How to Find Out the Electrical Service Ampacity & Voltage by Visual Inspection

AMPS & VOLTS, DETERMINE VISUALLY - introduction to visual inspection of the building electrical service to determine its capacity
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How to determine the electrical service capacity at a residential property by visual inspection alone: introduction.

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

"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.

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 (C) 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 (C) 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 Hazards and Safe Inspection Procedures at Residential Electric Panels.

We have divided this topic into the following articles.

AMPS VOLTS DETERMINATION
DEFINE AMPS VOLTS WATTS - basic electrical terms are defined: Amps, Volts, Watts
VISUALLY DETERMINE AMPS & VOLTS - Introduction to voltage and ampacity of an electrical service by looking
VOLTAGE at the SEC - how the service entry cable size limits the ampacity available at a building
VOLTS / AMPS MEASUREMENT EQUIP - how to use Volt-Ohm meters VOM's, Digital multi meters DMM's or multimeters
LIMITING FACTOR sets AMPS - explanation of limiting factors on the electrical amps available at a property
SE CABLE SIZES vs AMPS - table of electrical service entry cable sizes and ampacity ratings
UNDERGROUND SERVICE LATERALS - table of underground electrical service entry cable sizes and ampacity ratings
ALUMINUM SECs & WIRING - special concerns with aluminum service entry cables
MAIN DISCONNECT - how to inspect the main electrical disconnect switch
MAIN DISCONNECT AMPACITY - how the main electrical disconnect switch limits service ampacity
PANEL AMPACITY - how to recognize age, type and volts/amps capacity of older electrical service panels
ELECTRIC METERS & METER BASES - recognize age, type and volts/amps capacity of older electrical meters
ELECTRIC METER BASES
ELECTRIC METERS
INSPECTING METERS & BASES
AMPACITY - the LIMITING FACTOR - list of five key items to inspect when determining the electrical service size

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

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.
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.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
  Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
  Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
  Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
  Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com 11/06
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, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

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)
Rust and Corrosion in Electrical Panels, A Study and Report on Frequency and Cause for Electrical and Home Inspectors at Residential Electric Panels
"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.
"Simplified Electrical Wiring," Sears, Roebuck and Co., 15705 (F5428) Rev. 4-77 1977 [Lots of sketches of older-type service panels.]
"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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