Antiquated electric meter (C) Daniel FriedmanHow to Find Out the Electrical Service Ampacity & Voltage by Visual Inspection

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How to determine the electrical service capacity at a residential property by visual inspection alone: introduction.

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

Fuse panel (C) Daniel Friedman

"How do I determine the service amperage at a building?" This article series 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 also discussed, and we provide pictures and sketches that illustrate electrical panels, meter bases, and electric meters.

[Click to enlarge any image]

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.

The electrical masthead sketch at left, courtesy of Carson Dunlop Associates, shows where this electrical inspection starts - outside at the electric masthead.

Is the Electrical Service 120Volts or 240Volts?

240 vs 120 Volt Service Carson Dunlop Associates

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.

Sketch at left, courtesy of Carson Dunlop Associates, shows outdoor details of a 240V electrical service.

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.]

What is the Electrical Service Ampacity Provided by the Service Entrance Conductors

Determine service size at SEC Carson Dunlop Associates

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. As Carson Dunlop Associates sketch (left) illustrates, the electrical capacity that is provided by the service entrance conductors is determined by checking the wire size where they wires are accessible.

Keep in mind that while the SEC wires are a limiting factor in setting the ampacity and voltage available at a building, other electrical components (main breaker and panel size, for example) may set lower limits.

Also keep in mind that when looking at SEC wire sizes, we refer to the wiring from the masthead down to the electrical meter and the SEC wires between the electrical meter and the service disconnect or main circuit breaker or fuse. We do not usually inspect nor consider the diameter of the overhead or underground feeders coming to the building during a home inspection.

Watch out: inspecting and/or touching electrical components is inherently dangerous and there is risk of shock or death by electrocution. 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 for ELECTRICAL INSPECTORS at Residential Electric Panels.

We have divided the topic of determining the ampacity of electrical service at a building into the articles listed below at More Reading.

Articles on Determination of Ampacity & Voltage at Building Electrical Services


Continue reading at ELECTRICAL PANEL INTERIOR HAZARDS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see ELECTRICAL INSPECTOR SAFETY PROCEDURES important basic safety procedures, clothing, and equipment for home inspectors and electrical inspectors.

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