ECN Forum Forums General Topics of the Trade General Discussion Area Wild Leg

Although I've never seen it used as such, would there be anything to prohibit the use of the 208-grounded connection for a 208 1Ø load?? (Provided one used a single pole C/B that was rated for the higher voltage; ie 277)

The "high leg" of a 120/240 V 3Ø 4-wire delta source can be used to supply a 1 Ø 208 V load, but one wastes the kVA capacity of the power transformers. 1 kVA of 208 V load uses 1.73 kVA of the transformers capacity.

Al

If the 1ph 208v is not used, then why is it set up this way?

aldav53,
It is used for single and three phase 240 volt loads. It is just not normally used for line to neutral loads.
Don

Aldav53,
It's not set up that way, it's the only way to create a neutral with a delta configuration. The early history of commercial electrical use (AC) did not use a ground, as such, a bit later having already set up the configuration as Delta, manufacturers began making 110 volt equipment (even though this is REALLY a light explanation, no pun intended) so we began the center tap to create a stable reference point for 110 volt equipment.

Household stuff was not the first use remember, and 3phase is the economic choice of champions, households began being connected a bit later, from a distribution standpoint, you still fed the commercial with ungrounded (or at least, un-neutraled) 3 phase delta, and could tap the same transformer bank with the center tap for it's 110 volt loads.

The Wye, of star, was a later development, but it is not 120 degrees out of rotation like the delta. That can create it's own problems such as not quite as economical, nor pure on an oscilloscope, but is a great compromise and the one we are currently comfortable with.

That's also the reason you see so little being fed delta for the last 30-40 years. Delta primary is common, but delta secondary is pretty rare by comparison. Why delta primary? You don't have to have a ground with it, even though most utilities, at least in my area, utilize a grounded delta, but that is for lightning protection, not neutral creation. This also sets up it's own problems such as ground loops between primary and secondary, but again is a good compromise and one of the reasons power is so much more reliable than when I was.......er, younger.

Aldav53,

The way it was explained to me when I was a "young feller" went something like this.

In the 30's and 40's the standard voltage for 3-phase equipment in factories, mostly motors, was 240 volts. In those days transformers were much less reliable than today. The organic insulations in use then had a shorter life, and would last somewhat longer at lower voltages.

Delta-delta connections were almost always used because if one transformer of a 3-phase bank failed, the bank could be converted to open-delta by disconnecting the defective transformer, permitting service to the customer to be maintained (with reduced load capacity) until the new transformer could be installed.

The most practical way to supply the 120 volt lighting and receptacle loads in the building was to center-tap one of the transformers.
This one normally had a higher kva rating than the other two, as it supplied all of the single phase loads, plus one third of the three phase loads.

Electricians all were familiar with the "high leg" and were careful to not connect any 120 volt loads to it. The high leg buss was always colored orange to identify it.
There were very few 208 volt loads in existance at that time.

Ed

[This message has been edited by electric-ed (edited 08-04-2002).]

Thats for all the great info, I will print this out and keep it.
Al D