I always try to size the fuse to match the expected draw of the device I'm protecting. No idea what an RT-551 radio draws but a Becker 3201 draws a max of 1.4 amps when transmitting (according to their literature, anyway). So, if you protect it with a 2 amp rated fuse then Ohms law tells us that you'd only lose 0.11 volts across the fuse (E=IR or 0.109v = 1.4 A * 0.078 ohms). Negligible. I guess I don't really understand all of the concern over losing 1 tenth of 1 volt across the fuse.

Robert


On Sunday, July 14, 2019 at 1:40:14 PM UTC-5, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
On Saturday, July 13, 2019 at 11:24:31 AM UTC-5, soaringjac wrote:
Anyone know what fuse rating i would need for an old Edo-Aire RT-551 radio. Its marked inop now but i still want to replace the fuse in the panel for it

Between you, me, and the fence post, I prefer breakers rather than fuses. Breakers are easier to reset while in flight than replacing fuses.

Remember to use a minimum of 3A fuses/breakers or risk loosing precious volts. See page 86 of this presentation http://aviation.derosaweb.net/presentations/#wiring.

John OHM