Tag: amps

Have you ever noticed a blower motor rated for 120V draws about twice the amperage of  the same horsepower motor rated at 240V?

This is because motors are rated in Watts or Horsepower and according to Watts law Watts = Volts x Amps.

In order to keep the Wattage output the same at 120V, it draws twice as much current.

This is different than what happens when you drop the voltage of a motor below its rating.

Here is an experiment I did.

I took a regular 1/6 HP 208 – 230v condenser fan motor and tested it under normal conditions at my office and here is what I got


I then connected the common wire to neutral instead of L1 power which leads to approximately 120v applied and here is what I got.


By dropping the voltage by around 50% the amperage dropped slightly, the wattage went to less than half and the power factor also went in half and the motor slowed way down.

The motor slowing down is due to slip in the motor, meaning that the motor is running significantly slower than the speed it is designed for.

This means that not only is the motor running inefficiently, but it is also going to get hot because as the motor runs slower it has lower inductive reactance (the magnetic resistance in the windings). As the inductive reactance drops the windings have lower resistance and thus get hotter.

Even after all of this, the motor still consumes less than half the watts.

Rubber meets the road is that when a motor is designed for lower voltage it will draw more amperage to do the same work becasue it is designed to hit a wattage (horsepower) target at a designed voltage.

When you apply lower voltage you both decrease the work done as well as the efficiency and life of the motor because more of the energy goes to heat instead of mechanical work as the motor slips more and more. You also see higher power factor as the motor begins to slip resulting in even worse power efficiency.

This is one reason why voltage drop is a such an important thing to consider when sizing conductors and why 208-230V units are slightly derated or n both capacity and efficiency when installed on 208v.

Pay attention to Voltage, it can save a lot of money over time in both power efficiency and motor longevity.

— Bryan

We had a situation a few months back where we needed to monitor amperage on a grocery store panel over a period of time. The trouble was, we needed data logging capability as well as accurate measurement at 600+ amps.

Finding an all in one solution proved to be quite expensive. Luckily my friend Jim Bergmann happens to own an instrument company (Redfish Instruments)  and he had a simple solution.

Use the data logging capability of the Redfish multimeter with a Fluke I800 amperage to mA (milliamp) clamp to get the job done.

This particular clamp can be used with any good quality meter that reads in the milliamp range (not to be confused with uA which is microamps often used for testing flame rectification).

Like shown below, for this clamp you connect the meter leads to the correct jacks and select the mA scale. In this particular case 1mA = 1A so the image below is showing a 320 amp measurement.

There are other accessory clamps that use the mV (Millivolt) scale rather than mA like the Redfish IDVM333 shown below.

With this clamp the output scale can be adjusted with the selector on the clamp to either 10mV AC or 1mV AC.

Again you need to make sure the leads are in the right spots based on the type of clamp reading and make sure to get in in the right scale, but once you understand how to it, it’s surprisingly simple and you can use a wide variety of clamps and meters.

— Bryan


Most motors are designed to set amount of work, usually rated in either watts or horsepower, which is  746 watts per HP.

Watts law states that Watts = Volts x Amps. If a particular motor need to do 1 horsepower of work at 120 Volts it will draw about 6.22 amps. And yes in an inductive load like a motor it’s not quite as simple as VxA=P but we are keeping it simple here.

A motor designed to do the same amount of work (1HP) at 240v will draw half the Amps (3.11).

This does not make the second motor “more efficient” because the power company charges by the Kilowatt NOT by the amp.

) If you take a load that is designed for a particular voltage and you DROP the voltage it will also decrease the wattage according to Watts law (Watts = Volts x Amps) as well as decrease the amperage according to Ohm’s law (so long as the resistance remains the same).

Let’s say you take a 5KW heat strip that is rated as 5Kw at 240v and you instead connect it to 120v.

It would then only produce 1.25 kw and draw 1/4 the amps, this is because while we may call it a “5 Kilowatt heater” it is actually just a fixed resistor designed to do 5 kilowatts per hour of work in the form of heat at 240 Volts. Cut the Volts in half you also cut the amps in half the and you decrease the amount of work done down to 1/4.

— Bryan

 

Scroll to top
Translate »

Daily Tech Tip

Get the (near) daily Tech Tip email right in your inbox!
Email address
Name