Hokie said it “generally” in a simple-to-understand example… he just didn’t get specific.
Volts is like water Pressure. Amps is like Volume (gallons). When you combine them into a circuit you must consider them as “multipliers”….. Volts X Amps = Watts (Watts are a unit to describe the ability to perform Work……like Gallons-per-minute at Pressure.)
The more pressure in a pipe….the need for stronger pipe with thicker walls.…. Wire Guage.
A Fuse is a ”safety-valve”….Too much volume (Amps) and the fuse melts…. NOT to protect your pump motor….. it ”blows” to protect the WIRE….(not the pump). IF the wire were allowed to carry more Amps than it’s Gauge would allow…the wire would overheat and perhaps create a Fire.)
The pump is designed/rated to perform a certain amount of Work. (Watts) If the VOLTS supplied to the pump should be reduced (I.E. your battery become depleted, etc) then the pump will attempt to continue it’s Work (Watts)….by drawing more Amps. (remember, V x A - W)
When those demands for More Amps exceed the Fuse-rating…. it Melts/Blows.
Speed25‘s example assumes the motor doesn’t have an internal ”short”…in which case it Definitely will request More than 8A.
I suspect your pump is rated to draw 8 Amps as a Maximum….AT the Minimum-rated Voltage…. that calculation being 8A. But if your pump is pulling More than your Dynamo can RE-SUPPLY to your battery…..then eventually the Voltage of the source will DROP…until the required AMPS necessary to meet the pump-motor’s Work (WATTS) …exceed the Fuse-Rating.
(Those dropping Volts are why your pump motor is intermittent. The volts drop below the motor’s necessary requirement the battery/dynamo can supply ….so the pump stops….. But that then allows the dynamo to bring the battery back Up to sufficient Voltage….so the pump-motor starts working again…. repeat/repeat….. until the Fuse Finally Says NO! (Too many little events of borderline heating, so it finally “blows”.)
Increasing the rating of the Fuse MIGHT be a temporary ”Fix”….but at the risk of melting your wire.
You need a greater-capacity battery….or dynamo….or both. OR…. you need to turn other electrical devices OFF while using your pump...Or…you should use your pump less-frequently (let the battery re-charge when the pump starts going intermittently)**…..so your battery/dynamo can keep-up with pump demands. (are your lights “on” while pumping…?)
** This is another opportunity to mention the ill-advised practice of Re-setting Circuit Breakers repeatedly.
While C.B.s are a convenient way to replace a Fuse… one hazard of them is that they are so-easy to quickly re-set. Each Time the C.B. is re-set, any situation in which over-taxed the system is also re-enabled. If the C.B. is repeatedly re-set…the condition in which the wire was taken to an over-heat condition can re-occur.
If the system-design used the minimal-gauge wire protected by the C.B….then the wire will reach it’s temperature-limit over and over and over again…. Possibly melting it’s insulation…and the Next C.B. re-set….. can cause sparks and fire. (Pilots are instructed so that if they experience a “popped” C.B. while in-fight…to FIRST consider if they really NEED that appliance in order to complete the flight.
If they don’t…then Do Not Re-Set that C.B. (In earlier times they were instructed it was “OK” to re-set a C.B. ONE TIME ONLY. That former practice is now discouraged …in favor of …”Do I Really Need this thing…?