NEC 408.3.E: 3-Phase Busbar Arrangement

  • Thread starter MitYeltu
  • Start date
In summary: If one is a lot higher than the others, it could be the high leg delta configuration.In summary, NEC 408.3.E states that on 3-phase buses, the arrangement should be A, B, C from front to back, top to bottom, or left to right. The B phase should have the higher voltage to ground on 3-phase, 4-wire, delta-connected systems. However, other busbar arrangements are allowed for additions to existing installations and must be marked accordingly. The conversation also touches on the misconception that a 3-phase system is always balanced, when in reality, with high leg delta configuration, one phase will have a higher voltage to ground.
  • #1
MitYeltu
8
0
NEC 408.3.E reads as follows

The phase arrangement on
3-phase buses shall be A, B, C from front to back, top to
bottom, or left to right, as viewed from the front of the
switchboard or panelboard. The B phase shall be that phase
having the higher voltage to ground on 3-phase, 4-wire,
delta-connected systems. Other busbar arrangements shall
be permitted for additions to existing installations and shall
be marked.

Have I missed the boat on this? A 3-phase system with a higer voltage on phase B than on phases A and C? I don't understand this. I thought a 3-phase system was, barring unbalanced impedances, balanced. What did I miss here?
 
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  • #2
MitYeltu said:
Have I missed the boat on this? A 3-phase system with a higer voltage on phase B than on phases A and C? I don't understand this. I thought a 3-phase system was, barring unbalanced impedances, balanced. What did I miss here?
Nothing. You are right. I have no idea why they said B is higher.
 
  • #3
With High leg Delta configuration, ground is connected to the center tap of one of the transformers. It is then impossible to have all three legs at the same voltage in reference to ground. Which one you want to call A, B, or C is whatever someone decides.
-
When referring to buses, the terminology should be legs, not phases. A phase is a pair of conductors. Ideally all three phases will have the same voltage.
 
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Related to NEC 408.3.E: 3-Phase Busbar Arrangement

What is NEC 408.3.E: 3-Phase Busbar Arrangement?

NEC 408.3.E refers to a specific section of the National Electrical Code (NEC) that outlines the requirements for 3-phase busbar arrangements. These arrangements are used in electrical panels to distribute power between circuits and are an important safety consideration in electrical systems.

Why is 3-phase busbar arrangement important?

3-phase busbar arrangements are important for several reasons. They help to evenly distribute power between circuits, reduce the risk of electrical overloading, and improve the overall efficiency and reliability of an electrical system.

What are the main types of 3-phase busbar arrangements?

There are three main types of 3-phase busbar arrangements: vertical, horizontal, and flat. Vertical arrangements are the most common and involve placing the busbars in a vertical position within the panel. Horizontal arrangements have the busbars placed horizontally, and flat arrangements have the busbars laid out in a flat position.

What factors should be considered when selecting a 3-phase busbar arrangement?

When selecting a 3-phase busbar arrangement, several factors should be considered. These include the size and layout of the electrical panel, the type of equipment being powered, and the expected electrical load. It is important to consult the NEC and local building codes to ensure compliance.

Are there any safety considerations when installing a 3-phase busbar arrangement?

Yes, there are several safety considerations when installing a 3-phase busbar arrangement. These include proper insulation, grounding, and adequate spacing between busbars and other components to prevent electrical arcing. It is important to follow all safety guidelines and regulations to ensure the safe and efficient operation of the electrical system.

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