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Ammeter shunt

Your old Chris-Craft electrical system can be a challenge. If it runs on "juice" pose your questions and offer your advice here.

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island time
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Ammeter shunt

Post by island time » Mon Feb 12, 2018 7:13 pm

I'm working on the 23 Commander and getting near to rewiring. All wiring will be replaced, I am going to make a new wiring harness using all the original factory wire colors. Several changes have been made electrically during the engine replacement including replacing the old Motorola alternator with external regulator with a new Delco 1-wire alternator with internal regulator. With all I know about electricity, admittedly there are some things I don't understand about DC such as...
When it comes to the ammeter, I am using the original dash gauge, is the shunt required? why? I don't understand it's function. Also with this particular device comes a requirement for a "very" specific wire length between it and the gauge. I can only imagine this relates to resistance?
As always I thoroughly appreciate all help.

Craig
Craig Wilson
Churchville, NY
Alexandria Bay, NY

1968 23' Chris Craft lancer
1971 31' Revelcraft
1988 25' Fourwinns
1972 19' Chris Craft Lancer
1957 18' Chris Craft Sea Skiff
1958 18' Chris Craft Sea Skiff
1968 23' Chris Craft Commander

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Bilge Rat
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Re: Ammeter shunt

Post by Bilge Rat » Wed Feb 14, 2018 8:13 am

The gauge shunt's purpose is to reduce the current (amperage) to a more easily readable value. If say your alternator can put out 40 amps at high charge, a direct series reading ammeter would have 40 amps passing through it's windings. Typically, that would require at least a 10 gauge conductor for the gauge windings, that's quite large and difficult to make the gauge accurate. Also, that much current passing through a gauge with a metal shell presents a number of safety issues to ensure the positive current cannot accidentally short out to the shell's ground. There have been some spectacular short circuits because of gauge insulation failure trying to conduct the full battery ampacity (hundreds of amps!). This is known as a fault condition. The shunt essentially is a calibrated resistor that is "tapped" to measure how much voltage reduction is passing through it. This is a tiny current value that is proportional to the amount of current passing through at the given moment. The gauge then reads this smaller proportional current value. This is much easier and safer for the gauge to display and will be more accurate. This technology is at least 100 years old, but works quite well.

Not sure the reasoning for the wire length limitation but I suspect you are correct about total resistance of the circuit. Like any conductor carrying current, too small a conductor will cause the wire to overheat as it's carrying too many amps. That overheating leads to conductor failure and burn-down. Not a good day of pleasure boating. Also, too small a conductor can cause a phenomenon known as voltage drop; the conductor's resistance is too great (at the current it's trying to carry) to guarantee a minimum of loss of voltage at the end of the circuit. That Ohms law thing; voltage, current and resistance all figure into the equation.
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

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island time
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Re: Ammeter shunt

Post by island time » Wed Feb 14, 2018 9:13 pm

Many thanks, your detailed explanation makes good sense.
I will share a little detail found in the wiring diagram obtained from the museum, the notes on the drawing say wire size and lengths are as follows- #16 gauge wire to be exactly 11', #14 Gauge-18' and #12 Gauge 28'. The note says to coil and tie excess wire near shunt.

Craig
Craig Wilson
Churchville, NY
Alexandria Bay, NY

1968 23' Chris Craft lancer
1971 31' Revelcraft
1988 25' Fourwinns
1972 19' Chris Craft Lancer
1957 18' Chris Craft Sea Skiff
1958 18' Chris Craft Sea Skiff
1968 23' Chris Craft Commander

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Bilge Rat
Posts: 178
Joined: Thu Jul 16, 2015 4:02 pm

Re: Ammeter shunt

Post by Bilge Rat » Thu Feb 15, 2018 7:30 am

That makes sense, the wire's resistance is critical in the metering portion of the circuit to maintain accuracy. Been chasing electrons for over 40 years but I haven't caught them yet!
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

jim g
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Re: Ammeter shunt

Post by jim g » Tue Feb 20, 2018 10:27 pm

Like everything else.The 60's and 70's gauges were made cheaper. Without the shunt and the correct length of wire. Not only will it not read right but it will also fry the gauge inside.

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island time
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Re: Ammeter shunt

Post by island time » Wed Feb 21, 2018 9:39 am

Once everything is hooked up and running is there any way with common tools to confirm accuracy of the ammeter?

Craig
Craig Wilson
Churchville, NY
Alexandria Bay, NY

1968 23' Chris Craft lancer
1971 31' Revelcraft
1988 25' Fourwinns
1972 19' Chris Craft Lancer
1957 18' Chris Craft Sea Skiff
1958 18' Chris Craft Sea Skiff
1968 23' Chris Craft Commander

User avatar
Bilge Rat
Posts: 178
Joined: Thu Jul 16, 2015 4:02 pm

Re: Ammeter shunt

Post by Bilge Rat » Wed Feb 21, 2018 11:37 am

You would need a clamp-on type ammeter capable of measuring DC amps to measure the primary side (wire from the alternator to gauge shunt) and compare that reading to what your helm gauge indicates. You would need a clamp-on meter that can read DC current, most inexpensive ones are AC current only. The Extech MA445 is $89.00 at Home Depot and obviously can perform a lot of other electrical testing functions than just DC amp reading. A clamp-on ammeter just clamps around the conductor you want to read the current on, no need to break apart terminations to take a reading. As a note; the more expensive the testing equipment, the greater accuracy in the readings. This model has +/- 2% DC accuracy.
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

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island time
Posts: 131
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Re: Ammeter shunt

Post by island time » Thu Feb 22, 2018 6:52 am

I've got a very good quality Fluke clamp-on and I'm ashamed to say I never though about it because quite honestly I've never used it for DC. :oops:
I have all my new electrical equipment ordered and most of it is due in today. I'm building an electrical panel with a 12 space fuse holder, 2-bank digital volt meter and charger sockets for standard 12v cigarette lighter and dual USB ports. All new wiring will include #4 copper feeders from the battery switches to the panel. New circuits for the extra goodies besides all the regular circuits for normal loads. Why is it that every boat I work on has had wiring done before by the same idiot? Why does everyone think that it's OK to just tap on another wire to that terminal without regard for ampacity or wire gauge? Oh and no problem, I'll just put more tape on it!
When I removed the old wiring harness I cut it open in the neighborhood of a suspicious bulge in the outer jacket just to see the condition and found a knot of melted wires halfway in between the dash and the engine bay. This was probably caused by the trim tab circuit that was tapped into the bilge pump circuit that was tapped off from the bilge blower that was fed from the circuit that gets power from the loose connection on the instruments feed. No doubt the reason for the 3 additional #16 thhn conductors that were running loose in the general vicinity of the wiring harness. Fuses? who needs fuses?
I never cease to be amazed! If I had done something like that it would have caught fire!

Craig
Craig Wilson
Churchville, NY
Alexandria Bay, NY

1968 23' Chris Craft lancer
1971 31' Revelcraft
1988 25' Fourwinns
1972 19' Chris Craft Lancer
1957 18' Chris Craft Sea Skiff
1958 18' Chris Craft Sea Skiff
1968 23' Chris Craft Commander

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Bilge Rat
Posts: 178
Joined: Thu Jul 16, 2015 4:02 pm

Re: Ammeter shunt

Post by Bilge Rat » Thu Feb 22, 2018 7:12 am

Yeah, I've seen a lot of "cowboy" wiring on boats too (no offense to real cowboys!). It is a wonder there aren't more electrical fires on boats. I used a similar wiring type and ran a 2 gauge from the battery switches to the panel but I installed an in-line fuse after this wire left the switch. It is a high current 80 amp if memory serves me. Just in case!
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

User avatar
island time
Posts: 131
Joined: Mon Sep 28, 2015 9:11 pm

Re: Ammeter shunt

Post by island time » Sun Mar 11, 2018 6:59 am

So I've been a little busy with work so I haven't gotten much done with wiring yet,-damned work!. But I've gotten all the necessary parts collected so when I get time to work on it there won't be any more delays...I hope.
I got heat shrink tube for my label maker so I can label all my circuit wires and I ordered legend plates for the fuse panels identifying the circuits. I'm still working on a final layout of the fuse panel, terminal strips and wire routing.
I'm planning on getting started today measuring out wire to begin assembly of the new wiring harness.
I keep thinking back to the ammeter shunt, I'm guessing a shunt works like a current transformer and is calibrated to a specific gauge and if so this one is operating at a 10% conversion because the original alternator had a 60 amp maximum output and the gauge has a +60 to -60 amp scale.

Craig
Craig Wilson
Churchville, NY
Alexandria Bay, NY

1968 23' Chris Craft lancer
1971 31' Revelcraft
1988 25' Fourwinns
1972 19' Chris Craft Lancer
1957 18' Chris Craft Sea Skiff
1958 18' Chris Craft Sea Skiff
1968 23' Chris Craft Commander

User avatar
Bilge Rat
Posts: 178
Joined: Thu Jul 16, 2015 4:02 pm

Re: Ammeter shunt

Post by Bilge Rat » Mon Mar 12, 2018 6:23 am

You're on the right track about the shunt. A current transformer performs the same type of calibrated ratio function as a shunt resistor however a transformer can only work on AC systems. Current transformers are used in electrical metering (even your home residential power company meter has one) as well as fault current sensing and relay switching based on sensing current flow.
1966 Lyman Cruisette 25 foot "Serenity Now!"
1953 Chris Craft Sportsman 22 foot "Summerwind"

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