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ELECTRICAL INSPECTION, DIAGNOSIS, REPAIR
AFCIs ARC FAULT CIRCUIT INTERRUPTERS
ALUMINUM WIRING HAZARDS & REPAIRS
AMPS VOLTS DETERMINATION
AMPERAGE MEASUREMENT METHODS
APPLIANCE EFFICIENCY RATINGS
BOOKSTORE - ELECTRICAL
Cadet & Encore Heater Recall
CIRCUIT BREAKER SIZE for A/C or HEAT PUMP
Classified CIRCUIT BREAKER WARNING
CUTLER HAMMER PANEL FIRE
CORROSION in ELECTRICAL PANELS
DEFINITIONS of ELECTRICAL TERMS
DIRECTORY OF ELECTRICIANS
DMM Digital Multimeter HOW TO USE
ELECTRIC METERS & METER BASES
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
ELECTRIC PANEL AMPACITY
ELECTRIC PANEL INSPECTION
ELECTRIC PANEL MOISTURE
Electric Power Frequency Table
EMF RF FIELD & FREQUENCY DEFINITIONS
ELECTRICAL GROUND SYSTEM INSPECTION
ELECTRICAL SERVICE DROP
ELECTRICAL SERVICE ENTRY WIRING
EMF RF FIELD & FREQUENCY DEFINITIONS
FIRE SAFETY Checklist, CPSC
GFCI PROTECTION,Testing GFCIs AFCIs
HEATING COST FUEL & BTU Cost Table
HEAT TAPE USAGE GUIDE
Hertz - Definitions of KHz MHz GHz THz
KNOB & TUBE WIRING
LIGHTING, EXTERIOR GUIDE
LIGHTING, INTERIOR GUIDE
LIGHTNING PROTECTION SYSTEMS
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
MAIN ELECTRICAL DISCONNECT
MAIN DISCONNECT AMPACITY
MOISTURE SOURCES in PANELS
MURRAY SIEMENS Recall
PHOTOVOLTAIC POWER SYSTEMS
PUSHMATIC - BULLDOG PANELS
REMOTE ELECTRIC POWER, PHOTOVOLTAIC
RUST in ELECTRICAL PANELS
SAFETY for ELECTRICAL INSPECTORS
SE CABLE SIZES vs AMPS
SIEMENS MURRAY Recall
THERMAL EXPANSION of HOT WATER
THERMAL EXPANSION of MATERIALS
UNDERGROUND SERVICE LATERALS
VOLTS / AMPS MEASUREMENT EQUIP
VOLTAGE MEASUREMENT METHODS
WIND TURBINES & LIGHTNING
ZINSCO SYLVANIA ELECTRICAL PANELS
Electrical wire sizes & gauges: this article gives photos and tables of electrical service entry cable sizes, electrical branch circuit wire sizes, bell wire, telephone wire, thermostat wire, and ampacity or fuse/circuit breaker ratings.
In answering the question "How do I determine the service amperage?" start by taking a look at the service entry cables outside and at their entry into the electrical panel. A quick look can tell us if the property is served by 240V or only a 120V service, even before measuring the gauge or wire thicknesses that we discuss below.
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SE CABLE SIZES vs AMPS - How to Inspect Electrical Service entrance cables for Ampacity, Voltage, Condition
This article series 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. Readers of this article should also be sure to review SAFETY HAZARDS & SAFE ELECTRICAL INSPECTION PROCEDURES for examining Residential Electrical Panels.
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 that can make for big mistakes in your guess at the service ampacity for a property, and the process itself is dangerous.
At left we can see a three wire mast-head, suggesting that the building has a 240V service. But we did not like the position of that weather-head, and we considered that water may be entering the SEC.
The amperage provided by the electrical service entrance cable is a function of its materials and diameter. Often the actual cable type and size is printed right on the cable insulation. Otherwise some rough measurements of cable diameter are in order.
If, inside the panel, the inspector could see the ends of the entrance cable [Figure at left], measure metal wire diameter, and if the inspector knew the manufacturer of the cable and its specifications, a certain identification of the cable's ampacity could be made.
[Click to enlarge any image]
However a safer, faster and common practice is to examine the exterior of the cable at a point outside of the electric panel.
Many SECs include printing right on the external jacket of the cable that will tell you the number of conductors, the metal (AL or CU), and the wire size.
Watch out: Make sure you look at both outside (mast head down to meter) and inside (from meter into service panel).
They may be different! Inside the panel, stripped of ground and insulation, you may see only wires as in Figure 3. Don't confuse guides for external measurements of the whole cable with in-panel measurements of the wires themselves.
Carson Dunlop Associates' sketch (left) shows common electrical wire sizes for both service entry conductors (SECs) and in-building branch circuit wiring.
We use to use a plastic vernier caliper or other non metallic instrument to measure external (insulated) width of the whole cable as a reasonable guess at wire ampacity. Some inspection tool companies sell, and others give-away a plastic "wire gauge" with notches labeled to tell electrical inspectors the cable size for aluminum and copper SECs. Or you can make your own.
But this method is not precise. Plastic insulated cable, for example, is often thinner in total thickness than older fabric-covered cable. Look at the table of wire dimensions in the accompanying article.
Fortunately for building inspectors, even though wire thickness varies among manufacturers, you can generally find the measurement closest to one of the standard sizes and you're likely to be correct.
Table of Common Electrical Wire Sizes and Amps or Fuse Ratings - Residential
For copper wiring the following wire sizes and ampacity ratings or fuse/circuit breaker sizes are common on 120V residential electrical circuits. :
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
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