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

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

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?

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

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
VOLTAGE MEASUREMENT EQUIPMENT - 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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Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

• 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.
• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Daniel Friedman - InspectAPedia^® Website Author/Editor
• 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.

• 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)
• * Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
• 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.
• Ampacity of an Electrical Service: How to determine the electrical service size or ampacity entering a building
• 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.
• Lightning Strike Risk Assessment, Protection Systems & Services
• Multi-wire branch circuit inspection and defects
• 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.
• "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.
• Electrical System & Wiring Hazard Inspection, Detection, Cause, Remedy, Prevention - Main Electrical Page
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