Electrical Engineering

A CT, or current transformer, has a specified current ratio. The 5 in your question is the current supplied to the metering instrument at full scale. CT's current ratio is always full scale to 5, for instance - 200:5, 3000:5, etc.

So, if a CT has a ratio of, say, 1000:5, this means when the conductor being measured is supplying 1000 amperes to the load, the CT will output 5 amperes to the instrumentation. The relationship is linear, so if the current is 500 amperes, the output will be 2.5 amperes, at 200 amperes the output will be 1 ampere, etc.

This is done so the instrument can be designed always the same, with a 5 ampere input, no matter what size current is to be measured. Otherwise you would have to order (and manufacture) many different models of meter, one for each current range.

The metering must be set, or programmed, with the CT multiplier, so it knows how much current is really flowing when it sees 5 amperes on the input.

Similarly, we use PT's or potential transformers to reduce higher system voltages to a range that the instrument can also handle.

because of flux produced in coil of inducter

The potential difference (not 'potential') induced into a pure inductive component is proportional to the rate of change of current. The greatest rate of change of current occurs when the current waveform passes through zero (i.e. is at its steepest angle). So the voltage is maximum when the current is passing through zero -which means that the current is lagging the voltage by 90 degrees.

If the meter was set for "current" (amps) and the leads were connected to the current input, you hope it will read 0. -- It should blow the fuse inside.

If the meter is set for volts AC, it should read anywhere between 108 and 132 volts.

If the meter is set for volts DC, it should read close to zero.

If the meter is set for resistance (ohms) it would probably display an unstable reading, or could damage the meter.

And finally, is this a trick question, and you are really talking about a 120V AC powered benchtop multimeter? In that case it would read whatever current is flowing in the circuit under test, provided the meter is set up correctly.

A multimeter can test:

• ac current with a clamp on meter around one wire

  or

• ac or dc current in series with the load.

A multimeter cannot test dc current with a clamp on meter (there is no frequency in dc to create a field)

Another answer

I would think someone asking a specific question about a multimeter would assume that person knows what they're doing, and would connect it correctly. When I connect my multimeter to the 120V wall outlet I get 126 volts - the display is stable to within about a half a volt. I have a Keithley multimeter, as well as a Simpson analogue VOM.

I just measured line voltage again, this time getting 123 volts. This is reasonable, as there is no load on the line. With the toaster on, it drops to 118 volts. This is probably within whatever tolerance is built into the system.

Comment:

Because the standard tolerance for 120 volt services is " + or - 10%", those voltages you measured are certainly ok: those readings fall between the minimum tolerance limit of 108 volts and the maximum limit of 132 volts.

The actual voltage you are using at any instant in time doesn't only depend only on the load you have applied to the branch circuit, it also depends very much on:

• the length of cable-run from the step-down distribution transformer nearest to your home (located in an area substation or up overhead on a pole in the street)

  and

• how many of your neighbors are putting on their electric toasters and/or kettles at the same time as you are!

Voltage or current?

I notice the question mentioned both voltage and current. I don't know whether this was a typo. It would seem strange to ask how many volts you get from a 120V socket. The voltage version of the question has been answered quite comprehensively, so I'm going to answer the current version.

Voltage and current are different things, not just the same thing measured in different units. It doesn't make sense to ask "how many volts in an amp?" (which is essentially the question that's been asked). Electricity is analogous to water flowing through a pipe. Voltage is like the pressure difference between the two ends of a pipe; current is like the flow rate (in liters per second).

In an ideal world, an ammeter (current measuring device) has resistance 0, while a voltmeter has infinite resistance. Obviously, these are only ideals which can't quite be reached in practice. So the question is: Just how low is the resistance in your ammeter? Use the equation V=IR to calculate the current. I.e. the current is 120 volts divided by the resistance.

The National Electrical Code (US) list trade sizes in Chapter 9, Table 8, Conductor Properties: AWG (American Wire Gauge) 18, 16, 14, 12, 10, 8, 6, 4, 3, 2, 1, 1/0, 2/0, 3/0, 4/0 and by kcmil (thousand circular mils) 250, 300, 350, 400, 500, 600, 700, 800, 900, 1000, 1250, 1750, and 2000

I'm not sure there is a term other than "power"; V*A, or the vector sum of real and reactive power is equivalent to the power calculated by multiplying the voltage times the current, ignoring phase shift.

Answer

The product of voltage and current in an a.c. circuit is called 'apparent power', expressed in volt amperes, in order to distinguish it from 'true power' (in watts) and 'reactive power' (in reactive volt amperes).