The articles from which some 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
Readers of this article should also be sure to review Safety Hazards and Safe Electrical Inspection Procedures for examining Residential Electrical Panels.Also see the US DOE publication "How to Read Residential Electric and Natural Gas Meters".
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This article discusses the visual inspection of electrical meters and 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. We also discuss electrical arc burns at the meter base.
Visual inspection of the electric meter 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.
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ELECTRIC METERS & METER BASES - Inspecting Electric Meters & Meter Bases - Another Consideration When Determining Service Ampacity
Notice: recent discussion among various experts and one of the original reviewers (D.H.) raises question about the accuracy of estimating service size in volts or amps by inspecting modern electrical meters. While some of the cases illustrated here remain accurate, these methods may be unreliable when examining modern electrical meters and meter bases. This subtopic remains under review.
The photos and text below will assist in identifying a variety of electrical meters and service panels of various ages and capacities.
An article in the Winter, 1992 ASHI Technical Journal discussed the procedure and considerations for inspectors to determine the service ampacity of an electrical system. That article indicated that the safe and proper service amperage available at a property is set by the smallest of the service entrance conductor, the main disconnect fuse or circuit breaker, and the rated capacity of the panel itself. These are also issues of concern for the electrical contractor installing the service. ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E.
The concerns of inspectors must be broad in scope. Inspectors must look at the entire system, some of which may be dictated by the electric utility, and may not be a concern of the wireman or National Electrical Code. [National Electrical Code, article 90-2(b)(5)]
Because of this, there is one additional item that must be considered when determining service ampacity. This item is the rating of the electrical meter base and the meter itself. Many times there will be no sure way of determining this rating on a visual inspection, since the rating sticker would typically be inside the meter base and would require pulling the meter off to see it. However, there are some guidelines and "rules of thumb" that can be used.
Warning: Looking at meter bases and meters to guess at ampacity is questioned by some experts as too unreliable.
In 1976, an Underwriters Laboratory (UL) ruling required that all new meter bases be "continuous rated for 200 Amps," otherwise only 70% of the actual rating would be considered usable, (i.e., a 200 Amp meter base that was not "continuous rated" could only be used on a 160 Amp or smaller service). Inspectors can probably assume that a rectangular meter base is compatible with the system it is serving, unless a major change or a new panel has been installed.
The meter in our photo at left is obsolete.
Square meter bases such as the unit being pointed to with great caution by an ASHI Headquarters staff member on a field trip in the photo at below-left were normally rated at 100 Amps. Later generations of them were sometimes rated at 125 Amps.
Older electrical systems that were installed 30 or more years ago may be served by a square meter base with a round meter mounted on top of the square base as seen in the next photo at below-center - often called an "A-base" meter. Still older systems used the round meter base shown at below right. Round meter bases originally were rated for 60 Amps when installed 50 or more years ago. (Later generations of round meter bases were rated at 100 Amps.)
These meters can have a NMC main service wire running to and from them, or the wire may be installed in conduit. [Click any image for a larger, more detailed view.]
Most modern electrical meters for single family homes have the designation "CL200" somewhere on their face, which indicates they are rated for up to a 200 Amp service. The CL200 rating may not limit these to 200 amps. Cramer reports finding larger services with CL200 meter bases.
Inspectors will also occasionally find a "CL10" meter which is a transformer-rated meter for large houses with larger electrical systems or two separate main panels.
Some older meters have other designations such as "15 Amps" on their face. This was their test rating. These meters are only usable on systems up to 100 Amps.
Some older meters were also designated as "30 Amps" on their face and these are compatible with 200 Amp services.
For example, one may occasionally find an older house with an upgraded 200 Amp service that still has an old 15 Amp meter plugged into a new meter base.
This meter is not really compatible with the system and should be replaced. Since the responsibility for the meter varies throughout the country, inspectors should contact the local electric utility for their policies and procedures concerning meters.
INSPECTING Electrical METERS & Electric Meter BASES - Inspecting Electric Meters and Electric Meter Bases
Electrical inspectors need to examine the meter and meter base and should take them into account when determining service ampacity. The elements to consider are:
The weakest or smallest of these five items will determine the service ampacity of the system. This inspection will also determine whether or not all parts of the system are compatible with each other and proper for the installation. There are other good reasons to look at the meter base and meter during your inspection. Occasionally one may discover other meter defects or concerns:
The building inspector's responsibility goes beyond that of the electrical contractor and the local electric utility. Because of this, the inspection of the meter and meter base is an important part of your electrical system inspection, and must be done carefully. This will help to protect your client's interests and will help fully inform them about electrical system defects and the proper service ampacity of the house they are considering to buy.
Experienced Electrical Worker Warns of Electric Meter Base Arcing & Burns
Our photos (below) show a simple electrical meter base and meter connection mounts where arcing was not present. Field photos of electric meter damage or arcing are wanted. CONTACT us.
There's one important point about electrical meter sockets I'd like to make. I've been in the electrical trade since 1976, and have seen a lot of interesting situations over the years. One that I feel needs to be addressed is on existing installations, involving arcing at the meter stabs.
I live in California, and So. Cal. Edison loves their "vandal proof" meter locking rings, so in order to do a "proper" service entrance inspection, we have to call a service tech out to unlock the meter ring, they always pitch a "bad time", about coming all the way out to unlock a ring, but it's my reputation and the customer's safety that are both on the line.
More than once, but not really-often I've discovered electrical arcing damage to the electrical meter mounting stabs. Arcing and burning in this location can only be seen by removing the meter. When I find one of these electrical meter mount arc-burn cases, the Edison guy is always amazed.
Not too many sparky's pull the electrical meters to inspect the meter connections, but I recommend it.
And pulling all the breakers off the bus and checking for discoloration or arcing on the bus bars, and breaker clips is a good practice to live by, Especially Zinsco panels (see ZINSCO / SYLVANIA HAZARDS), and especially 30 and 40 amp breakers. - Kirk Schwoebel
Photographs of Arcing Damage & Heat Damage at Connectors in Electric Meter Bases
The electrical meter arcing damage photographs below were provided courtesy of Robert McBride.
At below left the photograph of an electrical meter base shows severe arcing burns at the connecting jaws. At below right you will note (arrow) arc burns at the upper left connector in the photograph. The contributor indicated that this arcing burn occurred because the meter was inserted into the meter base while the system was under load.
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