How to Measure Amps - electrical service, circuit or individual device electrical current usage rate
AMPS MEASUREMENT METHODS - How to measure amperage or amps: photos & text illustrate using the Digisnap DSA-500 snap-around digital multimeter from A.W. Sperry Instruments to measure the amps drawn by a simple electrical circuit, device, or appliance. How to measure electricity usage: measure amps or measure electrical current draw: how to use an ammeter, a clamp-on or snap-around ammeter or digital multimeter to measure amps or current draw at a building, on an electrical circuit, or at an electrical device such as an A/C compressor, electric motor, or appliance.
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How to measure electricity usage & measure electrical current or amps - the current drawn by an electrical circuit, device, or appliance.
Measuring the current drawn by an electrical device such as an air conditioner compressor motor, electrical motor, or an electrical circuit in a building can give useful diagnostic information and can also give insight into which electrical circuits or appliances are the heavy users of electricity.
This article describes using the Digisnap DSA-500 snap-around digital multimeter from A.W. Sperry Instruments to measure the amps drawn by a simple electrical circuit, device, or appliance.
Use a Clamp-on or Snap Around Digital Multimeter - Ammeter to Measure Amps (current draw)
Safety Warning: Home inspection standards for electrical inspections do not require the inspector to insert any instrument into the service panel. Therefore this testing is optional.
It's also a dangerous procedure that can damage electrical equipment or worse, cause electrical shock, or even death, and should not be undertaken unless the person conducting the examination is trained and competent to avoid electric shock. If the inspector is not trained for this procedure
s/he should never insert any instrument or tool into electrical equipment. See SAFETY for ELECTRICAL INSPECTORS.
Our photo (left) illustrates using Sperry's Digisnap DSA-500 to measure the current draw of an individual electrical device - in this case, the charging block for a laptop computer.
AMPS MEASUREMENT METHODS - How to measure amperage or amps: photos & text illustrate using the Digisnap DSA-500 snap-around digital multimeter from A.W. Sperry Instruments to measure the amps drawn by a simple electrical circuit, device, or appliance
How to measure electricity usage: measure amps or measure electrical current draw: how to use an ammeter, a clamp-on or snap-around ammeter or digital multimeter to measure amps or current draw at a building, on an electrical circuit, or at an electrical device such as an A/C compressor, electric motor, or appliance
What's the Difference between Determining Service Ampacity and Measuring Amps at an Electrical Service, Circuit, Appliance or Device?
In this article we describe various methods for actually measuring the amps or electricity usage at a building, circuit, or device. But first, let's not confuse the determination of the ampacity of an electrical service at a building with amps measurement at a building, electrical circuit, or device.
What's the difference between determining the ampacity of the electrical service at a building and making actual amps measurements at an electrical service, circuit, or electrical device?
Making an amps measurement on a building circuit or at an appliance, motor, or air conditioner etc.
Measuring electrical amps is the measurement of current draw or amps or amount of electricity being consumed is an actual instantaneous measurement of actual electrical power use at the moment, not a measurement of the design capacity or capability of the electrical service at a building or in an electrical circuit.
The actual current draw or total amps being consumed on an individual electrical circuit in a residential building should (with minor technical exceptions) be a number below the ampacity of the circuit breaker or fuse protecting that circuit - typically 15Amps, 20Amps, or for some appliances a larger number, e.g. 30A or 40A at an electric water heater and at some central air conditioner/heat pump systems.
Of course if no electrical devices or appliances are turned "on" at an individual electrical circuit, the amps or current draw measured there should be zero.
Determining the total electrical service ampacity for a building
Ampacity ratings describe the safe capacity of an electrical system, circuit, or device, not the actual amount of electrical energy that that system, circuit, or device may happen to be carrying or using at a particular moment. For example, a #14 gauge copper electrical wire is typically rated to safely conduct 15 Amps (15A) of electrical current while a #12 copper wire is rated to safely conduct 20A of electrical current.
Determining the electrical service ampacity means answering the question "how much total electrical power or current can a building's electrical system safely use at one time?"
We discuss this procedure in detail beginning at AMPS VOLTS DETERMINATION . Practically, the total capacity of an electrical system, measured in amps, sets the limit on how many electrical devices (lights, refrigerators, air conditioners) can be run simultaneously without overloading the system.
Overloading an electrical system or overloading an individual electrical circuit should trip a circuit breaker or blow a fuse. If those safety devices are subverted, not properly installed, are defective, or malfunction, the risk is that an overheated wire or connector or device ignites a building fire.
The service ampacity at an individual residential building typically will be somewhere between 60Amps (below current minimum standards for a home in the U.S. or Canada) and 200Amps.
What's the Difference between Measuring Amps and Actual Total Electricity Usage?
When we make an amps measurement at a building we are determining the current draw for all of the electrical devices that are in operation at that moment. We are seeing the rate of electricity usage.
Only by including the length of time that a building, circuit, or device is using electricity at a given rate (and voltage) can we know the total electricity used. That's the job of electrical meters. Typically we express electrical usage rates in watts, where Watts = Volts x Amps.
See Definition of Watts and also Definition of Power Factor, Real Power for examples of actual calculations of electrical energy usage that combines volts, amps, and other terms in order to accurately describe the electricity or energy used by a light bulb, an air conditioner, or other electrical circuits or devices.
Procedure for Measuring the Actual or Momentary Amps in Use (Current Draw) At A Building
In our page top photo and in our photos below you can see Sperry's Digisnap DSA-500 snap-around digital multimeter in use measuring the amperage level on a 120V electrical circuit fed into a building from the electric meter.
The jaws of the DMM-ammeter are closed around one 120-V service entry wire in its routing between the electric meter's output lug and the electrical panel's main input connecting lug (not shown in this photo).
In the U.S. and Canada it would usually be necessary to open the electric meter enclosure to access these wires. You can see that at the moment of measurement, this 120V circuit was drawing only 1.35A. Inside we observed that the only operating devices on this circuit were some electric lights and computer equipment.
As our at left illustrates, we could have made this same measurement inside the electrical panel itself by clamping the DMM's transformer jaws around the service entry wire just above its connection to the main lug in the panel.
In a crowded electrical panel and depending on just which wires are available with least disturbance it may be tempting to try measuring amps at the main neutral wire.
For technical and accuracy reasons we do not use the neutral wire for this purpose.
Watch out: fatal electrical shock hazards are present.
Do not open an electrical meter enclosure nor electrical panel if you have not had proper safety training and/or if you do not have proper safety equipment.
Procedure for Measuring the Amps or Current Draw on an Individual Electrical Circuit
Our photo at above left illustrates measuring the current draw in amps of an individual electrical circuit in the building.
Taking care not to disturb the circuit breaker and moving other wires as little as possible (note that gray twist-on connector and electrical splice in this panel), we clamp the ammeter jaws around one 120-V wire below its connection to the circuit breaker.
Of course this measurement is meaningless if nothing is operating on the circuit being tested and has little meaning unless we know what devices that electrical circuit is powering. For example, measuring a dedicated electrical circuit for an air conditioner or refrigerator will produce varying results:
Appliance off completely - zero amps
Appliance fan only or light only operating - typically very low, possibly < 1A
Appliance compressor motor in operation - highest amperage draw
Procedure for Measuring the Amps or Current Draw by an Individual Electrical Appliance, Device, Motor, etc.
At left we are measuring the current draw in amps for the charging block of a laptop computer.
Notice that the electrical wire was split so that the clamp-on ammeter's jaws surround just one of the two electrical wires. The transformer jaws or "clamp" must surround just one of the two 120V wires supplying the electrical device.
Also notice that we did not disturb nor damage the electrical wire insulation itself - doing so is dangerous and risks equipment damage or dangerous electrical shock as we cite just above.
At the moment of our measurement this electrical device was drawing 0.29A at 120V.
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N. Srinivasan, MSEE, is a senior member of IEEE with 30 years experience in the electrical industry. Mr. Srinivasan is in Vienna VA.
Louis P. Babin generously contributed technical editing about the effects of doubling ampacity in an electrical circuit (September 2007)
ASHI Technical Journal, Vol. 2. No. 1, January 1992, "Determining Service Ampacity," Dan Friedman and Alan Carson,
ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E.,
with subsequent updates and additions to the original text ongoing to 2/19/2006. Reprints of the originals and reprints of the Journal are available from ASHI, the American Society of Home Inspectors www.ashi.com.
 Digisnap DSA-500 snap-around digital multimeter, A.W. Sperry Instruments Inc., 2150 Joshua's Path, Suite 202, Hauppage NY 11788, Tel: 800-645-5398, Email: email@example.com, Website: www.awsperry.com
 Fluke Corporation, 6920 Seaway Blvd, Everett, WA 98203, USA, PO Box 9090 Everett, Washington 98206, Tel: +1(425) 347-6100, Technical support: 1(800) 44-FLUKE (1(800) 443-5853), Website: www.fluke.com,
Fluke Europe B.V,
PO Box 1186 Eindhoven, The Netherlands, Tel: +31 (0)40 2 675 200 +31 (0)40 2 675 222, Website: www.fluke.eu
Digital 287/289 Digital Multimeter, Users Manual, retrieved 9/5/21, original source: http://assets.fluke.com/manuals/287_289_umeng0200.pdf, [copy on file as Fluke_287_289_umeng0200.pdf]
Simpson 260® Series 6XLM
Volt-Ohm-Milliammeter Instruction Manual, retrieved 9/5/2012, original source: http://www.simpsonelectric.com/uploads/File/datasheets/260-6xlm.pdf, [copy on file as Simpson_260-6xlm.pdf]
Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN. Mr. Hankey is a past chairman of the ASHI Standards Committee. Mr. Hankey has served in other ASHI professional and leadership roles. Contact Roger Hankey at: 952 829-0044 - firstname.lastname@example.org. Mr. Hankey is a frequent contributor to InspectAPedia.com.
Arlene Puentes, an ASHI member and a licensed home inspector in Kingston, NY, and has served on ASHI national committees as well as HVASHI Chapter President. Ms. Puentes can be contacted at email@example.com
"Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"How to plan and install electric wiring for homes, farms, garages, shops," Montgomery Ward Co., 83-850.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"Home Wiring Inspection," Roswell W. Ard, Rodale's New Shelter, July/August, 1985 p. 35-40.
"Evaluating Wiring in Older Minnesota Homes," Agricultural Extension Service, University of Minnesota, St. Paul, Minnesota 55108.
"Electrical Systems," A Training Manual for Home Inspectors, Alfred L. Alk, American Society of Home Inspectors (ASHI), 1987, available from ASHI. [DF NOTE: I do NOT recommend this obsolete publication, though it was cited in the original Journal article as it contains unsafe inaccuracies]
"Basic Housing Inspection," US DHEW, S352.75 U48, p.144, out of print, but is available in most state libraries.
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