Re: panel meters

<SNIP> You pass the "big wire"

> carrying the current you want to measure through the "donut" (CT).

That is then

> a "one turn primary", and a voltage proportional to the current

flowing in

> that wire appears between the lead-wires coming out of the CT.

Not to split hairs but if you just pass the wire through the center
of the CT and don't take a wrap you actually have a 1/2 turn Primary.



This voltage is

> measured by a panel-meter which will "read" the amps flowing in

that wire

> through the CT. In many cases, this "big wire" might be passed

through the "donut"

> more than once, to DOUBLE or TRIPLE (etc.) the resulting voltage

from the

> CT's secondary.

The current in the secondary is porportional to the current in the
primary divided by the turns ratio. For voltage to develop the
current needs a load to work against. A resistor across the
secondary becomes a load and the voltage then can be determined by
simple ohms law. You can change the value of the resistor to get
different voltages but always be aware that the size of the resistor
(wattage) goes up with the square of the current in the secondary.
Let's say you have 20A flowing through the primary wire and you have
just 1/2 turn (wire goes straight through) and the transformer ratio
is 20:1 so you would have .5A flowing in the secondary. If you want
10VDC at that primary current you would need a 20 ohm resistor across
the secondary. The power flowing would be 5 Watts. The more voltage
you want, the larger the value of the resistor and the higher its
dissipation. Adding turns in the primary will give you more current
in the secondary and you can use a lower value resistor to get the
same voltage and a resulting smaller size for the power.


Sometimes you can spot CT's around the individual power lines

> entering an industrial building, say, a good-sized machine-shop.

These are

> often located overhead, but generally not on the "loose" part of

the "drop" that

> comes from the pole, but just these wires enter the

weatherhead/conduit-down.

> There will be smaller (say #14 or #12) wires from each CT (there'd

be THREE if

> the service-entry is 3-phase!), these leading down to a "box" next

to or

> right-behind the KWH meter. Connections in that "box" also read

the AC Voltage,

> and the meter "does the math" and figures what you owe Mr. Edison

(or whomever).

>
>