Analog AC Ammeter wiring

Thread Starter

duncan mccallie

Joined Apr 2, 2015
34
You guys were kind enough to walk me through an isolation transformer replacement recently so I’m back for more!
I have an analog AC ammeter mounted on my boats electrical panel. Currently (no pun intended) it is wired to a donut transformer on one leg of the 240 AC service. There is a second donut transformer on the second leg but it isn’t wired to anything. I’ve been reading and believing that gauge for years and now wondering if it was wired properly to monitor total amps?
Thanks in advance!
 

SamR

Joined Mar 19, 2019
1,842
Is it a clamp-on sensor or inline? Sounds like it's using a coil around the hot wire? It must be on the Vs as close to source as possible before any subfeeder circuits.
 

Thread Starter

duncan mccallie

Joined Apr 2, 2015
34
Is it a clamp-on sensor or inline? Sounds like it's using a coil around the hot wire? It must be on the Vs as close to source as possible before any subfeeder circuits.
Thanks
Yes, they are donut coils on each leg, before any loads. My question is whether I need a coil on each leg and/ if so, do both simply connect to the ammeter?
 

MisterBill2

Joined Jan 23, 2018
5,249
Is it a clamp-on sensor or inline? Sounds like it's using a coil around the hot wire? It must be on the Vs as close to source as possible before any subfeeder circuits.
The TS said that it was a donut coil, Those are not clamp-ons. And if the power in is single phase240 then you only need one. BUT if the shore power is 240 composed of two 120 volt lines and a neutral, then you would need to use both, and have a switch to read one or the other.
 

Thread Starter

duncan mccallie

Joined Apr 2, 2015
34
The TS said that it was a donut coil, Those are not clamp-ons. And if the power in is single phase240 then you only need one. BUT if the shore power is 240 composed of two 120 volt lines and a neutral, then you would need to use both, and have a switch to read one or the other.
Thanks! Would I need a special switch and diodes such as Blue Sea Systems refers to here?

Care should be taken to not leave a completely open circuit across the unused current transformer terminals. A shorting switch may be used, a resistor may be wired across the terminals or zener diodes may be used to limit the terminal voltage. Excessive voltage can cause circuit damage. Switching voltage also requires some special precautions.”
 

MisterBill2

Joined Jan 23, 2018
5,249
Thanks! Would I need a special switch and diodes such as Blue Sea Systems refers to here?

Care should be taken to not leave a completely open circuit across the unused current transformer terminals. A shorting switch may be used, a resistor may be wired across the terminals or zener diodes may be used to limit the terminal voltage. Excessive voltage can cause circuit damage. Switching voltage also requires some special precautions.”
Looking back at post #1, the answer depends on what the connections are to the presently unused current transformer on the second 240 volt power connection wire. If those wires are simply open then there is no reason to consider that any special treatment will be needed if switching between the two is added.
BUT if the only connection to shore power is 240 volts and the circuit is simply wired to the transformer primary, there will be no additional information gained from reading the current at a different point in the same loop. So there is no need nor benefit to be gained from adding a switch of any kind to monitor the second current transformer.
 

Reloadron

Joined Jan 15, 2015
5,140
The use of a CT (Current Transformer) with an analog meter movement is popular and simple. Current Transformers are normally specified out like the following:

Coil 1.jpg

What is important is the ratio and as seen here is 200:0 meaning for every 200 Amps (the maximum) the output on the black and white wires will be 5 amps into what is called a burden resistance. The 200:5 ratio tells us that 200/5 = 40 so the true turns ratio inside the transformer is 40:1 and the 1 being a single wire through the primary. If we open a CT up and look at the guts things go a little like this.

Coil 3.jpg

So in this case we take an analog meter movement which is really a 0 to 5 amp meter movement and we make a nice meter face scale of 0 to 200 Amps. The input to our meter movement serves as the burden resistance to our CT.

While of this is well and fine there is a caveat. While our current is divided down the voltage is multiplied up. So if we have a 40:1 ratio with a 120 VAC line running through the primary the open circuit voltage of the secondary is 120 * 40 = 4800 volts. This is why a CT is not run open circuit and while a single meter can be used to switch between several CTs the switches used are designed to place a short across any unread CTs. A Google of CT Open Circuit Voltage should find more information on that. Normally a CT is never left open circuit for that reason.

Since you already have another CT in place and if I recall correctly you have split phase 240 VAC I would think you have a CT on L1 and L2 the two 120 VAC hot lines. I would add another analog meter and if your current transformers are identical try to find and mount another identical panel meter. Your total load current will be the sum of both meter readings. Using a switch when switching between current transformers and this assumes a proper switch, you need to remember the first reading and add it to the second reading to know your total load draw.

Ron
 

crutschow

Joined Mar 14, 2008
24,408
You could use one meter by running both 240 wires through the same coil (one in the reverse direction).
That will give the sum of the currents.
But that will not tell you if one of the wires is being overloaded by a 120V load.
 

Thread Starter

duncan mccallie

Joined Apr 2, 2015
34
You could use one meter by running both 240 wires through the same coil (one in the reverse direction).
That will give the sum of the currents.
But that will not tell you if one of the wires is being overloaded by a 120V load.
Fascinating....thank you!
I do have some holes to fill in my electrical panel, maybe I’ll go with two ammeters.
 

MisterBill2

Joined Jan 23, 2018
5,249
The shunts are located on the PRIMARY side of the transformer, while the "split phase" connection is located on the secondary side. That is what was stated in post #1.

OK, I GOOFED and referenced the current transformers as shunts. But exactly the same situation exists, in that if they are on the primary side of the transformer then they will read the same current.
 
Last edited:

Reloadron

Joined Jan 15, 2015
5,140
Currently (no pun intended) it is wired to a donut transformer on one leg of the 240 AC service.
Uh Oh, I did a bad. The no, all you need is the meter on the one leg of the 240 VAC. Worst case here is even with no load on the transformer secondary there will always be a small, very small current draw on the primary side. Anyway I was thinking current transformers on the secondary side. All you need is what you have. Again, my bad on that, I simply read the wrong setup into it and was wondering why (thinking split phase secondary) didn't they connect the second CT. Little slow on the uptake here. You are fine with what you have.

Ron
 
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