[stevehull]

Measuring power in some circuits and appliances is confusing. Consider an electric water heater and a resistance type (not gas) electric clothes dryer.

First the water heater. This is a device with two active leads, L1 and L2 together with a chassis ground. The chassis ground never has any current on it and is only there as a true safety so that if one of the L1 or L2 leads shorts to the metal exterior, then that dangerous current is shunted to ground. You can disconnect this ground (stupid) and the electric water heater will work fine.

In the water heater, L1 and L2 are connected to a large resistor - the heating element. What goes through L1 is the same as L2. In this case, the total power is the amount in any leg multiplied by the voltage. If you put a current meter on L1 and get 20 amps, then the total power to the water heater is 4800 watts (240 v x 20 amps) or 4.8 kW when it is on. When it is off the power is 0 watts.

The electric dryer is a 240 V appliance with at least three leads, L1, L2, neutral, (and sometimes ground). It will have a plug with either three or four leads.

The large resistance element in the dryer is between L1 and L2 - just like the water heater (lets assume 20 amps). But there is typically also a 120 V electric motor on one of the legs that spins the drum. There is also a small 120 V electric bulb and a circuit board that runs off off 120. This circuit is between one of the legs (lets call it L2) and neutral. The neutral must be there for the 120 V circuit to work. This 120 V circuit alone draws 1 amp.

The ground (4th dryer wire) is there for safety and is attached to the metal of the dryer frame. It is there to prevent voltage on the metal dryer exterior if a fault occurred with L1 or L2 shorting to the metal skin of the dryer. Just like the water heater - safety.

As Jeff mentioned above, some dryers "cheat" and use the neutral and ground as the same wire (a three wire plug).

If you only have L1 and L2, the resistance element will have power, but there is no power for the drum 120 V motor as the return neutral for the 120 V circuit is absent.

With the electric dryer, the total current for the resistance element is the same as with the water heater. But there is also the extra current to run the 120 V electric motor (and also electric bulb and circuit board). In this case, the current through L1 might be 20 amps, but through L2 it is 21 amps. The extra current of 1 amp going through L2 is to power the dryer electric 120 V motor (and bulb and circuit board).

The total power of the dryer is 20 amps x 240 V (heating the resistance element) plus the 1 amp x 120 V (for the electric drum motor and bulb/circuit board).

Total power is 4800 watts for the resistance element plus 120 watts for the 120 V motor and bulb/circuit board or 4920 watts (4.92 kW).

A dryer is interesting. Much of the time, both the 240 V resistance element AND the 120 V drum motor are on. But there are times when the drum is spinning and the 240 V heater is off. This is an intermediate power load. So you need to average the power over the entire drying load to get the average power to do a load of laundry.

With the water heater, it is just off or on and there is no intermediate power load.

What I do is to measure current in both legs of a 240 V circuit. I take the current number that is highest and then multiply by the voltage. This is an approximation for a spot check. But to get accurate loads, you truly need to measure power on each 240 V leg for some period of time and know how much duration is on each.

Hope this helps.


Steve