What are the Limiting Factors That Determine the Electrical Capacity or Size at a Building? InspectAPedia® -
What are the Limiting Factors That Determine the Electrical Capacity or Size at a Building? Answers simplify the process of determining the actual electrical service capacity provided to a building
How to determine the size, capacity, or ampacity of electrical service at a building
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This article explains the Limiting Factors That Determine the Electrical Capacity or Size at a Building? 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
AMPACITY - the LIMITING FACTOR - Summary: What is the limiting factor that determines the actual electrical service ampacity at a building?
Remember you're looking for the limiting factor in determining
ampacity. Finding the electrical component in this list which has the smallest capacity means that you have found the "weak link in the chain"
of components bringing electrical service to the building. Particularly when inspecting older properties, where there is the chance that someone has
upgraded some but not all of the components in this list, there may be an inconsistency such as the installation of a larger electrical panel without
upgrading the main disconnect.
Electrical service entry cable or "SEC" size and amperage rating (we do not generally consider the overhead or service lateral wiring)
Electric meter (questionable but may assist, especially if the item's generation, age, and obsolescence can be determined)
Electric meter base (questionable but may assist, especially if the item's generation, age, and obsolescence can be determined)
Main electrical power switch fuse/circuit breaker ampacity
Electrical panel rated ampacity (and bus design may support only 120V)
Inspectors attempting to determine (or estimate) the ampacity of the electrical service at a building should report their
determination.
If conditions limit your inspection, such as in the photo at left, you can't identify the capacity of all the components described
in the text above. In that case you should report which determining components were or were not visible or identifiable.
We don't hesitate to tell our clients what the electrical service size (ampacity) appears to be from what we could see. But we make quite clear the difference between what we know and what we think.
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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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