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Old 17-01-2022, 03:18   #1
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Breaker Amperage for Autopilot

I feel like I'm missing something obvious here . Sorry, I'm not currently on the boat, so I'm running off of memory.

I've got one breaker on my DC panel (push button style) that is labeled "Simrad". When I turn that on, shockingly, my Simrad MFD turns on. Doing a little investigative wire following, it appears that the "Simrad" push button leads to a fuse panel that has a lot of items on it. Those items include:

* MFD
* AIS Transceiver/Receiver
* Autopilot Computer
* Radar Converter Box Thingy

Of particular note is the Autopilot Computer. There is a 30 amp fuse inside of the autopilot computer. Unfortunately, all of my push button breakers are attached with a solid wire rod, so I can't pull the breaker out to read its rating. Some previous owner did write "20" next to that breaker, and "10" next to all the other breakers. My educated guess is that the "Simrad" breaker is a 20 amp breaker.

Where my confusion comes in is that I was reading up on random electrical projects tonight and someone mentioned they had just bought a new breaker panel which came prewired with 15 amp breakers. So 10-20 amps seems like a sensible default for breakers. But 20 amps is well below what would be necessary to run all the MFDs and navigation equipment.

Do most people split their autopilot onto its own breaker? Or is it possible to get something like a 40 or 50 amp breaker?

Or am I wrong in thinking you need a breaker (and corresponding wire) that is capable of handling the total amperage of that circuit. I had an electrician onboard today to start planning out some other work, and we used his amp clamp and saw less than 10 amps of total draw on that circuit with radar, autopilot, and everything running. And it goes to reason that you would have a fuse much higher than normal load, so we wouldn't expect to see 30 amps out of the autopilot. I'm just wondering if I've gotten lucky that the 20 amp fuse hasn't blown yet, or if it's fairly standard practice to find circuits with a much higher theoretical load than what you see in normal use?

Also, bonus question, does anybody have any idea why the autopilot has a 30 amp fuse inside of it? When I looked it up online, I saw something in the manual about 15 and 30 amp models, so I assume some autopilot systems use 15 amp motors and some use 30 amp motors? My system is hydraulic, and the computer directly drives the hydraulic pump, if that helps out.
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Old 17-01-2022, 05:49   #2
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Re: Breaker Amperage for Autopilot

The only heavy draw in all that is the pump motor in the autopilot. It's peak draw will come with maximum water pressure on the side of the rudder to overcome steering you in a storm, so we don't know exactly what that will be. The thirty amp breaker on the pump is sized to flip when "This isn't a pump draw, there's a short circuit in here" or "If I draw any more I'm going to burn out the motor". In other words, well above routine use/draw.

In all probability the same computer is used by Simrad to control large (30 amperes) and small (15) pumps, neither of which draws 30 or 15 amperes.

Again in all probability, you don't have a problem. If you ever trip the 20 amp going in, then consider separating the autopilot from the rest of the load. The 20 amp appears to be sized with the rest of the users in mind, such that you won't burn out something like the MFD.
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Old 17-01-2022, 07:02   #3
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Re: Breaker Amperage for Autopilot

Not to sure about your Radar Box but other than the AP most of the other loads are typically rather small. I have a single 20 amp breaker feeding a small sub panel (just inside the laz) where there are breakers feeding 5 different loads all associated with steering pedestal electrical requirements. My AP loads are rather minor (3 to 4 amps peak) but this arrangement has worked well for a lot of years. Autopilot current draws are quite intermittent and variable, but the system should be capable of handling a worst case.

My thinking about this arrangement follows from many years deigning commercial electrical systems where this is SOP allowing a major feed to drive a sub panel (cuts down on the wire runs and the big copper get shared by all connected).


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Old 17-01-2022, 07:38   #4
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Re: Breaker Amperage for Autopilot

Breakers should be selected to protect the wire, not the end devices. If you have 5 wires and the smallest wire can only handle 15A, then it should be a 15A breaker regardless of the total load. If you have a 20A wire to your autopilot, then it should be a 20A breaker. It doesn't matter if the autopilot "needs" 30A. If you try to run 30A through a wire that can only handle 20A, you risk starting a fire. Devices frequently have an internal fuse that protects the device itself, normally much smaller than the breaker/wire rating.
If you can't support the devices this way, then add individual fuses to each wire according to the size of that wire. In this case you could use either a switch or a big breaker in lieu of a switch since each wire has its own fuse. Or upgrade the wire. Don't swap in a bigger breaker without considering the wire.
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Old 17-01-2022, 11:45   #5
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Re: Breaker Amperage for Autopilot

JPB is correct, fuse/breakers should be based on the wire, not the load. Most electronics draw very little. Big load would be the linear drive, wheel drive or hydraulic pump for the AP.



As far separate circuits. On my boat, and several others, the brain is on one breaker and the hydraulic pump for the AP is on a separate (20 amp) breaker. The brain just controls a relay to run the pump.



Mine breakers have one other one in between them on the panel. Was a pain when an electician (boat) moved the pump to be on the breaker next to the brain switch. Seems he didn't see that the pump was on a 20 or 30 amp breaker and the adjoining one he moved it to was only 10 or 15. AP worked fine in calm wx but kept blowing the breaker when the wind came up. Took a while for me to figure that out after a day + of manual steering half way from the from Chesapeake to BVI's. Crew, and I, were not happy campers.
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Old 17-01-2022, 13:53   #6
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Re: Breaker Amperage for Autopilot

There are some spiffy charts on Google Chrome showing compatible fuse sizes for each gauge of wire. Some of the charts factor in length, as length enters into the equation. Isn’t the internet great, at times !
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Old 17-01-2022, 14:02   #7
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Re: Breaker Amperage for Autopilot

Don't forget with 12 Volts, voltage drop can be the short pole in the tent.

Thus my choice of a big wire to get to the loads and then individual circuits for the last short step.


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Old 18-01-2022, 01:35   #8
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Re: Breaker Amperage for Autopilot

Thanks for all the input!

I didn't phrase my question very well, but you folks were able to get me the info I was after. Amazing.

For clarification, I'm totally on board with having fuses between the battery and "feed wire" that are sized appropriately for the wire (so that the wire does not become the fuse). The bit that was throwing me off was placing a 30 amp fuse downstream of a 20 amp breaker. It seems that the breaker should be the first thing to go if anything over 20 amps worked its way down the wire. I wouldn't do it, but in theory, I could put a solid piece of metal (a jumper pretty much) in place of that 30 amp fuse and get the same protective value from the 20 amp breaker.

I was trying to match my expectations of theory vs reality. If I sum up all the fuses in that sub panel, there is probably north of 40 amps of fused output in that sub panel. Neither the feed wire or the 20 amp breaker are rated anywhere near to that. So it sounds like I have a risk of "annoyance" tripping that 20 amp breaker, when none of the individual devices have actually broken their contract, especially that 30 amp autopilot by itself.

And apart from Moontide's incident of self steering, it seems like it is fairly common practice to size the breaker and wire for the "expected" draw (plus overhead) versus "worst case" draw where every individual fuse could simultaneously blow.

I'm very new to boating and am trying to get a prioritized list of things to fix. When I first saw this undersized breaker, I thought I had found something that needed to be swapped right away. But instead, it sounds more like I have something that I can use in the meantime and when I get far enough down the line (and enough things that need to be rewired), I can look at upsizing that breaker and wire or splitting the circuit out.
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Old 18-01-2022, 02:24   #9
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Re: Breaker Amperage for Autopilot

First thing you have to do is add up the ACTUAL current draw of all your devices on thaat circuit, fuses mean NOTHING!
Then you can go about properly designing the circuit to deal with the expected loads AND wire sizes.
Evidently it is under 20 amps or you'd be blowing the 20 amp breaker, the fact there's 40 amps worth of fuses means NOTHING in regards to what you actually using.
I could wire my cabin lighting with 1/0 wire and put a 75 amp fuse in there and I'd still only be using about 1.5 amps to drive the leds.
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Old 18-01-2022, 02:47   #10
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Re: Breaker Amperage for Autopilot

[QUOTE=Barkingmad;3556983] ACTUAL current draw of all your devices on thaat circuit, fuses mean NOTHING!

Is there a way to accurately calculate the draw on a circuit? Should I look for documentation on the devices? Putting an amp clamp around the feed line and turning everything on would give a number, but how accurate is that number? Moontide's example of the autopilot under load comes to mind. I've also read stories about motors that require 3 or 4 times their rated amperage on startup, and that amp clamps sometimes can't capture that peak draw.

Or am I stuck too much in the theory of electronics, and in the real world, measuring the draw and building in a bit of overhead is the best way to go about it?
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Old 18-01-2022, 03:09   #11
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Re: Breaker Amperage for Autopilot

Quote:
Originally Posted by workmaster2n View Post
Thanks for all the input!

For clarification, I'm totally on board with having fuses between the battery and "feed wire" that are sized appropriately for the wire (so that the wire does not become the fuse).
yes
put for your electronic sized appropriately fuse for the wire so that the wire does not become the fuse and burn.
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Old 18-01-2022, 03:37   #12
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Re: Breaker Amperage for Autopilot

The manuals will give you the info you need, you could measure I suppose but that is more effort than required. Also usually on the back of the equipment as part of the certification process.
You will see some spikes but they are within the specs unless something is wrong. If your pilot is rated at 15 amps it's NOT going to draw 60 at start up.
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Old 18-01-2022, 07:53   #13
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Re: Breaker Amperage for Autopilot

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Originally Posted by workmaster2n View Post
ACTUAL current draw of all your devices on that circuit, ...

Is there a way to accurately calculate the draw on a circuit?
It's pretty much impossible to get an exact number. You will never actually use all devices at one time. Add them up as best you can, there are tables you can look up the average draw of a VHF, fridge, etc. Does the number seem right? Less than 100A for a small boat. More for a larger boat. That's good enough. (Always considering wire capacity).

Quote:
Or am I stuck too much in the theory of electronics, and in the real world, measuring the draw and building in a bit of overhead is the best way to go about it?
Exactly. There are a lot of helpful folks here, post your results and people will tell you if they think they are reasonable.

(BTW, there are also a lot of people here who will tell you that you will instantly burn up your boat, die, and be forever mocked by the world if you don't do things THEIR WAY. You will have to sort them out.)
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Old 18-01-2022, 08:25   #14
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Re: Breaker Amperage for Autopilot

AS JPB points out it's a very good idea to do an "energy audit" of your entire boat and see if the entire electrical system is up to par.
Don't assume it's good from the factory either! Mine came with about a 35 foot total circuit length of 14 gauge wire to run the Jabsco shower drain pump. WAY undersize, too much voltage drop.
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Old 18-01-2022, 09:33   #15
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Re: Breaker Amperage for Autopilot

Yes, the autopilot will draw more than 20 amps. Not at the dock, but out in heavy weather, when you really want it to work.

You need a separate 30 amp breaker for the pilot alone, and at least 10 gauge wiring from the panel to the autopilot box.

This breaker will work, and is cheap enough to buy a spare.

https://www.amazon.com/STETION-Reset...27&sr=8-4&th=1

How do I know this? I jerry rigged a Simrad pilot into a 70 ft race boat to bring it back from Hawaii. On the delivery to the Transpac start, I had it wired to an unused 20 amp breaker on the panel. It was an experiment and I had a crew of 7 so I was ready to take over steering if the 20 amps proved insufficient. It tripped going around Pt Conception, and we hand steered to quieter conditions. I installed the above breaker direct to the panel power feed wire in Hawaii and the pilot never tripped again.
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