240 vs 120 Volt Service Carson Dunlop Associates Electrical Service Entry Cable Visual Inspection Can Determine Voltage Range

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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?" Our page top sketch of a 240V service entrance is provided courtesy of Carson Dunlop Associates.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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.

Readers of this article should also be sure to review SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for examining Residential Electrical Panels.

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 MastheadProbable Voltage
30 or 1120/240, 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

Articles on Building Electrical Service Wiring - SEC Cabling


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Publisher - Daniel Friedman