Delta xfrmr secondary: Line to Neutral Relationship 2

[bbleiler]

What is the line to neutral mathematical relationship in a delta connected transformer secondary? As in, why is the "wild leg" voltage 208 volts, when the line to line voltage is 240 volts?

 

[jghrist]

There is no 3ø neutral as such in a delta connected transformer secondary. The "wild leg" voltage to the grounded center tap is (sqrt(3)/2)·240. This can be seen by drawing an equilateral triangle where each side is 240. Calculate or measure the distance from the midpoint of one side to the opposite corner.

 

[cuky2000]

From vector diagram, the red-leg is the side opposed to 60 o triangles between the voltage phase to ground and the voltage phase-to-phase. Therefore, the high leg can be calculated multiplying the voltage line-to-neutral (120V) times 1.73 (SQRT 3).
If the high-leg is at the mid-tap connection between phase A & B the vector relation could be as follow:

V_red-leg = V_cn = V_bc<60^o – V_bn<0^o = 1.73|V_bn|<90^o
<sub>NOTES:
- Single pole breakers should not be connected to the high leg.
- Must be permanently market permanent per NEC 215-8.
- Red-leg is also known as wild leg, odd leg, high leg, and delta mid-tap.

REFERENCES:
- IEEE std 142- 1.5.2
- NEC 1999 and previous version: 210.4(D), 215-8, 230.56, 240.83 (E), 240.85, 250-5 and 384-3(E)). 408.3(E), FPN
- For 2002 NEC version: 200.6, 250.119, and 310.12.</sub>

 

[busbar]

Illustrations at

http://home.att.net/~benmiller/elecsys.htm