What are the Limiting Factors That Determine the Electrical Capacity or Size at a Building?
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What are the Limiting Factors That Determine the Electrical Capacity or Size at a Building? Answers simplify the process of determining the actual electrical service capacity provided to a building
How to determine the size, capacity, or ampacity of electrical service at a building

Residential Electrical Service Amps Limitations: This article explains the Limiting Factors that determine the electrical capacity (amps) or size at a building. We describe five components to examine, the risks or shortcomings of relying on each of these individually to estimate the service amperage at a property, and how the service ampacity at a property is set by the lowest-capacity item among the five things we examine.

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One of the most frequently asked questions at ASHI Education Seminars and Conferences is "How do I determine the service amperage?" This article series includes photographs and sketches that illustrate different ages and capacities of residential electrical panels, meter bases, and electric meters.

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AMPACITY - the LIMITING FACTOR - Summary: What is the limiting factor that determines the actual electrical service ampacity at a building?

Remember you're looking for the limiting factor in determining ampacity. Finding the electrical component in this list which has the smallest capacity means that you have found the "weak link in the chain" of components bringing electrical service to the building.

Particularly when inspecting older properties, where there is the chance that someone has upgraded some but not all of the components in this list, there may be an inconsistency such as the installation of a larger electrical panel without upgrading the main disconnect.

Electrical service entry cable or "SEC" size and amperage rating (we do not generally consider the overhead or service lateral wiring)
Electric meter (questionable but may assist, especially if the item's generation, age, and obsolescence can be determined)
Electric meter base (questionable but may assist, especially if the item's generation, age, and obsolescence can be determined)
Main electrical power switch fuse/circuit breaker ampacity
Electrical panel rated ampacity (and bus design may support only 120V)

Inspectors attempting to determine (or estimate) the ampacity of the electrical service at a building should report their determination.

If conditions limit your inspection, such as in the photo at left, you can't identify the capacity of all the components described in the text above. In that case you should report which determining components were or were not visible or identifiable.

We don't hesitate to tell our clients what the electrical service size (ampacity) appears to be from what we could see . But we make quite clear the difference between what we know and what we think.

Readers of this article should also be sure to review Safety Hazards and Safe Electrical Inspection Procedures for examining Residential Electrical Panels. T

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 www.ashi.com

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AMPS VOLTS DETERMINATION
DEFINE AMPS VOLTS WATTS
VISUALLY DETERMINE AMPS & VOLTS
VOLTAGE at the SEC
VOLTAGE MEASUREMENT EQUIPMENT
LIMITING FACTOR sets AMPS
SE CABLE SIZES vs AMPS
UNDERGROUND SERVICE LATERALS
ALUMINUM SECs & WIRING
MAIN DISCONNECT
MAIN DISCONNECT AMPACITY
PANEL AMPACITY
ELECTRIC METERS & METER BASES
  ELECTRIC METER BASES
  ELECTRIC METERS
  INSPECTING METERS & BASES
AMPACITY - the LIMITING FACTOR
APPLIANCE EFFICIENCY RATINGS

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.
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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AMPACITY - the LIMITING FACTOR - Summary: What is the limiting factor that determines the actual electrical service ampacity at a building?

Questions & Answers regarding this article

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Technical Reviewers & References

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