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Full Version: St4000 - wiring and voltage drop
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I think I know my answer, but i thought I would post and that way someone else might avoid having to ask.

I have just installed a pedestal in an old sailboat. Along with that I installed an ST4000 system that I had. It all works fine, except...

The system wiring comes off a 8 awg 30amp fused line from the battery switch, that also hosts the engine systems, gauges. I then run 14 awg less than 8ft to a junction that connects the positive for the St4000(fused@15amp as 12amp agc could not find), Garmin GPS(fused inline as well) compass and cockpit lighting(LED). As you can see the only item that would draw significant power at times would be the drive motor.

When the system is put into Auto - In 30sec to a min I will get a brown out that will turn off the power to the GPS and reset the ST4000.

If I am under power and the alternator is charging - no brown out - I have run for hours in this mode.

So it would seem obvious I am getting voltage drop because of the potential draw of up to 10amps from the drive motor. I am going to assume because of the systems age - it could be spiking higher.. though it has not blown the agc 15amp fuse. 14 awg should be enough to handle 10amp draw on that short of run. My batteries pass load testing, but they are deep cycle not starters.

Is my answer to simply run a single line to the drive motor from one of the batteries? I would prefer to draw after the battery switch so that if I am on 1 battery or the other or both and can use that.


Welcome to the Raymarine Forum James,

As a best practice, it is recommended that autopilots be installed on a dedicated power circuit. When determining the appropriate gauge of wire for a given amount of current, it is important to understand that it is the round trip circuit (i.e. from the battery/power supply to the device being powered and then back to battery/ground) which must be considered. As such 14 AWG reduce the power supplied over the round trip distanced involved. Please see the table from Blue Sea Systems below addressing this subject:

[Image: DC_wire_selection_chartlg.jpg]

In this case, I would have recommended that either 12 or 10 AWG (preferred) be used. It is also recommended that an clamp meter (instrument capable of sensing current (ex. Extech 400A Dual Input AC/DC Clamp Meter + NCV) to measure the current draw of the pilot under various operating conditions.
Thanks Chuck, so.. last thing I wanted to add.. when it was making course changes and therefore loading the circuit, the temp gauge would flutter in sync with the change... voltage spikes I assume in that circuit.

So I must go all the way back to the first junction from the battery switch and run a clean positive to the autopilot, agreed?


While your battery(s) may have load tested satisfactorily, the problem appears to be one of insufficient power to the entire system. Contributing factors may include insufficient battery capacity, corroded conductors, posts, or connector, loose connections at battery, bus bar, and panel. As these are easy items to check, these should be examined first. Adding additional battery storage should also be considered ... recommend changing all batteries whenever adding or replacing a battery. A

s the autopilot is not reporting any problem with the drive (i.e. Drive Stopped) and is not blowing any fuses, the reported browning out is resulting from not having electrically isolated the marine electronics from the vessels other systems and placing a higher power demand on the supply circuit than can be accommodated.
OK, I will run a separate circuit for the AP.

Finally - when I did the calibration on the FG compass it came up with 23 in the deviation? I know from the manual that this is high.. and it says for me to relocate the compass.. but I will be honest I dont know of any better area anywhere close to the centerline of the boat.

What causes the deviation? I am wondering if there are things I could move or change to reduce it?

Also what is the negative effect of it .. if I just change the compass to match the magnetic and it is holding a course???


Deviation is the effect of the magnetic field generated by the vessel or by objects witin the vessel upon a magnetic compass. Objects which have magnetic fields associated with them include objects constructed from ferrous materials, magnets, electrical coils, or those that consume or carry high current loads. The fluxgate compass should not be installed within three feet of such objects. In some vessels, particularly steel hulls, the fluxgate will be installed above deck (ex. on the mast, pole, etc.). The sensed deviation may differ based upon heading and as such vessels will "swing the compass" (a procedure by which the vessel will be steered slowly in a circle to determine the amount of deviation at various sample points of the compass. For this generation of autopilots, the installation location for the fluxgate should ideally sense a maximum deviation which is less than ten degrees. Excessive deviation sensed by the fluxgate compass may adversely affect the performance of an autopilot.
Hmmm.. ok.. its 3 feet from another compass (main ritchie) its 4+ from the GPS, its about 4 feet from the alternator - maybe 3+ from the closest point of the engine block. - the excessive wire the compass comes with.. I only need about 4 feet.. could that be an issue?

There are two compasses on the boat other than the flux.. they are about 5ft apart and are within 5deg of each other with no adjustments required. Ive held a handheld compass beside the fluxgate and get the same reading at the compass 4ft about it? SO should I worry about this deveation?


The sensed deviation is too high. Before relocating the fluxgate compass, it is recommended that the fluxgate compass be tested per the FAQ found herehere and replaced if found to be faulty. Should the test not indicate a faulty fluxgate compass, then the fluxgate compass should relocated further from the engine block and alternator.
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