Alternator B-Lead Circuit Breakers

Russ Erb

Originally published June 1998

(The following is an e-mail question I sent to Bob Nuckolls of The Aeroelectric Connection and his response.)

Our local Chapter 1000 electrical guru raised the question of why people put large breakers on the B-lead of the alternator. His point was that it is impossible for the alternator to put out in excess of its rated amperage (excess voltage, of course, is possible).

I thought about it for a while, and came up with this: Is the purpose of the B-lead breaker to protect against the power windings or the diodes shorting to ground and thus the battery popping the breaker?

(The response:)

You are entirely correct . . . the fuse/breaker is used to protect the b-lead wire from BATTERY current should a diode or wire short in the alternator. This is such a rare event these days, I've moved b-lead protection out of the cockpit and onto the firewall in the form of an 80 amp fuse for a 60 amp alternator.

- Bob Nuckolls

EAA Chapter 1000 Home Page
E-Mail: Web Site Director Russ Erb at

Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 14 March 1999