Chapter One

Electrical Design and Component Basics

Introduction

• Rural and domestic electrical systems cause many electrocutions each year.

• It is important to know how to work with electricity safely.

Electrical Safety

• Safety is freedom from accidents.• Accidents are caused by hazards.• To be safe, the risks associated with

electricity must be maintained at an acceptable level.

Managing Electrical Risk

• All electrical installations must meet current standards—National Electric Code.

• Insure you include the appropriate people in the design and installation of electrical service and components. (textbook)– Power supplier– Qualified electrician– Electrical equipment supplier– Insurance company

• Electrical service high voltage conductors are especially dangerous.– 10 foot rule

Codes and Safety

• The source of information for the safest installation of electrical service and components is the National Electric Code (NEC).– Published by the National Fire Protection Association (NFPA).

• Note: many rural areas do not have local or state agencies to enforce NEC regulations. But:– The NEC should be used for safety reasons and because insurance companies

may deny coverage or charge a higher rate for buildings that do not met NEC standards.

• Example: Minimum clearance distance for grain storage bins or buildings (Table 1-1.)

Building Groups

• Safe wiring is influenced by the building environment.– Different environments have different electrical code requirements.

• Agricultural buildings are defined as any structure used to house farm implements, hay grain, animals or other agricultural produce.– No habitation or product processing, treatment or packaging.

• Agricultural buildings are categorized into three groups:– Dry– Damp– Dusty

Building Groups—cont.

• Dry buildings: machine storage, shops and garages not attached to a residence.

• Damp: Livestock, milking centers, ventilated manure pits, well pits, silos, silo rooms, high humidity produce storages and any structures that are washed periodically.

• Dusty: fertilizer, dry grain, dry hay storage and grain feed processing centers.

Note: these building groups do not include buildings with explosive environments.

Building Materials for all Buildings

• For the safest electrical system, only use components approved by the Underwriters Laboratories Inc. (UL)– Develops specification for minimum safety standards.– Specifications are based on building environments

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Surface mount Waterproof

Building Groups--Dry

• Dry buildings do not require special electrical materials.

• Surface wiring is recommended– Wiring and devices should be mounted out of the

way.

Surface mounted wiring must be protected from physical damage.

Building Groups—Dry—cont.

• NM-B cable is acceptable indoors and were it is not subject to long periods of high humidity.

• Type UF-B cable is used for damp or wet indoor locations, underground use, and out door locations.

• Solderless connectors (wire nuts) are the preferred means of splicing conductors.

• Insure the correct size is used and that all of the conductors are tight.

Building Group—Damp

• Damp agricultural building require wiring practices, wire, fixtures and devices engineered for that environment.

• If surface wired, UF-B cable must be used.• High levels of moisture, dust and gas can quickly corrode metallic

equipment.• All boxes and fixtures must be dust and water tight, and corrosion resistant.

(Fig. 1-6 & 1-7)– Dust and moisture accumulation leads to fire and safety hazards by creating

short-circuits or heat buildup in fixtures.

Building Group-Damp-Incandescent lights

• Incandescent lights must use appropriate fixtures and heat resistant globes to cover the bulbs. (Fig. 1-4)

Building Group-Damp-Fluorescent lights

• Fluorescent lights must use appropriate fixtures with gasketed cover. (Fig 1-5)

• Also note the seal around the wire inlet.

Building Group-Damp—Wire Nuts

• Solderless connectors used in damp environments must be designed for the environment.– Silicone filled.

Building Group-Damp—Cable Connectors

• The correct cable connectors must be used.• Metallic can be used in non corrosive

environments.• Non metallic is used in corrosive

environments• Must be thread into box.• Uses a gasket that compresses and seals

when nut is tightened.

Building Group—Damp--Conduct

• PVC is the recommended conduct for damp buildings.– Available in different lengths up to 20 feet.– Available in diameters from ½ inch to 6 inches.

• Recommended conductors for plastic conduct include:– THWN– RHWN– THHW– XHHW

Table 1-4.

Building Group--Dusty

• Commercial dusty buildings such as grain elevators require additional precautions. [NEC 502.10(A)]

• On farm grain storage and handling facilities are usually wired according to the damp building requirements.

Building Group—Dusty-explosive

• Intrinsically safe circuits are used. – Current and voltage are limited so that the minimum ignition energy and the

ignition temperature of an explosive mixture cannot be reached.• The production of sparks, arcs or excessive temperatures which might act

as a source of ignition are prevented by additional measures and an increased degree of safety.– Use non sparking tools

• The components which could cause the ignition of a potentially explosive mixture are built into an enclosure which is capable of withstanding the pressure of an explosion. – It must be ensured that the explosion inside the enclosure cannot be

transmitted beyond the enclosure and ignite the external potentially explosive atmosphere

Box Size

• Box must have adequate volume for the devices and conductors.• Characteristics of inadequate box size.

– Difficult to work in– Increases work time– Concentrates heat– Increases chances of short circuits

Box Size—cont.

• Box size is determined by multiplying the equivalent number of conductors by the conductor volume values in Table 1-2.

Conductor Size (AWG) Box Volume (Cubic inches per

conductor)14 2.0012 2.2510 2.508 3.006 5.00

Box Size—cont.

• The number of equivalent conductors is determined by 5 criteria:1 Each conductor passing through a box without being spliced or connected to

a device is counted as one conductor.2 Each conductor connecting to a splice or device is counted as one conductor,

unless the conductor is totally contain inside the box.3 All grounding conductors are counted as only one conductor unless a second

set of equipment grounding conductors are used.4 A switch or receptacle is two conductors.5 Each of the type of fitting or fixture is a box is counted as one conductor:

Box Size—cont.—box fittings

• Cable clamp

• Fixture stud

• Fixture strap

Box Size--Example

• Determine the minimum size of box for the illustration when 12 AWG is used.

€

9Conductors

2Switch

Equivalent

11Conductors

€

11 2.25x =24.75 in 3

Box Size—Example 2

• Determine the minimum size of box for the illustration when 12 AWG is used.

€

2Conductors

1Ground

1Cable clamp

2Receptacle

Equivalent

6Conductors

€

2.25 6x =13.5 in 3

Wires per Conduct

• The NEC limits the number of wires that can be installed in each size of conduct.

• For a size of conduct the limit is determined by the type of insulation and the wire size. (Table 1-5)