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Electriciansparadise -- Articles

Multiwire Branch Circuits

By David Herres

The following article was written when NEC 2005 was in force. NEC 2011 contains one significant Code change. Instead of only certain sensitive multiwire branch circuits, now all multiwire branch circuits must be protected by a double-pole breaker, not two single-pole breakers. In all cases, two single-pole breakers, linked by a listed device, are equivalent to a double-pole breaker.
Another important NEC 2008 change is that the ungrounded and grounded conductors of each multiwire branch circuit must be grouped by wire ties or similar means in at least one location within the panelboard or other point of origination. An exception provides that the requirement for grouping does not apply if the circuit enters from a cable or raceway unique to the circuit that makes the grouping obvious.

The very concept of a multiwire branch circuit, sharing a neutral between two circuits, appears designed to throw apprentice electricians into severe anxiety and disapproval.

It would seem obvious that two hot legs, both loaded close to maximum, would heat their shared neutral to such an extent that it would glow red hot as it passed through all that flammable material present in the walls of a wood frame building. Wouldn't it be best to call it "a multicircuit branch wire" and ban it forever?

Savvy electricians know otherwise and indeed the careful NEC allows a multiwire branch circuit if used properly. Actually such an arrangement, besides being Code compliant, has a few advantages and efficiencies which make it an attractive option. But there are some severe drawbacks as we shall see, and it may be that the outraged apprentices are right.

The whole idea is that the two hot wires have to be connected to different phases. Then, the neutral carries only the unbalanced current, the difference at any moment, between the more heavily loaded leg and its lighter colleague. In a single-phase circuit the neutral carries the most current when one leg is fully loaded (just under the amount that would trip the circuit breaker) and the other leg is not loaded at all. Then, as the amount of current flowing through the other leg increases, the neutral cools down. When both legs are equally loaded the two return flows in the neutral cancel and it is like the white wire is not there. In a run of conduit there is actually less heat than if the same two loads were on separate two-wire circuits. Besides saving wire, it is sometimes possible to use smaller conduit and on long runs voltage drop can be significantly lower. Additionally, 12-3 type NM is easier to install because it is round. There is definitely money to be saved.

Before considering the downside of these circuits that claim to eliminate a redundant neutral, let's see what NEC 2005 has to say.

The first mention is in Article 100-I Definitions. (Its presence there indicates that multiwire branch circuits are going to appear in more than one place in the Code. In fact, requirements are in eight locations.) "Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system."

The main concept embodied in this definition is that the ungrounded conductors have to have voltage between them; that is they cannot be connected to the same leg. If they were connected to the same leg, the apprentice's worst fear would be realized - the shared neutral's ampacity would soar. Most electrical mistakes resulting in overloads will trip out the circuit breaker providing warning that something is wrong. But a misfed multiwire branch circuit will silently heat the neutral until it either burns clear inside an enclosure or ignites flammable building material resulting in property loss or worse.


NEC 2005 On Raceways

Article 100 Definitions

Chapter 3 Wiring Methods and Materials

The 2005 NEC defines raceway as "an enclosed channel ... for housing wires, cables or busbar." It can be metal or nonmetallic. Besides the familiar rigid and flexible pipe, which comes in liquidtight versions, there are also underfloor raceways, surface raceways, wireways and busways. Cabletrays are not considered raceways.

For specifics on individual raceways, we consult Chapter 3, Wiring Methods and Materials, which is divided into 43 articles. Articles 320 through 340 deal with various types of cables in alphabetical order. Articles 342 through 362 are the pipe-type raceways, not in alphabetical order. Finally, articles 366 through 390 deal with 13 less commonly used raceways, predominately of rectangular cross section and having more specialized application, again alphabetized.

The 2005 NEC has a new common numbering system within each article that is quite user-friendly and makes it easy to find answers to exam questions. For example, in the articles on individual raceways and cables, Uses Permitted is under the article number followed by .10. Uses Not Permitted is under the article number followed by .12. These universal subheadings range from .1 Scope to .150 Conductor Fill. (Incidentally, the Code is arranged not according to a decimal system where .15 would come between .1 and .2. Instead all numbers to the right of the dot are whole numbers.) If you become familiar with this numbering system by looking at any of the raceway articles, you will become adept at locating the answers to many code questions very quickly and with consistent accuracy.

Question 1.

Both Intermediate Metal Conduit (IMC) and Rigid Metal Conduit (RMC) shall be securely fastened within ___ feet of each outlet box, junction box, device box, cabinet, conduit body, or other conduit termination. Both types of conduit shall be supported at intervals not exceeding ___ feet. (Under certain conditions greater intervals are permitted.)

A. 2, 5.

B. 2, 6 12.

C. 3, 10.

D. 5, 10.

Correct answer: C.

#2

Chapter 3. Wiring Methods and Materials

The first step in designing a raceway installation is to ascertain the type of raceway to be used. Then determine the raceway size.

In some occupancies, such as residences, raceways are not required since type NM Nonmetallic Sheathed Cable is permitted and is appropriate. At the other end of the scale, Hazardous Class I locations where volatile flammable liquids or flammable gases are present, Rigid Metal Conduit (RMC) or its close relative Intermediate Metal Conduit (IMC) is required in most instances.

To determine which if any raceway is required, consult Uses Permitted (.10) and Uses Not Permitted (.12) under the individual raceway types.

Generally, a stronger type of raceway like Rigid Metal Conduit (RMC) can be substituted for a lighter pipe like Electrical Metallic Tubing (EMT) but the rigid is more expensive and the labor to install it is much greater.

Plastic pipe -- Rigid Nonmetallic Conduit (RNC) -- is permitted in many applications but has more tendency to buckle and sag than EMT. This is because of its greater coefficient of thermal expansion as detailed in Table 352.44(A). Where total expansion is expected to be 14 inch or greater in a straight run between securely mounted items such as boxes, cabinets, elbows or other conduit terminations, expansion fittings have to be provided. Another weakness of RNC is that it may contribute greatly to the amount of smoke generated in a fire. RNC is at its best in dairy barns and areas of frequent washdown where its corrosion resistance is a plus. Also it is frequently used for underground lines.

Under no circumstances is waterpipe to be used as a substitute for UL listed plastic or metal raceways.

Prior to 2002 NEC, buildings over three stories could not be wired in type NM, therefore type MC (Metal Sheathed - in some ways resembling the now obsolete type BX) and conduit (mostly EMT) were used.

2002 and 2005 NEC permit NM in over three story buildings but not in all cases - it depends upon the type of construction and fire rating. See Article 334.10 and 334.12 for details.

Similarly, NM is permitted in residential garages. Commercial garages are hazardous areas, Class I Division 2 up to 18 inches above the floor and raceways are required above classified areas. Basically the whole commercial garage has to be in conduit. (Walled off areas such as offices do not come under this requirement).

What about the backyard garage where mechanical work will be done? It should be considered commercial since there exist the same hazards as in an auto dealership on Main St.

Question 2. Which of the following raceways cannot be used for direct burial? `

A. Liquidtight Flexible Metal Conduit (LFMC)

B. Liquidtight Flexible Nonmetallic Conduit (LFNC)

C. Electrical Metallic Tubing (EMT)

D. Electrical Nonmetallic Tubing (ENT)

Correct answer: D.

#3

Sizing Raceways - Part 1

Chapter 3 Wiring Methods and Materials

To calculate the size of a given raceway, you have to have a list of the conductors that are to be installed. Consulting tables 310.16 through 310.19, make a list of conductors with insulation type and size. There are two conditions that require reducing the maximum number of amps that each conductor can carry. Both of these are multipliers that are applied in the same way. The first is ambient temperature. If the conductors pass through a hot area, they are permitted to carry less current since this ambient heat adds to the heat created by current flow. The correction factors shown at the bottom of each table are applied to the ampacity of any given size and wire type. The resulting corrected ampacity is subject to another derating detailed in Table 310.15 (B) (2) (a) with notes and exceptions. This has to do with the number of current-carrying conductors in the raceway. Where more than three current-carrying conductors occupy the same cable or raceway, heat buildup can occur and so the Code requires a second reduction in ampacity. Notice the equipment grounding conductor is not counted as a current-carrying conductor. The neutral may or may not be counted, depending on whether it carries current. A balanced single-phase 240 volt load, for example, will not put current through the neutral and so the neutral will not be counted toward the number of current-carrying conductors.

Where conductors are derated by these two factors, you have to go back to the ampacity tables to see if the wire size has to be increased to handle the circuit size as determined by the the overcurrent device. Even though some of the conductors are not counted as current carrying, they still are subject to size increase after the two derating factors are applied. Now you can complete the first step in sizing conduit and tubing by compiling a list of all conductors.

Question 3. Three 240-volt single-phase 30 amp balanced loads are going to be on three separate circuits, using THHN copper conductors with neutrals and three separate equipment grounds. The powerline will pass through an area where the ambient temperature reaches 120 degrees Fahrenheit. Make a list of the required conductors.

A. 6 THHN 8 AWG and 6 THHN 12 AWG.

B. 12 THHN 8 AWG.

C. 12 THHN 6 AWG.

D. 6 THHN 10 AWG and 6 THHN 12 AWG.

Correct answer: B.

#4

Sizing Raceways - Part 2.

Annex C Conduit and Tubing Fill

Chapter 9 Tables

The final stage in sizing raceways is to choose the correct minimum size from Code tables and guidelines. It is one easy step if the conductors are all the same size and if the raceway is conduit or tubing. Go to Annex C, choose the correct table based on conduit or tubing type, refer to conductor size and insulation type, and the size is given.

If the conductors are different sizes, it is a two-step calculation.

First, make a list of the conductors involved, with AWG size and insulation type. Then, consult Chapter 9, Table 5, and get the area of each conductor. Add these figures together to get the total cross-sectional area of all conductors that are going in a given conduit or tubing. If one or more of the conductors is bare, consult Table 8, Conductor Properties. Use the figure under overall area.

Take this total area of all conductors to Table 4, find the desired conduit or tubing, feed the total area into the table and you get the correct size. Notice that the permitted raceway fill for one, two, or over two wires (as required by Chapter 9 Table 1) is given in separate columns so you do not have to make that calculation. The 60 percent column is when you have to comply with Article 310.15 (B) (2) Exception 5.

The foregoing procedure will provide correct size for conduit or tubing. Other raceways are figured as outlined in the appropriate article. For example, Cellular Metal Floor Raceways can be filled to 40 percent of the interior cross-sectional area of the cell or header, as stated in Article 374.5. Here again, make a list of all conductors and get the total area from the tables in Chapter 9.

Question 4.

What size Liquidtight Flexible Metallic Conduit (LFMC) will accomodate three THW 8 AWG and two THW 10 AWG conductors?

A. 12 inch.

B. 34 inch.

C. 1 inch. D. 1 14 inch.

Correct answer: B.

#5

Installing Electrical Metallic Tubing (EMT)

Chapter 1 General

Chapter 3 Wiring Methods and Materials

A widely used raceway is Electrical Metallic Tubing (EMT). And while this material is user friendly and easy to work with, advance planning and careful work are necessary to create an efficient and code-compliant installation.

Article 110.10 says "Electrical equipment shall be installed in a neat and workmanlike manner." To comply with this requirement, conduit must be straight, horizontal runs must be level, vertical runs plumb, and all runs free of unnecessary bends. If six runs emerge from the top of a panel, they have to be in the correct order so that they can peel off in different directions and reach their final destinations without crossing or having a problem with other utilities. Numerous requirements enumerated in respective articles in Chapter 3, grounding and overcurrent protection in Chapter 2 and special requirements in later Code chapters must be addressed.

State licensing examinations can be passed by learning the National Electric Code and amassing the required hours of practical experience. Most state licensing exams are open book - a copy of the National Electric Code can be brought into the exam and consulted freely. Therefore it is not necessary to memorize all rules and tables. What is important is to know the organization and structure of the Code so that questions can be quickly and accurately answered.

As for raceways, information can be found in all chapters of the Code, but the place to start is the respective articles in Chapter 3, especially Uses Permitted and Uses Not Permitted.

Question 5. Which of the following is untrue?

A. EMT shall be permitted as an equipment grounding conductor.

B. There shall be not more than the equivalent of three quarter bends (270 degrees total) between pull points, for example conduit bodies and boxes.

C. The maximum size of EMT shall be metric designator 103 (trade size 4).

D. Ferrous or nonferrous EMT, elbows, couplings and fittings shall be permitted to be installed in concrete, in direct contact with the earth, or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the conditions.

Correct answer: B.

David Herres

e-mail: electriciansparadise@hughes.net


Books for electricians --

Here is a selection of the most significant electricians' books available online today, at the best prices around. Clicking on any logo provides access to reviews and ratings by electricians. A good place to start is with the 2008 NEC Handbook, which contains the complete text of the current code plus extensive commentary, diagrams and illustrations. Other books of interest for the electrician are available as well.

Low Voltage, Telecom, Fire Alarm Books --


HOME | Best Web Host | Question of the Week | Archived Questions | More Archived NEC Questions | Still More Archived Questions | Still More Archived Questions-2 | Still More Archived Questions-3 | Articles | Electrical Deficiencies | More Electrical Deficiencies | Electricians Tools | Online computers | Cybercorner | Electrician's License | Electronics Tutorials | Electricians' worksaving ideas | Electronic Theorems | Satellite Dish | Digital Cameras and Equipment | HTML Color Chart | Electronic Acronyms | Electronic Definitions | Electrician's Soldering Tutorial | Photovoltaic Power | Wind Power | Fire Alarm Basics | More Fire Alarm Info | Working with MC and EMT | Electricians' Color Code | Wiring Commercial Garages | Managing Your Emergency Lights | Lighting Design | Industrial Wiring | Wiring Ethernet | Residential Wiring | Low Voltage Wiring | PLC Overview | Electrical Troubleshooting Techniques | Using Loop Impedance Meter | Ten Common Grounding Errors |NEC and Low-Voltage Wiring | Raceway Protection and NEC | Working with Metal Raceway | Inductance and Characteristic Impedance | Understanding Capacitance | History of the Ethernet | Twisting Data Conductors | NEC Article 800, Communications Circuits | NEC Article 810, Radio and Television Equipment | NEC Article 820, Community Antenna and Radio Distribution Equipment | NEC Article 830, Network-Powered Broadband | Troubleshooting Submersible Well Pumps | Wiring Healthcare Facilities | First Edition National Electrical Code 1897 | Books for Electricians | Links

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