Breaker size for Raymarine ACU-200

[benzy]

Hi all,

Just want some help with clarification on something.

We are installing a Raymarine Evolution autopilot, with the Type 1 Mechanical Linear drive and the ACU-200 unit.

According to the documentation, the maximum continuous amp draw by the drive is 15amps.

The ACU-200 has an internal 20amp fuse.

I’ve calculated wire size to limit to 3% voltage drop and am going with AWG8 wire.

So, wire can take about 80amps - no problem there.

Question is, what size breaker should I use? I’ve read to size the breaker to 150% of max amps, which I guess is 20amps since the ACU-200 internal fuse is 20amps?? But then again, I want the breaker to trip before the internal fuse blows. So maybe 15amps for the breaker is better?

But I’m not totally sure. Any guidance appreciated.

Thanks

[_andi_]

Hi,

no definite tip here but something to consider: Which type of fuse. Old "melt" type automotive fuses will take 200% rated amperage for minutes, newer electronically ones might trip quite fast. Since the linear drive will produce some spikes this might be something to consider.

As a rule of thump, fuses protect the wiring, so there should be nothing wrong with going 20A (or more) as well.

Andi

[_andi_]

To clarify the rule of thumb: Nobody fuses an outlet to 1A when running a phone charger and then changing to 10A for the cooler. Internal fuse protects the device, external fuse the wiring. Conditions may apply :-)

[benzy]

Thanks Andi, makes sense.

Maybe I go the 150% rule: so 30amp breaker (well within the 80amp limit for the wire). Allows for spikes from the motor. Internal fuse handles device protection.

[_andi_]

Sounds reasonable, manual says 20A (rather slow tripping blade fuse) as a guideline. (page 24).

http://www.jgtech.com/pdf/87180-2-EN...structions.pdf

Ah, one thing: Often folks don't calculate the full cable length (plus and minus cable length added) but just the distance from junction box to appliance, being off on the final result by the factor of two. Mostly no biggie but a common mistake anyways.

[benzy]

Ok cool.

Yep accounted for full length of run. It’s 7 meters.

According to a few books we have, 7m run with max 3% voltage drop falls between AWG 10 and AWG 8.

So for us metrics, a 6mm^2 cable should do.

[Cadence]

Originally Posted by benzy

Hi all,

Just want some help with clarification on something.

We are installing a Raymarine Evolution autopilot, with the Type 1 Mechanical Linear drive and the ACU-200 unit.

According to the documentation, the maximum continuous amp draw by the drive is 15amps.

The ACU-200 has an internal 20amp fuse.

I’ve calculated wire size to limit to 3% voltage drop and am going with AWG8 wire.

So, wire can take about 80amps - no problem there.

Question is, what size breaker should I use? I’ve read to size the breaker to 150% of max amps, which I guess is 20amps since the ACU-200 internal fuse is 20amps?? But then again, I want the breaker to trip before the internal fuse blows. So maybe 15amps for the breaker is better?

But I’m not totally sure. Any guidance appreciated.

Thanks

I think you are on target with the 15A breaker. JMHO

[benzy]

Hey Cadence,

Fair enough. Could you explain why?

Cheers

[Dsanduril]

The standard breaker curve for DC applications allows between 1.05 and 1.3 x rating for 3600 seconds (one hour). So, a 15A breaker will pass between 15.75A and 19.5A for up to an hour. They'll also provide 20A for just about a minute if you need it for some reason. This is true for B, C, and D curve breakers which pretty much covers anything you'll find easily on the market (the various curves diverge at times below 3 seconds).

Since the AP should only draw max amps for short bursts a 15A breaker should be fine and will most likely trip before the fuse blows (no guarantees - without the trip curves for both devices and information on the available short circuit current no trip coordination can be calculated).

[benzy]

Thanks Dsanduril

Guess I can try with the 15amp breaker and if it trips prematurely then upgrade to a bigger breaker.

[Cadence]

Originally Posted by Cadence

I think you are on target with the 15A breaker. JMHO

My rational is the spec. was max.. A breaker, in all likelihood, won't trip instantaneously. I certainly wouldn't want to be trying to replace the internal fuse for a momentary need. It is probably there as a fail safe. JMHO

[benzy]

Yep understand. Thanks

[_andi_]

I'd just stick to the manual...

Quote:

Fuses and circuit protection

There are 3 levels of power protection in the autopilot system.
The motor and associated cables are initially protected by the
current sensing and stall condition detection within the ACU
hardware and software. A second level of protection is provided
to these parts and the ACU components by the unit's main
power fuse. Protection for the entire autopilot system and its
wiring back to the main power distribution panel is provided by
your vessel’s circuit breaker / fuse.

Power supply circuit protection
Protect the power supply for the Evolution system at the
distribution board with a fuse or circuit breaker that is suitable for
the ACU. For guidance, refer to main power fuse rating stated on
the ACU connector panel. If in doubt, consult your local dealer.
Internal fuses

The ACU uses standard automotive blade fuses. Spare fuses
are located on the underside of the removable cover.