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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
How to determine the electrical service capacity at a residential property by visual inspection alone: introduction.
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Determining the Electrical Service Ampacity and Voltage at a building by visual inspection - Introduction
"How do I determine the service amperage at a building?" 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.
Visual inspection and use of digital multimeters(DMMs), Volt-ohm meters (VOMs), neon testers, and electrical inspection safety are also discussed, and we provide pictures and sketches that illustrate electrical panels, meter bases, and electric meters.
[Click to enlarge any image]
When a homeowner or client asks you "how much" electrical service is provided s/he needs to know the service ampacity and voltage. In the US and Canada service voltages are commonly (nominally, that is, not exactly) 240 volts at the electrical panel, a system which supports both 120V and 240V circuits in the building.
The electrical masthead sketch at left, courtesy of Carson Dunlop Associates, shows where this electrical inspection starts - outside at the electric masthead.
Is the Electrical Service 120Volts or 240Volts?
Typically, two 120V hot wires entering the building provide 120V for circuits connected from an individual entering wire and the neutral bus, and 240V for circuits connected between the two incoming individual 120V circuits.
Older building electrical services, typically those installed before 1940, may support only 120V and as little as 30-amps. We still find these services on small cabins and on a few old residences in the U.S., Canada, and Mexico and elsewhere.
Sketch at left, courtesy of Carson Dunlop Associates, shows outdoor details of a 240V electrical service.
For normal residential use, excluding small vacation cottages, a 120V or 30A service would most likely be considered obsolete and has been since 1959 (Hansen). [NOTE: 1992 ASHI Standards of Practice, 8.2.1. While ASHI and other building inspection standards are updated periodically, the basic requirements addressed in this article should not change.]
What is the Electrical Service Ampacity Provided by the Service Entrance Conductors
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. As Carson Dunlop Associates sketch (left) illustrates, the electrical capacity that is provided by the service entrance conductors is determined by checking the wire size where they wires are accessible.
Keep in mind that while the SEC wires are a limiting factor in setting the ampacity and voltage available at a building, other electrical components (main breaker and panel size, for example) may set lower limits.
Also keep in mind that when looking at SEC wire sizes, we refer to the wiring from the masthead down to the electrical meter and the SEC wires between the electrical meter and the service disconnect or main circuit breaker or fuse. We do not usually inspect nor consider the diameter of the overhead or underground feeders coming to the building during a home inspection.
Watch out: inspecting and/or touching electrical components is inherently dangerous and there is risk of shock or death by electrocution. Electrical inspections are inherently dangerous to the inspector and potentially dangerous to inspection clients and building occupants.
People conducting these procedures must first be familiar with safe electrical practices before attempting any inspection of electrical equipment. Such familiarity is essential to protect all parties concerned. Inspectors: proceed at your own risk. See SAFETY for ELECTRICAL INSPECTORS at Residential Electric Panels.
We have divided the topic of determining the ampacity of electrical service at a building into the articles listed below at More Reading.
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Frequently Asked Questions (FAQs) about methods for determining the electrical service ampacity size at a building
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