Electrical Service Entry Cable Visual Inspection Can Determine Voltage Range InspectAPedia® -
How to inspect the electrical service entry cables 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.
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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?" Our page top sketch of a 240V service entrance is provided courtesy of Carson Dunlop Associates.
VOLTAGE at the SEC - How to figure out Electrical Service Voltage by visual examination at the service entry
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. This article explains the visual examination of overhead electrical wires at the service entry. This procedure pertains
to overhead wires, not underground conduit. Examination is made at
the point of connection of the ELECTRICAL SERVICE DROP to the service
conductors.
In lay terms, the service conductors are also called
the service entry cable or SEC. Underground wiring up to the building
is called a service lateral. A 240V electrical service [three-wire figure at left] will include
three wires connected to the building - the two 120V "hot"
legs which together provide 240V, and a third grounded conductor.
A 120V service [second, two wire figure at left] will have only two wires,
a 120V power line and a grounded line or neutral.
However simple visual examination from the ground is not a certain proof of voltage available
in the building. In a few unusual circumstances, all three wires may
be present but one of them may be disconnected at the utility pole
or masthead. Such "mistakes" occur during temporary hookups
in new construction or renovation.
Could there be 240V service to a building with only two wires? Yes but not normally in the U.S.
Some Canadian rural properties served by long private wires may have
only two overhead conductors on a 240 volt service. For these systems ground connections are made only at the building, not back to the
utility company's lines. In the U.S. this would be unusual but be alert for it, especially at rural properties.
Table 1. Service Voltage Configurations
Number of Visible Wires at Masthead
Probable Voltage
Insulated
Uninsulated-Ground
1
1
120
2
0
120/240
2
1
120/240
3
0 or 1
120/240, 120/208 3-phase [1]
4
.
3 phase, need to measure volts [2]
Notes to Table:
1. This could also be a delta 3-phase with 3 120V legs; 3 phase power not common at residential properties.
2. 208 Volt service may be present on 3rd red wire.
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
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Additional technical contributors & reference sources for this article are listed below.
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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.
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)
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
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)
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.
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.
"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.
"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.
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