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• 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

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. The articles from which much of this online material originated appeared first in the 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

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

We look for an indication right on the 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. Look at the fuse/breaker for this data. 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).

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.

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.

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.

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.

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Technical Reviewers

• 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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