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How to Measure the Electrical Voltage Entering a Building using a VOM, DMM, etc ElectricAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Basic introduction to use of simple instruments like neon testers, VOM's, DMM's or multimeters to measure electrical voltage at a building
• How to determine the size, capacity, or ampacity of electrical service at a building

This article explains how to estimate the electrical service size, (or "electrical power" or "service amps") at a building by visual examination of the service entry cables, electric meter and meter base, electrical service panel, main switch, and other details. Visual inspection and use of digital multimeters(DMMs), Volt-ohm meters (VOMs), neon testers, and electrical inspection safety are discussed. Photographs and sketches illustrate electrical panels, meter bases, and electric meters. One of the most frequently asked questions at ASHI Education Seminars and Conferences is "How do I determine the service amperage?"

It's not as difficult as one may think to get a reasonable handle on the electrical service capacity at a building without sophisticated analysis. But there are some pitfalls, and the process itself is dangerous. The articles from which much of this online material originated appeared first in the ASHI Technical Journal, Vol. 2. No. 1, January 1992, "Determining Service Ampacity," Dan Friedman and Alan Carson, and the 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

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

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 some basic types of electrical test equipment defined:

• An ammeter measures current
• 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.
• A neon tester simply measures for the presence of live electrical power.
• 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.

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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Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• 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.com ^TM Website Author/Editor
• Douglas Hansen, Robert Stead. Mark Cramer. Photographs: Daniel Friedman.
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
• 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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More expert information on Inspecting Electrical Equipment and Electrical Safety for Inspectors

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• * Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
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• "Electrical System Inspection Basics," Richard C. Wolcott, ASHI 8th Annual Education Conference, Boston 1985.
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