ENGINEERS CENTER

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Thursday, 28 July 2016

Learn Electrical Wiring Work Part 02 - ENGINEERS CENTER

The Basics of Panelboards

It's easy to make mistakes when installing and wiring a lighting and appliance panelboard. Let's look at some basics that apply to its installation. You can use this article as an abbreviated checklist to keep the electrical inspector from citing your installation.

• Short-circuit rating of bus and overcurrent devices. Apply interrupting ratings to branch-circuit and main circuit breakers and fuses. This minimizes excessive movement (deflection) and overheating (hot gas/shock wave formation) of conductive parts and prevents damage to adjacent insulating materials. (NEC Sec. 110-10).

• "Slash" voltage ratings. Watch for multipole, molded-case circuit breakers rated "120/240V" or "480Y/277V" compared to "240V" or "480V." Usually, the Code does not permit applying "slash" ratings to ungrounded, resistance-grounded, or grounded-delta systems. (Secs. 100, 110-9, 240-85).

• Bus overcurrent protection/main lugs only (MLO) versus integral molded-case circuit breaker or fusible switch. Do you have an upstream overcurrent device or disconnecting means? If so, then determine whether you need a panelboard with these components integral to the board. If you are feeding the panelboard from "tap" conductors, then you usually must install an external-fused switch or circuit breaker upstream of the MLO panelboard. (Secs. 215-3, 240-21, 384-14, 384-16)

• Attached insulated-conductor temperature rating. When determining conductor ampacity, use the temperature column that matches the temperature rating of the weakest (lowest temperature) connected termination, conductor, or device in the circuit. That is, if you have 60C lugs, don't use the 75C ampacity tables for your conductors. You can use conductors whose temperature ratings exceed those of other components for ampacity adjustment. And, there are limits to ampacity/temperature ratings when you go beyond 100A. Be sure to review the Code's extensive instructions on this. (Sec. 110-14(c), Table 310-16).

• Main breaker feed direction. Does your panel main circuit breaker come with an interchangeable trip mechanism? If so, then terminate incoming cable only to the line side of the breaker. This limits a degree of shock/flash hazard during service or replacement of the trip unit.

• Bolted versus "stab" bus-to-MCCB pole connections. When you have a bolted connection, each ungrounded (phase) overcurrent-device pole secures to the panel bus by a machine screw; as opposed to a spring-clip-type conductor. Make sure you have the right breaker before getting started!

• Main bonding jumper (install or not?) The main bonding jumper gives you "system grounding." If a transformer is immediately upstream of the panelboard, you must bond the neutral bus or neutral conductor to the panel enclosu re and to a (bare) grounding-electrode conductor. Generally, exceptions are for ungrounded service, like 480V, 3-phase, 3-wire service, and some 2-wire control circuits under special conditions. For most feeders serving this installation, like 120/240V, 3-wire, or 4-wire 208Y/120V and 480Y/277V circuits, you must keep the two electrically separate. You can place the bonding jumper in the transformer enclosure or panelboard. (Sec. 250).

• Concentric knockouts versus field-cut openings/51/2-in.-deep enclosures.Don't depend on concentric knockout rings (or reducing washers) to give you adequate grounding continuity. The Code requires bonding jumpers around them over 150V. If you have several enclosure openings, all those jumpers can be a headache. Specifying panelboards furnished "without knockouts" or "with blank-end walls" can eliminate this concern; while cutting conduit openings at the installation site eliminates these problems. Generally, panelboard enclosures 51/2 in. deep or more are available without factory knockouts. (Sec. 250).

Learn Electrical Wiring Work Part 01 - ENGINEERS CENTER

What is Current? Types of Current Flow.

What is CURRENT?

The flow of free electrons in the same general direction from atom to atom is referred to as current and it is measured in amperes (“amps” or “A”). The number of electrons that flow through a conductor’s cross-section in one second determines amps. Current can be expressed in a number of different ways, such as:

Quantity Symbol Decimal

1 milliampere 1 mA 1/1000 A
1 ampere 1 A or 1 amp 1 ampere
1 kiloampere 1 kA 1000 amperes

When discussing current, the direction of current flow needs to be considered. There are two different theories about this: · Conventional Flow · Electron Flow

Conventional Flow: This theory states that electrons flow from positive to negative. Benjamin Franklin theorized this when very little was known about electricity. It states that an invisible fluid known as electricity tended to flow through a wire from the positive to the negative. Ben’s theory became the convention (hence the term “conventional current”) in electrical theory, mathematics, textbooks and electrical equipment for the next hundred years.

Electron Flow: This theory states that electrons flow from negative to positive. When more was known about the behavior of electrons, scientists discovered that electrons actually flow from negative to positive. Since electrons are negatively charged, it follows that they are attracted by positively charged bodies and repelled by negatively charged bodies.

Despite the fact that it has been positively determined that electron flow is the correct theory, the conventional flow theory still dominates the industry. Either theory can be used as long as the orientations are correct. Conventional flow will be used from this point on in these training modules unless otherwise stated.

How to Read HVAC Drawings and Symbols - ENGINEERS CENTER

How to Read HVAC Schematic Drawings - ENGINEERS CENTER

Wednesday, 27 July 2016

HVAC Ducting Part 1 - ENGINEERS CENTER

Tuesday, 26 July 2016

How to Troubleshoot a Gas Furnace - ENGINEERS CENTER

Saturday, 23 July 2016

HVAC BASICS 02 - ENGINEERS CENTER

HVAC BASICS 01 - ENGINEERS CENTER

Friday, 22 July 2016

Electrical Engineering Passive Components 03 - ENGINEERS CENTER

Electrical Engineering Passive Components 03 - ENGINEERS CENTER

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Electrical Engineering Passive Components 02 - ENGINEERS CENTER

Electrical Engineering Passive Components 02 - ENGINEERS CENTER

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Electrical Engineering Passive Components 01 - ENGINEERS CENTER

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Thursday, 21 July 2016

Selection of Air Conditioning Systems 03 - ENGINEERS CENTER

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Selection of Air Conditioning Systems 02 - ENGINEERS CENTER

Selection of Air Conditioning Systems 03 - ENGINEERS CENTER

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Tuesday, 19 July 2016

Temperature Difference

According to Newton's Law of Cooling heat transfer rate is related to the instantaneous temperature difference between hot and cold media

- In a heat transfer process the temperature difference vary with position and time Mean Temperature Difference.

- The mean temperature difference in a heat transfer process depends on the direction of fluid flows involved in the process. The primary and secondary fluid in an heat exchanger process may

- flow in the same direction - parallel flow or co-current flow
- in the opposite direction - counter-current flow or
- perpendicular to each other - cross flow

Cooling and Heat Load Calculations Lecture 03 - ENGINEERS CENTER

Cooling & Heat Load Calculations Lecture 03 - ENGINEERS CENTER

Cooling & Heat Load Calculations Lecture 02 - ENGINEERS CENTER

Cooling & Heat Load Calculations Lecture 02 - ENGINEERS CENTER

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Cooling & Heat Load Calculations Lecture 01 - ENGINEERS CENTER

Cooling & Heat Load Calculations Lecture 01 - ENGINEERS CENTER

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Cooling & Heat Load Calculations Lecture 09 - ENGINEERS CENTER

Cooling & Heat Load Calculations Lecture 09 - ENGINEERS CENTER

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Cooling & Heat Load Calculations Lecture 08 - ENGINEERS CENTER

Cooling & Heat Load Calculations Lecture 08 - ENGINEERS CENTER

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