How to Estimate the Electrical Capacity or Size of an Electrical Panel 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
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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?"
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.
Older 30-amp 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 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 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.
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.
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.
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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