How to Estimate the Electrical Capacity or Size of an Electrical Panel - Amps Rating

ELECTRICAL PANEL AMPACITY- 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
Questions & Answers about how to estimage the electrical panel size or capacity - amperage rating of an electrical panel
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Electrical Panel Amps: How to determine the residential electrical service panel age, type, and size or capacity in Amps & Voltage: this article explains how to estimate the electrical service size, ampacity rating, (or "electrical power" or "service amps") at a building by visual examination of the electrical service panel, main switch, and other details.

In this article series we describe how to recognize different vintages, ages, types of electrical panels and how to find the components that define the ampacity rating of the service equipment. Visual inspection and use of digital multimeters(DMMs), Volt-ohm meters (VOMs), neon testers, and electrical inspection safety are also discussed. We include photographs and sketches illustrate electrical panels, meter bases, and electric meters.

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

Electrical Panel Ampacity - Guide to Inspecting Electrical Service Panels for their Ampacity Rating

A frequently asked question at home inspection education conferences is "How do I determine the electrical service amperage or size?" This article describes inspecting the ampacity or size of the main disconnect and/or main electrical panel.

Safety concerns attend any electrical system inspection. See Safety Hazards and Safe Inspection Procedures at Residential Electric Panels.

Watch out: Do not attempt to open an electric panel without making yourself familiar with safe procedures.

Watch out: 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 Determine the Electric Service Panel Ampacity, Age, Type by Visual Inspection of Components

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.

Older 30-amp fuse panel

Older 30-amp fuse panels typically have round 15-amp or 20-amp fuses installed on both hot and neutral conductors,[Figure at left] and sometimes a separate main fuse switch.

There are safety issues with fused-neutral equipment - not discussed in this article.

Simpler 30-amp panels or sub-panels

Simpler 30-amp panels or sub-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 fuse panels

Older 60-amp fuse panels [first panel shown above] and some 100-amp panels [Figure at left] often have a single or a pair of Bakelite^TM 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.

Single-Pull-Out 100A Fused Electrical Panels

100-Amp fuse panels: 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 disconnect CB 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.

100-Amp Circuit Breaker Panels

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.

Multi-Pull-out 100-Amp Fuse Panels, Split Bus

In the multi-pullout 100-amp fuse panel shown next at left, all four of the top pullouts are mains.

The top two switch 240-volt appliances and the bottom two power the left and right hand groups of screw-in type fuses in the bottom of the panel. (The upper left-hand 240 V appliance circuit's fuse is not connected so is not in use.)

All four should be labeled mains.

This is a tougher one since, as Hansen points out, [Hansen, op. cit.] the sum of the ratings of the different disconnects may exceed the capacity of the conductors.

For example you can have a 125A service with two main disconnects, one rated at 100 amps and one rated at 50 amps. In this case we'd look at the rating of the service conductor and the service equipment (panel) for help.

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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
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  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.
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  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.
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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.

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Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd

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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