Amp draw is a measurement of the power being consumed by a blower motor to move the air through your HVAC system. It’s one of the lesser-used diagnostic tests, but perhaps if we focus on how to measure amp draw and learn to better interpret what it can tell us, we will expand our ability to provide comfort to those we serve.
To interpret the meaning of an amp draw reading, you need to know the capacity of the blower motor that’s turning the fan. This is found in the nameplate of the motor and is called “full load amps,” designated by the initials FLA.
When a motor operates at 100% of its FLA, it’s operating at 100% of its capacity. The goal of the test I’ll present here is to compare the measured amps that the motor is using to the FLA capacity of the motor.
When a system has been renovated and system performance has been maximized, it’s common to read amp draw at 80% to 90% of FLA on a most direct-drive residential units.
If the measured amp draw exceeds the capacity of the motor, the motor will fail prematurely. Excessive heat is what usually causes damage to a motor, and when a motor is working beyond its FLA, its life expectancy is decreased significantly.
If the measured amp draw is only half of the FLA, the chances are that you have a serious air flow problem that will require additional work to get the system to perform properly.
Most of us carry one of many electrical multimeters that are on the market to test electrical properties. On these meters, there is a selection for AC Amperage that allows for a wide range of testing sufficient for all sizes of residential blower motors. An electrical meter will range in cost from $60 to $500. As with all tools, you get what you pay for.
Some meters come with a built in AC current clip that looks like two fingers at one end of the meter that wrap around the wire being tested.
To measure ampacity on most smaller systems, you clamp onto a wire in the blower compartment, then close the blower compartment door during the test.
To get access into the blower compartment requires a remote amp clamp attached to the end of a 3-ft. to 4-ft. cord. The amp clamps look like a clothespin with two insulated wires that attach it to the meter. This clamp snaps onto the power lead of the motor and measures the flow of the current pulled by the motor.
Remember to have your test instruments calibrated annually so that you can have trust and confidence with your readings and diagnostics.
Before we discuss taking measurements, let’s look at safety. This amperage measurement is taken with the power on and a fan that often moves from 500 to 2,000 rpm. With some systems you may have fingers and test leads dangerously close to this moving fan.
Also, whenever you’re measuring electricity, whether this is a new test for you or if you’ve successfully performed it a thousand times, please pay attention to your personal safety.
Here’s my favorite safety quote: Whenever taking electrical measurements, or working with fans and pulleys, remember that even a fleeting misunderstanding or inattention to energized equipment may easily carry the punishment of death, enforced promptly, without the chance of appeal anywhere on Earth.
No diagnostic measurement is worth a finger.
Should you ever be tempted to reach in and pull out a wire to test with your bare fingers, be aware that insulation may have been stripped from the wires and you could be headed for some serious pain, or even the punishment mentioned above.
Having said that, let's take a look at how to measure amp draw of a residential direct drive blower. Even if you often deal with large commercial systems, read on, the same principles apply.
The Test Procedure
1. Disconnect power from the blower motor by shutting it off at the disconnect box to the unit or unplugging the power source.
2. Open up the blower compartment of the furnace or air handler. Find the Full Load Amp (FLA) rating of the blower motor. If the motor is direct-drive these numbers can be difficult to read. They are usually located on the nameplate located on the side of the motor. An inspection mirror may be required, just be careful not to read the numbers backwards!
Some air handlers now have the FLA of the blower motor listed right on the nameplate of the equipment. These manufacturers have gone the extra mile to understand the needs of their HVAC contractors.
3. Check the rotation of the fan. “Bump” the fan by depressing the blower compartment safety switch. Check that the blower is rotating in the right direction, that it is tight on the shaft, and that there is no restriction or grinding as the motor turns.
4. Locate the power wire leading to the blower motor. This is normally the wire leading to the air conditioning or high-speed wire on the speed tap. (Remember to normally balance in cooling mode.) If you have a single-phase, multiple-speed motor with speed taps, place the unit in cooling mode with the fan switch in the “on” position and the temperature set to 55F.
One rule of reading amp draw is that only one wire at a time can be measured. Testing two or more at a time will cancel each other out or the reading will be false.
If there is not an exposed wire within the unit, you will have to find an exposed single lead to the motor where you can test. This may be in the electrical disconnect, or at a junction box.
5. Connect the amp clamp from your electrical meter around the power wire.
If the only access to the wire is in the blower compartment, you must be sure that the test is taken with the blower door on, or at least closed as much as possible.
If the door is left off during the test, the fan will move more air than it would if it had to pull all its air through the return air ducting. Therefore, the fan will work harder and the amp draw will be higher than it would be under normal operating conditions.
6. Read the amp draw of the blower motor. Record the reading and compare it to the FLA listed on the motor.
Check the amp draw against the manufacturer's fan performance data to verify the airflow that the fan is producing. A rule of thumb is that on high speed most motors will draw 70-100% of the FLA if it is moving the required 400 CFM per ton on residential systems.
Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in a no cost, one-page Amp Draw Measurement Procedure, contact Doc at firstname.lastname@example.org or call him at 800/633-7058.