Multiwire Branch Circuits

(محمد سميري) #1

Multiwire Branch Circuits
Multiwire branch circuits are widely used in all types of installations, because they offer several advantages: fewer conductors (three instead of four, when compared to using a pair of 2-wire circuits), smaller raceway sizes, reduced circuit voltage drop, and the ability to supply two different voltages from the same circuit. However, because multiwire branch circuits also have potential hazards associated with them, the NEC®includes several applicable safety rules. These rules deal primarily with two subjects.
Ensuring safe disconnection of ungrounded (phase) conductors
Ensuring continuity of the grounded conductor

What Are Multiwire Branch Circuits?
According to the definition of Branch Circuits in Article 100, a multiwire branch circuit consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor with 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 most common type is a 120/240-volt branch circuit consisting of two phase conductors and one grounded conductor. (There are also 277/480-volt, single-phase, 3-wire circuits, but for convenience, the rest of this article will focus on 120 and 240 volt circuits.) Section 210.4(A) requires that all conductors of a multiwire branch circuit originate from the same panelboard or similar distribution equipment.</SPAN>

Uses of Multiwire Branch Circuits
120/240-volt single-phase 3-wire circuits are often used to supply appliances, such as electric ranges and dryers, that have both line-to-neutral and line-to-line loads. Multiwire branch circuits can also be used to supply a combination 250- and 125-volt receptacle. </SPAN>

Because 210.4(A) states that multiwire branch circuits can be considered multiple circuits, they can be used, for example, to satisfy the requirement for wiring kitchen countertop receptacles in a dwelling unit on two different small-appliance branch circuits.

Safe Disconnection of Ungrounded (Phase) Conductors
The 2008 edition of the NEC requires that all multiwire branch circuits “be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.” This can be accomplished with a double-pole breaker or two single-pole breakers with an identified handle tie. Prior to 2008, only multiwire branch circuits that supplied more than one device or equipment on the same yoke required a means of simultaneous disconnection.

Grounded Conductor Continuity
In multiwire branch circuits, the removal of a receptacle or other device must not interrupt the continuity of the grounded conductor [300.13(B)]. Disconnecting the neutral can cause voltage fluctuations that create a serious shock hazard and can damage electrical equipment. This means that receptacles fed by a multiwire branch circuit cannot be “daisy-chained” together using the terminal screws as splice points. Instead, grounded circuit conductors must be tied together in the outlet boxes, with pigtails run to the terminal screw on receptacles.
The reason this rule applies only to grounded conductors is that opening one of the phase conductors of a multiwire branch circuit while replacing a receptacle doesn’t cause a safety hazard. Likewise, opening either conductor of a 2-wire circuit doesn’t cause a hazard, so pigtailing these conductors isn’t required (although pigtailing would still be considered good workmanship