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What limits the electrical service ampacity & service voltage entering a building? InspectAPedia®  -     What limits the electrical service ampacity and service voltage entering a building? A list of limiting factors useful when estimating building electrical service capacity How to determine the size, capacity, or ampacity of electrical service at a building

What determines the actual limits of available amperage at a residential property?

Readers should also be sure to see AMPACITY - the LIMITING FACTOR. 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.

One of the most frequently asked basic electrical questions is "How do I determine the service amperage?"

© Copyright 2012 InspectAPedia.com, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website.

LIMITING FACTOR sets AMPS - Checking the Electrical Service amperage - what is the limiting factor?

The safe and proper service amperage available at a property is set by the smallest of: the service conductors, the main disconnect fuse or switch, or the rated capacity of the electric panel itself. The inspector should consider all three of these and report any inconsistencies among them.

Sketch at left, courtesy of Carson Dunlop Associates, illustrates a definition of amperage or current flow. Details are at DEFINITIONS of ELECTRICAL TERMS

The main fuse/ circuit breaker (CB) is the only component which actively limits amperage at a property by shutting off loads drawing more than the main fuse rating. (Undersized conductors or service panels, if switched by a mismatched and oversized fuse or breaker, will permit more current to flow than their design intended, and can then overheat causing a risk of fire.)

The main breakers or fuses are permitted to be smaller (have lower overcurrent protection) than the capability of the service equipment (panel) and conductors (entrance cable). For example, a 2AWG copper service conductor (rated for 125 Amps, or in Canada for 120 Amps) to a panel using a 100-Amp main (service disconnect) is not a defect.["Residential Electrical Inspection," Douglas Hansen, ITA class material, Ver. 6.1 Oct. 1991, Los Altos, CA] You may want to point out to your client that such a service can be upgraded to higher amperage simply by installing larger main fuses/breakers.

Note that the main disconnect may be a fuse, breaker, or switch which is physically separate from the distribution panel. This is more common in older equipment installations and in very large commercial high-amperage installations.

Watch out: for a new higher amps-rated electrical panel connected to old, smaller-capacity service entry cables. A common defect found in upgrades of older equipment is the installation of a new 100-Amp panel and main breaker while failing to replace the old 60-amp service entry cables. This is either incomplete work or work by an untrained person.

Keep in mind that by service entry cables we're referring to the cables from the mast head (point of attachment of overhead wires to the building) down to the meter and from meter into the main switch or panel. Overhead wires, being cooled by open air, may be of a smaller diameter determined to be safe by the utility company for open air use.

Electrical Services providing less than 100 Amps are found in older properties but for a detached single-family dwelling these are considered obsolete and are a financing issue with some US lenders.[NOTE: NEC-230-42(b,1) and some FHA, VA, and private Banks in some states.]

Electrical services of 60 Amp capacity are still allowed in at least some Canadian areas and in the US for loads such as individual apartments. Inspectors should not write up 60-Amp services as a defect without first checking these points. In cabins or similar vacation structures in the U.S. and Canada, and in buildings in some other countries such as Mexico, much smaller amperage services are often brought into the building.

Home Inspectors should make clear to all parties that generally, electrical defects are unsafe conditions, can be quite dangerous, and deserve prompt attention.

One exception: an undersized service, if it is properly fused, will be inconvenient but is not itself inherently unsafe. For such systems the safety and fire risks come more often from the temptation of frustrated owners to subvert or bypass the safety devices the classic "penny in the fuse base" problem.

See Safety Hazards and Safe Electrical Inspection Procedures for examining Residential Electrical Panels.

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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.

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
AMPACITY - the LIMITING FACTOR

• 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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