installation, applies to "AIC" or amperage interrupt current for a safe trip. Let's say you have a bank that can throw 10,000A of short circuit current (eg: two G-31 Odyssey AGM's), then you'll need a fuse that can handle 10,000A of short circuit so it can trip and so safely on your 12V system. The AIC current rating should never be confused with the trip current. Eg: a 300A fuse would need to be Class T if the bank has the capability of creating 20,000A of short circuit current. AIC is all about the over-current device not failing when trying to do its job.

As the fuses voltage rating goes up the AIC rating goes down. For example the very common MRBF fuse is AIC rated for a 10,000A safe trip at 14V but only 5000A, or half, at 32V.

Standard Marine Fuses & Their AIC Ratings:

*Class T = 20,000A AIC @ 125V - (Significantly higher at 12V) (*Class T Complies with ABYC for LiFePO4 batteries)

MRBF = 10,000A AIC @ 14.0V

ANL = 6000A AIC @ 32V (Higher at 12V)

We've seen a fair number of circuit breakers literally welded shut because the AIC rating was not considered when choosing the over-current protection for the bank. We've also seen un-safe failures of ANL fuses when the AIC rating was not applied correctly.
 
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Jul 4, 2015
436
Hunter 34 Menominee, MI; Sturgeon Bay WI
The voltage rating, in regards to a marine installation, applies to "AIC" or amperage interrupt current for a safe trip. Let's say you have a bank that can throw 10,000A of short circuit current (eg: two G-31 Odyssey AGM's), then you'll need a fuse that can handle 10,000A of short circuit so it can trip and so safely on your 12V system.

As the fuses voltage rating goes up the AIC rating goes down. For example the very common MRBF fuse is AIC rated for a 10,000A safe trip at 14V but only 5000A, or half, at 32V.

*Class T = 20,000A AIC @ 125V - (Significantly higher at 12V) (*Class T Complies with ABYC for LiFePO4 batteries)

MRBF = 10,000A AIC @ 14.0V

ANL = 6000A AIC @ 32V (Higher at 12V)

We've seen a fair number of circuit breakers literally welded shut because the AIC rating was not considered when choosing the over-current protection for the bank. We've also seen un-safe failures of ANL fuses when the AIC rating was not applied correctly.
What is MRBF? ANL?
So, if I have a fuse stamped 10A at 24V, it is going to blow at 5A at 12V?