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• Table of Electrical Service Entry Cable Sizes & Ampacity assists home inspectors in determining the electrical service size at buildings
• 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

SE CABLE SIZES vs AMPS - How to Inspect Electrical Service entrance cables for Ampacity, Voltage, Condition

The amperage provided by the electrical service entrance cable is a function of its materials and diameter. Often the actual cable type and size is printed right on the cable insulation. Otherwise some rough measurements of cable diameter are in order.

If, inside the panel, the inspector could see the ends of the entrance cable [Figure at left], measure metal wire diameter, and if the inspector knew the manufacturer of the cable and its specifications, a certain identification of the cable's ampacity could be made. However a safer, faster and common practice is to examine the exterior of the cable at a point outside of the electric panel. Make sure you look at both outside (mast head down to meter) and inside (from meter into service panel). They may be different! Inside the panel, stripped of ground and insulation, you may see only wires as in Figure 3. Don't confuse guides for external measurements of the whole cable with in-panel measurements of the wires themselves.

We use a plastic vernier caliper or other non metallic instrument to measure external (insulated) width of the whole cable as a reasonable guess at wire ampacity. Some inspection tool companies sell, and others give-away a plastic "wire gauge" with notches labeled to tell electrical inspectors the cable size for aluminum and copper SECs. Or you can make your own. But this method is not precise. Plastic insulated cable, for example, is often thinner in total thickness than older fabric-covered cable. Look at the table of wire dimensions in the accompanying article. Fortunately for building inspectors, even though wire thickness varies among manufacturers, you can generally find the measurement closest to one of the standard sizes and you're likely to be correct. [See sidebar, Electric Wire Sizes in this article]

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Technical Reviewers

• 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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• "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.]
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