VOM in use measuring live voltage (C) Daniel Friedman

How to Measure the Electrical Voltage Entering a Building using a VOM, DMM, or Voltage Detector Pen
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Basic introduction to use of simple instruments like neon testers, voltage detector pens, to detect the presence of live electrical voltage
Use of Volt-Ohm meters VOM's, Digital multi meters DMM's or multimeters to measure the electrical voltage and amperage of an electrical service or at individual electrical circuits in a building
How to measure the size, capacity, or ampacity of electrical service at a building

This article describes how to safely use digital multimeters(DMMs), Volt-ohm meters (VOMs), neon testers to check for live electrical circuits and to measure electrical system voltage and amperage levels.

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One of the most frequently asked questions at home inspection education conferences is "How do I determine the service amperage?" Photographs and sketches in this article series also illustrate electrical panels, meter bases, and electric meters.

Readers of this article should also be sure to review Safety Hazards and Safe Electrical Inspection Procedures for examining Residential Electrical Panels.

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VOLTAGE by EQUIPMENT - Determining Electrical Voltage (240V/120V) using a neon tester, a VOM, DMM, Volt Meter, or Voltage Detector pen.

Instruments as simple as the neon-tester shown here or a digital multimeter (DMM), or an analog volt meter (volt-ohm meter) such as those made by Triplett, Fluke, and Simpson can be used to measure the voltage level between the two power feed lines to confirm the presence of 240 Volt service. (Some versions of this equipment can also be configured to measure amperage or amps).

Using a DMM, analog volt meter, or a neon tester, voltage between the two apparently "hot" wires in the panel will confirm 240-volt service. If only one cable is "hot" (testing cable to ground) it is a 120 V system. If both cables are "hot" to ground but there is no voltage between them it is a 120V system which has been "split" to look like 240V but it is not 240V. A volt meter is required to test for other voltage levels.

Safety Warning: The ASHI Standards of Practice and other 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 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 Hazards and Safe Inspection Procedures at Residential Electric Panels.

Here are photographs of the basic types of electrical test equipment useful for a building inspector, electrical inspector, contractor, or homeowner

If you're unsure about the definitions of amps, volts, ohms, resistance, watts, etc. See DEFINITIONS of ELECTRICAL TERMS.

A voltage detector of voltage sensor detects the presence of live AC voltage (some equipment also detects DC voltage), with a sensitivity typically between 5 and 1000 Volts. This equipment can sense the presence of live electrical devices or wiring without touching the device or wire.

Shown here (left) is the Greenlee® GT-16 adjustable voltage detector. Greenlee and other manufacturers recommend that you test the operation of the voltage detector on a known live circuit both before and after using it to test for the presence of electrical voltage at a wire or device. This device uses an AAA battery and has a sensitivity adjustment (the yellow dial in the center of the green patch on the device).

Some voltage sensors such as the FCB Voltage Pen operate without requiring a battery.

An ammeter measures electrical current. Our photo at left shows a multi-meter that can measure amps as well as serve ad a VOM - a volt-ohm volt meter.
A voltmeter measures the potential difference (voltage) between two points
An ohmmeter measures resistance. An ohm meter measures resistance in a circuit or on a component and is not normally connected to a circuit carrying live voltage.

A multimeter combines these functions into one piece of equipment and is often called a VOM or volt-ohm meter. Our little VOM shown at left is being used to check for voltage at an electrical receptacle. If you click and enlarge the photo you can see the meter reading.
A neon tester simply measures for the presence of live electrical power as we demonstrated above.
A continuity tester is a simple substitute for a more sophisticated ohm meter, and is used simply to confirm that there is electrical continuity on a circuit, that is, that it is not interrupted by a switch or open connection.

Some simple ohm-meters or multimeters include a switch position that emits a tone if there is good electrical conductivity (low resistance) between the two points being touched by the probes.

Analog VOMs: A VOM or volt-ohm meter may also be found in an analog form such as our favorite and now collector-item little Jensen VOM that we've used for nearly 30 years. Here our photo shows the Jensen analog VOM with its probes attached but not connected to anything.

In 30 years of use we needed one repair of this instrument - an internal fuse which we couldn't find locally. On contacting Jensen Corporation, the company kindly sent us a few spare fuses. The total repair bill? $0.00.

Safety Warnings for Electrical Inspectors & Home Inspectors inspecting electrical services, wiring, or equipment

See Safety Hazards and Safe Inspection Procedures at Residential Electric Panels

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AMPS VOLTS DETERMINATION
DEFINE AMPS VOLTS WATTS
VISUALLY DETERMINE AMPS & VOLTS
VOLTAGE at the SEC
VOLTAGE MEASUREMENT EQUIPMENT
LIMITING FACTOR sets AMPS
SE CABLE SIZES vs AMPS
UNDERGROUND SERVICE LATERALS
ALUMINUM SECs & WIRING
MAIN DISCONNECT
MAIN DISCONNECT AMPACITY
PANEL AMPACITY
ELECTRIC METERS & METER BASES
  ELECTRIC METER BASES
  ELECTRIC METERS
  INSPECTING METERS & BASES
AMPACITY - the LIMITING FACTOR

The Original Authors: Alan Carson is an ASHI Member, national home inspection educator, author and building failures researcher in Toronto, Ontario. Daniel Friedman, an original author of this article and the editor and producer of InspectAPedia where this article now appears is an ASHI Member, first ASHI Technical Committee chairman, editor and publisher of the ASHI Technical Journal, licensed home inspector, educator, and building failures researcher in Poughkeepsie, NY. Robert Klewitz is a licensed professional engineer, a professional home inspector, an ASHI Member, and has served on the ASHI Technical Committee as well as in other ASHI activities. His practice is in Issaquah, WA.
Daniel Friedman - InspectAPedia^® Website Author/Editor
Douglas Hansen, Robert Stead. Mark Cramer. Photographs: Daniel Friedman.
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)

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Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Recommended books on electrical inspection, electrical wiring, electrical problem diagnosis, and electrical repair can be found in the Electrical Books section of the InspectAPedia Bookstore. (courtesy of Amazon.com)
* Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
Aluminum Wiring Information Website Aluminum Electrical Wiring Hazards and Repairs: in-depth authoritative info, photos, documents including selection of proper vs. ineffective repair methods. E.g.: Ideal 65 "Twister" purple connector fails in field and lab testing with aluminum wire.
Ampacity of an Electrical Service: How to determine the electrical service size or ampacity entering a building
Circuit Breaker, a bad one fails to trip failure at aluminum bus-to-circuit breaker connection - field report and photographs
Electrical Panels, How to Inspect in buildings, safety for electrical inspectors, electrical panel, fusing, wiring defects, defective products. Inspection Class Presentation
Federal Pacific Electric (FPE) Stab-Lok Circuit Breaker Panel Hazards Website - Latent fire hazards, in-depth authoritative research, documents, advice on Stab-Lok electric panel and circuit breaker failures and what to do when this equipment is found in buildings.
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Rust and Corrosion in Electrical Panels, A Study and Report on Frequency and Cause for Electrical and Home Inspectors at Residential Electric Panels
"Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
"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.
"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.
"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.
Electrical System & Wiring Hazard Inspection, Detection, Cause, Remedy, Prevention - Main Electrical Page
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VOLTAGE by EQUIPMENT - Determining Electrical Voltage (240V/120V) using a neon tester, a VOM, DMM, Volt Meter, or Voltage Detector pen.

Here are photographs of the basic types of electrical test equipment useful for a building inspector, electrical inspector, contractor, or homeowner

Safety Warnings for Electrical Inspectors & Home Inspectors inspecting electrical services, wiring, or equipment

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair