How to Estimate the Electrical Capacity or Size of an Electrical Panel - Amps Rating InspectAPedia® -
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
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.
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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.
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.
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 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.
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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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.
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)
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.
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.
"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.
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