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

By looking at just the number and size of wires we can find the electrical service voltage and amperage delivered to a building.

While there may be other limiting factors yet to be found (such as panel or main breaker size), just by eye, no equipment, no touching, no testing, we can find this basic information.

Page top photo: an obsolete GE electric meter.

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

Can I determine the service amperage at a building simply by looking at the service entry wire or panel?

When a homeowner or renter asks "how much" electrical service is provided, they want to know the buildings's service ampacity and voltage.

Here we explain that by simply looking at a few components of the system without touching them, we can make a reasonably accurate answer to the question.

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

Is the Electrical Service 120Volts or 240Volts?

Typically, two individual 120V hot wires entering the building provide 120V for circuits connected from an individual entering wire and the neutral bus, and 240V is also available for circuits that are connected between the two incoming individual 120V circuits.

A 240V electrical service will show at least these three wires

1. First 120VAC service wire
2. Second 120VAC service wire
3. Third a service neutral or ground wire. The incoming neutral wire is grounded at a utility pole and may also serve to support the incoming hot wires.

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.

A 120V electrical serives will show just two wires

1. A 120VAC servcie wire
2. A service neutral or ground wire

Sketches above and below are provided courtesy of Carson Dunlop Associates.

The sketch beleow shows outdoor detailed components 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 for home inspectors: 1992 ASHI Standards of Practice (and later) , 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 Size or Ampacity Provided by the Service Entrance Conductors

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 just one 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.

So you'll also want to see ELECTRICAL SERVICE AMPS LIMITING FACTOR

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 Continue Reading .

...

Continue reading  at ELECTRICAL PANEL INTERIOR HAZARDS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see

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Suggested citation for this web page

VISUALLY DETERMINE AMPS & VOLTS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to ELECTRICAL INSPECTION & TESTING

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