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what is a high leg in delta system? how to color it?

Allan2008

可以用中文问吗

Allan2008

A high leg delta (also known as wild-leg or red-leg delta) is a type of electrical service connection sometimes found in older three-phase electric power installations. It was used where both lighting and three-phase motor loads were to be fed from the same distribution system.
Center Tapped Delta Transformer
: O0 J' i( F% C5 u" C9 v& ECenter Tapped Delta Transformer Voltages

" a+ Y! [! S# k# R+ K/ d# qThis type of service is supplied by a transformer having four wires coming out of the secondary: the three phases, plus a neutral that is used to center-tap one of the windings. The voltages between the three phases are the same in magnitude, however the voltage magnitudes between a particular phase and the neutral vary. The phase-to-neutral voltage of two of the phases will be half of the phase-to-phase voltage. The remaining phase-to-neutral voltage will be √3 times half the phase-to-phase voltage. Typically, the transformer is connected such that the 'B' phase is the 'high' leg. According to Article 110.15 of the 2005 National Electrical Code, panelboards connected to this type of transformer must explicitly identify the high leg, preferably by coloring it orange.
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Contents
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; W8 e) }% o2 o; d    * 1 Example
    * 2 Advantages
% |5 H7 }; M. l( Q3 W    * 3 Disadvantages
% [) q+ L2 j! G( K) ^- O    * 4 References

4 r4 L0 A! Z" Q, O[edit] Example
Phasor diagram showing three 240V phases, and center-tapped 120V lines.

5 u7 Y6 ?3 l6 P5 P* cConsider the low voltage side of a 480/240 V high leg delta connected transformer, where the 'B' phase is the 'high' leg. The line to line voltages are all the same:
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: N, w; }  I# b% O  l2 S4 Y    Vab = Vbc = Vac = 240V
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) e9 i+ M7 @4 O3 |7 k2 w, NBecause the winding between the 'A' and 'C' phases is center-tapped, the line-to-neutral voltages for these phases are as follows:
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# b. k0 @$ F6 \4 y, g    Van = Vcn = 120V

But the phase-neutral voltage for the 'B' phase is different:
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3 [" F( y' ?  L  g; g    V_{bn} = 120\sqrt{3}\ V \approx 208\ V

This can be proven by writing a KVL equation starting from the grounded neutral:

    0 + 120 \angle 0^\circ + 240 \angle 120^\circ = 120\sqrt{3} \angle 90^\circ
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Note: Writing the KVL equation going the other way, the same magnitude is found, though the phase angle will of course be different.
# }1 J, d& ]+ \0 p' E% k% {6 z[edit] Advantages

& G# H) A" I; h. q& H6 hThis type of services is usually supplied using 240V line-to-line and 120V phase to neutral. In some ways, the high leg delta service provides the best of both worlds: a line-to-line voltage that is higher than the usual 208V that most three-phase services have, and a line-to-neutral voltage (on two of the phases) sufficient for connecting appliances and lighting. Thus, large pieces of equipment will draw less current than with 208V, requiring smaller wire and breaker sizes. Lights and appliances requiring 120V can be connected to phases 'A' and 'C' without requiring an additional step-down transformer.
[edit] Disadvantages
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6 w4 Y0 x# G0 S! L5 V2 tSince one phase-to-neutral voltage (phase 'B') is higher than the others, no single phase loads can be connected to this phase. This eliminates the ability to use one third of the breakers in a panel for single-phase loads. Further, the lack of loading on the high leg will result in an unbalanced load. However, many new fluorescent ceiling fixtures utilize ballasts which automatically sense line voltage. These fixtures are designed to operate on voltages from 110 to 277 volts. Use of modern fixtures in conjunction with code compliant wiring may allow selected lighting to be placed on the high leg, thereby eliminating this drawback.