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How to Find Out the Electrical Service Ampacity & Voltage by Visual Inspection ElectricAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Introduction to visual inspection of the building electrical service to determine its capacity
• 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

Determining the Electrical Service Ampacity and Voltage at a building by visual inspection

When a homeowner or client asks you "how much" electrical service is provided s/he needs to know the service ampacity and voltage. In the US and Canada service voltages are commonly (nominally, that is, not exactly) 240 volts at the electrical panel, a system which supports both 120V and 240V circuits in the building. Typically, two 120V hot wires entering the building provide 120V for circuits connected from an individual entering wire and the neutral bus, and 240V for circuits connected between the two incoming individual 120V circuits.

Older building electrical services, typically those installed before 1940, may support only 120V and as little as 30-amps. We still find these services on small cabins and on a few old residences in the U.S., Canada, and Mexico and elsewhere. For normal residential use, excluding small vacation cottages, a 120V or 30A service would most likely be considered obsolete and has been since 1959 (Hansen). [NOTE: 1992 ASHI Standards of Practice, 8.2.1. While ASHI and other building inspection standards are updated periodically, the basic requirements addressed in this article should not change.]

Electrical inspections are inherently dangerous to the inspector and potentially dangerous to inspection clients and building occupants. People conducting these procedures must first be familiar with safe electrical practices before attempting any inspection of electrical equipment. Such familiarity is essential to protect all parties concerned. Inspectors: proceed at your own risk. 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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• * 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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