Planning the electrical system
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Planning The Electrical System. Safe means for the entrance of electrical supply into a building Means of disconnecting all electrical power in a building Means of limiting to a safe level the maximum amount of electrical current that can enter a building

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Planning the electrical system
Planning The Electrical System

  • Safe means for the entrance of electrical supply into a building

  • Means of disconnecting all electrical power in a building

  • Means of limiting to a safe level the maximum amount of electrical current that can enter a building

  • Common point for grounding the electrical equipment

  • Safe means of subdividing the electrical supply to serve individual loads or group of loads

May 2007


Types of service entrance panels
Types of Service Entrance Panels

  • Circuit Breaker Type

  • Fuse Type


Circuit breaker type service entrance panel
Circuit Breaker Type Service Entrance Panel

  • When breaker trips, it may be reset after the problem has been corrected

  • Circuit breakers are easy to switch off

  • Circuit breakers are available in sizes to protect any circuit

  • Circuit breakers will take a short periods of overload without tripping

  • Circuit breakers cannot be tampered with

  • A larger size circuit breaker cannot be installed accidentally


Fuse type service entrance panel
Fuse Type Service Entrance Panel

  • Cost less to purchase

  • Less convenient to use

  • When fuses “blows”, It must be replaced unless equipped with a special reset-type fuses


Determining the size service entrance switch
Determining the size Service Entrance Switch

  • Size of dwelling

  • Appliances

    • Large Appliances (Ranges, Dryer, Water Heater, ect.)

    • 2 Small Appliance loads Circuits

  • Laundry Load Circuit

  • General Lighting load Circuit


One family dwelling
One Family Dwelling

  • 2000 square feet

  • Appliances

    • Range – 240 Volts – 12 kilowatts

    • Dryer – 240 Volts – 5.5 kW

    • Water Heater –240 Volts – 5 kW

    • 2 Small Appliance Load Circuits – 120V

    • Laundry Load Circuits – 120V

    • General Lighting Load Circuit – 120V


Computed load for dwelling
Computed Load for Dwelling

  • General Lighting load:

    • 2000 sq. ft. X 3 volt-Amperes = 6000VA

    • 6000VA divided by 120 Volts = 50 Amps


Laundry circuits
Laundry Circuits

  • At least one laundry circuit is required by NEC

  • Must be 20 amp

  • The circuit may not serve any lighting outlets or outlets in any other room in the house

  • 3 circuits X 1,500 Watts = 4500 Watts

    • 4,500 Watts divide by 120 Volts = 37.5 A


Major appliances
Major Appliances

  • Refrigerator 350 Watts

  • Freezer 350 Watts

  • Dryer 7,000 Watts

  • Dish Washer 700 Watts

  • Range/Oven 1,150 Watts

  • Water Heater 4,500 Watts

    Total 24,050 Watts

    Well Pump 2,000 Watts


Heating and cooling
Heating and Cooling

  • 100% of the nameplate rating of Central Heating and Air Unit

  • 65% of the nameplate rating of central electric space heating, including supplemental heating elements

  • 65% of the name plate rating of electrical space heating if less than four separately controlled units

  • 40% of the nameplate ratings of electric space heating of four or more separately controlled units

    9,600 Watt Central AC/Heat Pump


Loads except heating and cooling
Loads except Heating and Cooling

  • Small Appliance Load 4,500 Watts

  • General Purpose Load 6,000 Watts

  • Fixed Appliance Load 24,050 Watts

  • Other Items (Well Pump) 2,000 Watts

    Total Load 36,550 Watts


Calculate total load
Calculate Total Load

  • Heating/cooling Load @100% 9,000 Watts

  • First 10 KW of other Load @100% 10,000 Watts

  • Remainder of other Load @40% 10,620 Watts

    36,500 – 10,000 = 26,550 Watts

    26,550 X 40% = 10,620 Watts

    Total Load 30,220 Watts


Total amperage requirement
Total Amperage Requirement

Total Load divided by 240 volts

30,220 Watts divided by 240 Volts = 125.9 Amps



Computing farm shop load
Computing Farm Shop Load

  • 1200 sq. ft. Farm Shop

    • 1 - 240 Volt, 50 amp Welder 50 A

    • 1 – 230 Volt, 5 hp Air Compressor 28 A

    • 1 – 230 Volt, 2 hp Bench Grinder 12 A

    • 1 – 230 Volt, 3 hp Radial Arm Saw 17 A

    • 1 – 230 Volt, 3 hp Tilting Arbor Saw 17 A

    • 1 – 115 Volt, ¾ hp Drill Press (13.8)

    • 1 – 115 Volt, 1/3 hp Exhaust Fan (7.2)

    • 6 – Additional 120 Volt Outlet (6 X 1.5 A = 9 A)

    • 3 – Lighting Circuits with 15 outlets = 22.5 A


Converting 115 volt items to 240 volts
Converting 115 Volt items to 240 Volts

  • Drill Press 13.8 Amps

  • Exhaust Fan 7.2 Amps

  • Additional Outlets 9.0 Amps

  • Lighting Outlet 22.5 Amps

    Total Amperage 52.5 Amps


To convert 115 volts to 240 volts
To Convert 115 Volts to 240 Volts

  • 52.5 Amps X120 Volts = 6300 VA

  • 6,300 VA divided by 240 Volts or ½ of 52.5 amps= 26.25 A


Total service load
Total Service Load

  • Welder 50 Amp

  • Air Compressor 28 Amp

  • Bench Grinder 12 Amp

  • Radial Arm Saw 17 amp

  • Tilting Arbor Saw 17 amp

  • Converted 115 Volt items 26.25 Amp

    Total Load 150.25 Amp


Load without diversity
Load without diversity

  • Loads most likely to operate at one time to produce the heaviest loads are:

    • Welder 50 Amps

    • Exhaust Fan 3.60 Amps

    • Half of Lighting 11.25 Amps

      Total 64.85 Amps

      Largest Motor is 5 hp; 28 amps X 125% = 35 Amps


Computing total demand
Computing Total Demand

  • Not less than the first 60 amps of all loads

    • 100% of largest demand load 64.85 Amps

    • 50% of the next 60 amps 30 Amps

    • 25% of remainder of other load

      150.25 Amps – 124.85 Amps= 25.45

      25% of 25.4 = 6.36 Amps

      Total Demand 101.21 Amps