Panelboard Busbars NEC 408

[mityeltu]

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.

What am I missing here? A 3-phase system that has a higher voltage on one phase? How is this possible? What am I missing? I thought one of the points of a 3-phase system was to have 2 distinct voltage levels (phase-phase, phase-gnd). How can I have a phase-gnd voltage that is different on only ONE phase?

 

[don_resqcapt19]

You have a high leg on a 3 phase, 4 wire delta system. Take a look at this drawing.

 

[mityeltu]

I have seen a similar drawing, but I can't igure out how to get the voltage. If I have a 3-phase distribution line at 7.62kv with this 3phase transformer, each phase should be 7.62k/240. That will give me the 240V p-p and the 120 p-g. The center tap though makes it kind of like an autotransformer, but not really.

I'm confused. I can't even figuer out how to analyze this thing. What is the mathematical relation here?

208V is p-p on a 3p 120V wye system (120*sqrt(3)), but this is confusing.

 

[masterinbama]

Here's a 120/240 Delta transformer bank on a project I am working on.

 

[jim dungar]

I have seen a similar drawing, but I can't igure out how to get the voltage. If I have a 3-phase distribution line at 7.62kv with this 3phase transformer, each phase should be 7.62k/240. That will give me the 240V p-p and the 120 p-g. The center tap though makes it kind of like an autotransformer, but not really.

I'm confused. I can't even figuer out how to analyze this thing. What is the mathematical relation here?

208V is p-p on a 3p 120V wye system (120*sqrt(3)), but this is confusing.

First, a 240/120 3PH 4W delta system has nothing to do with a wye configuration so please stop trying to convert p-p voltage to p-g using the factor 1.732.

Now, what you have left is (3) single phase transformers connected into a delta. One of the these transformers has a center tap which provides 120/240 (this is the big one in masterinbama's picture). Working around the delta arrangement gives you these:
p-p voltages
A-B = 240V
B-C = 240V
C-A = 240V
and if the center tapped transformer is connected A-C:
A-N = 120V
C-N = 120V
B-N = 208V

Now the confusing part, the voltage B-N is 208V, not because of the p-p and p-n relationships in a wye, but rather because of the Pythagorean Theorem. If you have an equilateral triangle (a delta) and you bisect one leg (the center tap), the angle from the bisected point to the opposite corner (phase B) will be 90?. Solving "a? + b? = c?" where a = 120 and c = 240, gives us b = 208.

 

[iwire]

As a guy that does not know the Pythagorean Theorem I look at this bottom diagram

and notice that neutral is only half a winding away from A and C but it is one and half windings from B. That, in my mind explains why the voltage from N to A and C are different from N to B.

 

[mityeltu]

I have been familiar (been a long tim) with vector addition etc., so applying Pythagorean's Theorem should have been second nature. Thank you. Now, does the connection make any difference? Geometrically (d-d or y-d), I would say not, but electrically, that doesn't make sense.

If I have a y-d, the p-p voltage changes based on the turns ratio and sqrt(3). So, would I have to have a different transformer ratio to achieve the same result? Based on my transformers being 7.62/240-120, having them connected strictly y-d gives me a delta p-p of 139V, right? So with one leg center-tapped, the voltages would be 139/120 (a little weird that one).

 

[jim dungar]

I have been familiar (been a long tim) with vector addition etc., so applying Pythagorean's Theorem should have been second nature. Thank you. Now, does the connection make any difference? Geometrically (d-d or y-d), I would say not, but electrically, that doesn't make sense.

If I have a y-d, the p-p voltage changes based on the turns ratio and sqrt(3). So, would I have to have a different transformer ratio to achieve the same result? Based on my transformers being 7.62/240-120, having them connected strictly y-d gives me a delta p-p of 139V, right? So with one leg center-tapped, the voltages would be 139/120 (a little weird that one).

The primary being connected in a delta or a wye configuration has no affect on the secondary connection, it only impacts the voltage across the secondary windings, so for your transformer you will end up with (3) secondary windings of 120/240V each. Your secondary will then be connected in a delta arrangement of 240V p-p. The center tap of the winding connected between A-C will be used to supply your 120V p-g voltage (this connection will look like a standard 120/240V single phase system), the center taps of windings A-B and B-C will not be used.