How to Inspect the Size of the Main Electrical Disconnect, Fuse, or Breaker

AMPACITY, MAIN DISCONNECT - How to Inspect the Main Electrical Disconnect, Fuse, or Breaker as part of determining the capacity of electrical service at a building; How to determine the size, capacity, or ampacity of electrical service at a building.
MAIN DISCONNECT - separate article
Questions & Answers about how to inspect the main electrical disconnect switch or fuse
References

Click to Show or Hide Related Topics

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Electrical Service Main Disconnect or Main Breaker: This article explains how to estimate the electrical service size, (or "electrical power" or "service amps") at a building by visual examination of the main electrical service disconnect switch, and related details. In this article series on determining the ampacity and voltage provided by different residential electrical equipment, we also 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.

MAIN DISCONNECT AMPACITY - How to find the Main electrical service disconnect ampacity determination

In determining the service amperage at a property a key component to examine is the main disconnect switch such as we illustrate at page top and at left, thanks to Carson Dunlop Associates' sketch.

We look for an amperage indication right on the main disconnect switch. If no markings are present, document that observation.

Markings in or on the panel box about ampacity indicate what current limiting device (such as a fuse) size could or should be installed, not what fuse or breaker size is actually present.

Main service disconnect switch size (C) Carson Dunlop Assoc

Look at the main electrical disconnect switch fuse/breaker for this ampacity number.

Beware: I [DF] have found buildings with mismatched SEC and main panel switch, with errors in both directions: in one instance a new 100A SEC was installed but connected to an old 60A service panel (safe but not delivering 100A to the building). In a second case, a new 150A panel was installed but connected to an old 60A SEC (unsafe)

Split Bus Electrical Panels: Multiple Main Disconnects

Watch out: some electrical panels, especially models using a split-bus design may not have a single main disconnect switch in the panel. There may be multiple "main" disconnects in the panel, usually grouped together.

Our Federal Pacific Electric panel (photo at left) appears to be one of these models [click to see an enlarged detailed view of this panel]. By its internal label (not shown) this panel was rated for 200 Amps.

Several of these switches may power individual high-ampacity electrical circuits in the building (air conditioner, electric range, electric water heater, for example) and one will power the lower bus in the panel to which branch circuits are wired for lighting and electrical receptacles.

Also check for a separate individual main disconnect switch between the panel and the electric meter.

Also see FEDERAL PACIFIC FPE HAZARDS

On some fuse systems the main fuses, usually a cartridge type, may be visible and the fuse ratings may be read on the fuse itself. [Figure at left] There are varying opinions among inspectors about pulling out a fuse block to read a hidden fuse value.

Mark Cramer reminds us that You can usually see the color of the fuse through the little hole. Also note that you generally can't put a fuse that's too large into a block. For example, you can't put a 150 amp fuse in a 60 amp block.

However I have seen smaller fuses inserted in larger-fuse blocks using an adapter, and I've seen copper tubing and copper pipe inserted where a fuse belonged!

So if you can't see the fuse, it's risky to assume the right thing is in place. In Canadian and in some US panels fuses can be changed individually and may be more visible.

Rusty unsafe fuse panel (C) Daniel Friedman

In an occupied building we do not pull a fuse or shut off any component without permission.

While these devices are designed to be pulled by the homeowner and thus fall under ASHI's definition of "normal user or operator controls," we've seen and had reports of accidents including broken fuse pullout blocks, arcing, loss of power, and angry owners whose computers or kidney dialysis machines were in operation.

And if we find a horrible rusty fuse or breaker panel such as the fuse panel shown at left, we would be reluctant to handle it - this equipment is unsafe and likely to be easily pushed "over the edge" into collapse simply by touching it.

Main disconnect switch amperage (C) Carson Dunlop Assoc

We do pull fuses in unoccupied buildings and may pull fuses in an occupied building with permission, provided in both cases our visual inspection of the equipment does not reveal any obvious unsafe condition.

On the occasion when we pulled a fuse which disintegrated all parties agreed that it was better to discover the need for that repair now than in an emergency when power was needed.

If you find a fuse smaller than the maximum permitted by the conductors and panel equipment you should inform your client. But this is not a defect any more than it is unsafe to screw an 60-watt light bulb into a light designed to permit 100 watt bulbs.

Watch out: the opposite is not OK - a fuse that is rated at larger capacity than permitted for the wire size, such as a 30Amp fuse on a #14 wire branch circuit, would be unsafe. #14 copper wire is rated for a 15-Amp circuit and should be protected by a 15-Amp fuse or circuit breaker.

If you cannot see the actual fuse, you may be able to see a rating on the fuse holder itself. However be warned that there are methods (not recommended) for modifying fuse holders to carry fuses other than those intended. You may even find pieces of copper pipe used as "jumpers" to replace the original fuses. These modifications are unsafe. If you do not pull the fuse and cannot see the ampacity, report this limitation to your client.

...

Frequently Asked Questions (FAQs)

...

Ask a Question or Search InspectApedia

Questions & answers or comments about how to inspect the main electrical disconnect switch or fuse.

Ask a Question or Enter Search Terms in the InspectApedia search box just below.

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
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
  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.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
  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.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
  Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
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.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. 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.

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.
...


	InspectAPedia^®	InspectAPedia
	Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair	Ask a Question or Search InspectAPedia

Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair

Ask a Question or Search InspectAPedia