InspectAPedia ^®

Electrical Meters - how to estimate the electrical service size InspectAPedia®  -     Inspection of Electrical meters or "electric meters" - can electric meters be examined or used to determine the electrical service capacity at a building? How to determine the size, capacity, or ampacity of electrical service at a building

This article discusses the visual inspection of eletrical 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. 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

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".

© Copyright 2010 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

ELECTRIC METERS & METER BASES - Inspecting Electric Meters & Meter Bases - Another Consideration When Determining Service Ampacity

Notice: June 2006 editors's note: 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.

A Broad View of Service Ampacity Determination Must Also Consider Electric Meters & Meter Bases

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.

Electric Meter Bases as Indicators of Electrical Service Ampacity

Warning: Looking at meter bases and meters to guess at ampacity is questioned by some experts as too unreliable.

Systems that have been installed for the last 20 years or so will typically have a rectangular meter base of various sizes as shown in the rectangular-base meter at left. These are usually compatible with 200 Amp services, although undoubtedly there are exceptions to this. NOTE: Did you notice the measuring tape marking the 3" gap between the siding "J" channel and the top of the meter? This electric meter had been pulled downwards by backfill settlement which pushed down the horizontal portion of the buried conduit bringing the service laterals to the building. I [DF] considered this a reportable defect as it was possible that the buried conduit was broken and subject to water entry from this condition and since the wires entering the building panel from the meter box have were disturbed. However M. Cramer points out that most conduit is not continuous, so breaking it is not really an issue aside from damage to conductors underground.

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.

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 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.

ELECTRIC METERS - as Indicators of Service Ampacity

The meter itself is also a consideration when determining the service ampacity of an electrical system. Most modern 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 METERS & 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:

1. The size/rating of the service entrance conductors.
2. The rating of the panel.
3. The rating of the main breaker, if any.
4. The rating of the meter base.
5. The rating of the meter.

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:

1. Moisture or water inside the meter face and/or rust on the meter base.
2. Missing knockouts on the meter base.
3. Broken or cracked conduit or hubs.
4. Loose connections of the meter.
5. Inadequate fastening of the meter base and/or mast to the building.

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.

Share this Article      

...

Technical Reviewers & References

• InspectAPedia.com^® - Daniel Friedman
• InspectAPedia Bookstore lists recommended books, organized by topic & available for purchase. Most of our articles also include a list of recommended books for the specific article topic as well as other references, and information sources.
• Critique, contributions wanted: Contact Us to suggest corrections or additions to articles at this website, and if you wish, to receive online listing and credit as a contributor. Particular thanks are due to the many experts and also consumers who read and critique technical articles at InspectAPedia.com.
• Additional technical contributors & reference sources for this article are listed below.

Use links just below or at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

• 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^® Website Author/Editor
• 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)
• "How to Read Electric and Natural Gas Meters", U.S. DOE original source: www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=13120?print

More Information on Building Diagnostic Inspections and Repairs

• * Safety Hazards and Safe Inspection Procedures for Electrical and Home Inspectors at Residential Electric Panels
• Aluminum Wiring Information Website Aluminum Electrical Wiring Hazards and Repairs: in-depth authoritative info, photos, documents including selection of proper vs. ineffective repair methods. E.g.: Ideal 65 "Twister" purple connector fails in field and lab testing with aluminum wire.
• Ampacity of an Electrical Service: How to Estimate the electrical service size or ampacity entering a building
• Circuit Breaker, a bad one fails to trip failure at aluminum bus-to-circuit breaker connection - field report and photographs
• Electrical Panels, How to Inspect in Buildings, safety for electrical inspectors, electrical panel, fusing, wiring defects, defective products. Inspection Class Presentation
• Federal Pacific Electric (FPE) Stab-Lok Circuit Breaker Panel Hazards Website - Latent fire hazards, in-depth authoritative research, documents, advice on Stab-Lok electric panel and circuit breaker failures and what to do when this equipment is found in buildings.
• Lightning Strike Risk Assessment, Protection Systems & Services
• Multi-wire branch circuit inspection and defects
• "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.
• 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 & Wiring Hazard Inspection, Detection, Cause, Remedy, Prevention - Main Electrical Page
• Directory of Home Inspectors accessible via internet

InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.

The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems

Use this simple, economical mold test kit by following our instructions on how to collect and mail mold samples to our lab

Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.

Building Inspection, Problem Diagnosis, Forensic Investigation & Testing, Repair Consulting

Contact Daniel Friedman for website content suggestions or for fee-paid consulting