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How to Estimate the Electrical Capacity or Size of an Electrical Panel ElectricAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• How to Estimate the electrical capacity or size of an electrical panel by visual inspection of the panel and its components
• How to determine the size, capacity, or ampacity of electrical service at a building

This article 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 discussed. Photographs and sketches illustrate electrical panels, meter bases, and electric meters. One of the most frequently asked questions at ASHI Education Seminars and Conferences is "How do I determine the service amperage?"

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. But there are some pitfalls, and the process itself is dangerous. Readers of this article should also be sure to review Safety Hazards and Safe Electrical Inspection Procedures for examining Residential Electrical Panels. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links 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.

PANEL AMPACITY - Inspecting Electrical Service Panels for Ampacity

Many safety concerns attend this step. See Safety Hazards and Safe Inspection Procedures at Residential Electric Panels. More safety guidelines are in other publications. Do not attempt to open an electric panel without making yourself familiar with safe procedures.

Do not open or even touch electrical equipment if you find dangerous conditions, or simply uncertain conditions which give cause for alarm. If you elect not to examine a component, indicate to your client that you did not inspect the component, explain why, and recommend appropriate steps to make access and inspection safe and possible. Document this information in your report.

HOW TO ESTIMATE - Electric Service Panel Ampacity

We look for a tag (usually paper) or embossed rating (on fuse pull outs) on the panel itself which often includes the amperage rating of the panel. This information is usually present in newer panels on a panel side, or on the panel cover.

Home inspectors or electrical inspectors who touch electrical wires or move them in order to better read the data tag are performing a dangerous act.

Actual dimensions of an electric panel are not a reliable determinant of ampacity. For example, many larger panels can be fitted with a variety of bus-bar and main switch assemblies of varying ampacity.

Simpler 30-amp panels like the smaller four-fuse panel shown at immediate left are usually auxiliary rather than main service panels in most buildings. An exception which the author's have found is that this panel was used as a main service for small lake and other remote cottages. Where such buildings have in more recent years had additional electrical circuits installed, for use of electric heaters, for example, such panels are often overcrowded and very often the individual circuits are over fused - a fire hazard.

Older 60-amp panels [first panel shown above] and some 100-amp panels [Figure at left] often have a single or a pair of BakeliteTM fuse holders, one marked "main," the other marked "range." In typical installations, the "main" fuse pullout shuts off power in the panel except the range circuit. This main should contain 60-amp or smaller fuses. Both main and "range" pullouts must be removed to cut all power. The example at left used a 60A fuse supplying the lighting circuits (fuses in row along the panel bottom) and a 40A 240V appliance circuit is supplied by the "range" fuse pair.

The next panel shown at left is a 100-amp (or larger) fuse panel using a main fuse holder centered in the top of the panel. The two fuse holders for two 240V circuits at the panel bottom are not mains and were often used to supply a range circuit and clothes dryer circuit, or range and sub panel circuits. 125-amp CB panels are often fitted with a 100-amp main disconnectCB or fuse-pair. In this case the electrical inspector should ignore the larger equipment panel size (perhaps indicated by an inside-panel label) and instead s/he should rate the electrical service ampacity based on the main CB or fuse-pair.

Based only on apparent panel size and provision for 24 circuits, this larger panel at left, showing a modern main CB at the top and two 12-breaker columns is at least a 100 Amp box, maybe 150 Amps. Better check the service conductors, main CB, and panel label.

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Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our 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.
• Daniel Friedman - InspectAPedia.com ^TM Website Author/Editor
• Douglas Hansen, Robert Stead. Mark Cramer. Photographs: Daniel Friedman.
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
• 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)

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More expert information on Inspecting Electrical Equipment and Electrical Safety for Inspectors

More Information on Electrical Inspections & Building Diagnostic Inspections and Repairs

• * 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 Estimate 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
• "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.
• Rust and Corrosion in Electrical Panels, A Study and Report on Frequency and Cause for Electrical and Home Inspectors at Residential Electric Panels


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