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Service Voltage
Note that some commercial and industrial systems do not carry standard 240-volt services, and may not be compatible with ordinary 240-volt household appliances.
Service Entrance Amperage The service entrance amperage is sometimes difficult to determine. The amperage can be derived from the size of the service entrance cable (SEC) by: (a) looking for the size printed on the cable insulation; or (b) comparing the cable sizes to the chart listed below, using a gauge to determine size. Electric Meter
Although the meter is usually owned by a service power company, it should be checked for defects. The
following are some areas that should be evaluated.
Service Disconnect Inspection When the service disconnect is located on the exterior of the structure, the following areas should be closely evaluated:
Service Panel Inspection After locating the main disconnect, main service panel and sub-panels that may be present, remove the covers very carefully and evaluate the interior components for the following items:
Bonding of Service Equipment Noncurrent-carrying metal parts and equipment, such as the panel, service cable, armor or sheath, equipment enclosures, meter boxes, and fittings are required to be bonded to the metal enclosures which are enclosing electrical components. They may be bonded with approved connectors or locknuts. Bonding jumper size is based on the size of the over-current protection device. They are the same size as the equipment grounding conductors. Other systems with current-carrying potential (i.e., water and gas piping) should be bonded back to ground. Sub-panels should be bonded to the main panel box to ensure a single, low impedance ground. Bonding should be provided where necessary to ensure electrical continuity and the capacity to safely conduct any “fault current” that is likely to be imposed. Equipment and/or piping that may become energized should be bonded to the service equipment, the grounding conductor at the service, the grounding electrode conductor where it is of sufficient size, or to one or more grounding electrodes. Grounding Systems and circuit conductors are grounded to limit voltages due to lightning, line surges, or unintentional contact with higher voltage lines, and to stabilize the voltage to ground during normal operation. Equipment grounding conductors are bonded to the system grounded conductor to provide a low
impedance path for fault current that will facilitate the operation of
over-current devices under ground
fault conditions. The protective devices should be selected and coordinated to permit the circuit protective devices that are used to clear a fault without the occurrence of extensive damage to the electrical components of the circuit (i.e., circuit breakers, GFCIs, proper grounding and bonding). Grounding should: (1) Be permanent and continuous; (2) Have the capacity to safely conduct any fault current that is likely to be imposed on it; and (3) Have sufficiently low impedance (resistance) to limit the voltage to ground and to facilitate the operation of the circuit protective devices. Driven rods are the typical choice for grounding conductors when a plastic water service pipe is present, since plastic does not conduct electricity.
How Much Amperage Is Enough? The NEC outlines ways to calculate the service load requirements. The following information is an outline of one of the options, and an understandable way to determine the load requirements from a building inspector’s perspective. Article 220 of the NEC provides the necessary information for calculating loads. This is a summary using assumptions in an effort to understand the general requirements of developing load calculations. Loads Small appliance circuits should be calculated at 75% of the circuit rating. A 20-amp branch circuit would be computed at 1500 volt-amps or 15 amps. A 15-amp branch circuit would be computed at 1125 volt-amps or 11.25 amps. General lighting and general use receptacles are calculated at 3 volt-amps per square foot (SF) of living space. Appliances that are fastened in place, permanently connected or located on a specific circuit, ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers and water heaters should be counted as the actual nameplate rating. Motors should also be counted at the nameplate rating. Summary: On-Site Method Watts = Amps X Volts Example Watts divided by Voltage = Amps, or a 100 watts (ie., a light bulb) divided by 120 volts (typical circuit voltage) = 0.8 amps. (amount of draw) You should also understand that 60-amp or higher single-phase services allow you to access 120 and 240-volt circuits. A 100 amp single-phase service will allow you to access 100 amps at 240 volts, or 200 amps at 120 volts, or a combination of 120 and 240-volt circuits. The latter is commonly found in most residences. Simply add the wattage or amperage required by the existing or probable appliances. Assume a house with 2000 SF of living space, a fossil fuel heating system, central air, electric range/oven, electric dryer and an electric water heater. 120-volt appliances and equipment:
Total of the 120-volt circuits = 130.15 amps 240 volt appliances and equipment:
Total of the 240-volt circuits = 99.41 amps These items represent many of the appliances normally contained in a home. There will be other items such as: well pumps, freezers, saunas, welders, generators and multiple systems, etc. Such items can be added, as long as you know the voltage and wattage. The 130.15 amps will be distributed across both of the 120-volt poles. This means that the 130.15 has to be divided by 2, suggesting that 65.07 will be taken from each side. We know that the distribution will not be even, however, it will be reasonably close. If we add the approximate 65 amps from the 120-volt circuits to the approximate 99 amps from the 240-volt circuits, we get 164 amps at 240 volts. This suggests that the maximum current that could be drawn in this sample house, assuming every electric device was on simultaneously, is 164 amps. We know that it is nearly impossible to have a situation where every electric component in the house will be on at the same time. This situation will provide a design cushion. An arbitrary figure would be in the 20% range. 164 amps x 80% = 131 amps. The example above, according to the Summary method, requires 131 amps to service this house. You will typically find a 200-amp service in this situation, however, 150 amps would be adequate. 100 amps would not be acceptable. Electrical
Main Page Method of
Inspection Common Defects
Clearances
and Fastener Requirements
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The service voltage can be determined by counting the number of wires connected to the mast or weather
head. A 2-wire drip loop indicates a single-phase, 120-volt electrical service. A 3-wire drip loop indicates a
single-phase, 240-volt electrical service. A 4-wire drip loop indicates a 208 or 240-volt three-phase service.
Three-phase electrical systems are typically found commercially, such as in a supermarket or someplace
with commercial refrigeration equipment. Two-phase services have 5-wire drip loops and are typically
found in industrial applications. A licensed electrician should evaluate three and two phase systems.
Check for excessive rust or corrosion on the interior and exterior of the box. 
indicate water penetration via the service entrance cable or a condensation problem in the area.