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ANL Size for 60a Alternator

rockitdoc

Well Known Member
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I'm struggling with this.

Nuckolls' book says 60a is too small, that a larger one is recommended to avoid 'nuisance' trips. But, I heard 'somewhere' that a 60a ANL won't blow until WAY more than 60a passes it, which makes me think even a 60a ANL might not protect the avionics in case of an alternator surge. Am I overthinking this?

My alternator is a Plane Power that came with the FWF -14A kit.

Another question:

Doesn't the internal voltage regulator in the PP alternator protect the circuits downstream?
 
I'm struggling with this.

Nuckolls' book says 60a is too small, that a larger one is recommended to avoid 'nuisance' trips. But, I heard 'somewhere' that a 60a ANL won't blow until WAY more than 60a passes it, which makes me think even a 60a ANL might not protect the avionics in case of an alternator surge. Am I overthinking this?

My alternator is a Plane Power that came with the FWF -14A kit.

Another question:

Doesn't the internal voltage regulator in the PP alternator protect the circuits downstream?

ANL's tend to take forever to blow anywhere near their rating. Might want to find a chart for it. They list time vs amperage to open. In my experience, a CB will open WAY faster than an ANL at their rated capacities. One of the reasons that CB's or fast blow fuses are recommended for sensitive applications.

The blue sea chart shows that their ANL won't trip (i.e. melt) until 140% of rated capacity and will take 20 seconds to do it at that current. It will trip in 5 seconds at 200% of rated current.

Larry
 
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A 60 amp circuit breaker might be too small if the alternator output current is 60 amps. However, a 60 amp ANL current limiter will not blow when the alternator is putting out a full 60 amps. Neither a circuit breaker nor an ANL current limiter will protect against an over voltage condition. If you are talking about a current surge, the aircraft load determines the current. The alternator will only put out current that the load demands. An alternator is not capable of outputting much more than its rated capacity. The purpose of the alternator "B" lead circuit breaker or current limiter is to protect the B lead and battery and aircraft electrical system against short circuits of the B lead or alternator.
 
Neither a circuit breaker nor an ANL current limiter will protect against an over voltage condition. If you are talking about a current surge, the aircraft load determines the current. The alternator will only put out current that the load demands. An alternator is not capable of outputting much more than its rated capacity. The purpose of the alternator "B" lead circuit breaker or current limiter is to protect the B lead and battery and aircraft electrical system against short circuits of the B lead or alternator.

This........
 
... The purpose of the alternator "B" lead circuit breaker or current limiter is to protect the B lead and battery and aircraft electrical system against short circuits of the B lead or alternator.

Which is why the protection, be it A Bussmann ANL current limiter, Littelfuse MIDI fuse, or fuselink wire, is placed as close as practical to the starter contactor in the Z schematics. The power source is the battery and the protection disconnects the power source from the fault.

A 60A Bussmann ANL current limiter can be used for the B lead of a 60A alternator but I chose to use a 100A Littelfuse MIDI fuse because it is physically smaller, 30 mm vs 61 mm bolt center to center. How I came up with 100A for the MIDI.

You can ask electrical questions on the Aeroelectric List hosted by Matronics, Bob Nuckolls hangs out there.
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