Bob,
What you have is a 240/120 Volt, 3-Phase, 4-Wire grounded delta service.
You did not state what the phase-to-phase voltage is but it probably is 240
Volts. Not knowing where you are located you have what we would call out
west a "Farmers Service". Years ago they provided that service to farmers
so they could have 120/240 Volt service for the normal residential
equipment, dryers, ranges, air conditioners, etc. With this you would get
120 Volts from either phase A or C too neutral. They then had 3-phase delta
available for irrigation system pumps and heavier equipment.
You hardly ever see this service in modern facilities because it creates an
unbalance for the utility company. It takes three single-phase transformers
to provide this type of service. All three transformers are connected in a
loop creating a delta service. Consider the three connection points between
each transformer as Phase A, B and C. The mid point of one of the
transformers (normally the "B" transformer) has a terminal for grounding the
mid point of the transformer coil. Between the center tap and each end of
this transformer you get the 120 Volt service, and between each end of the
transformer you get the 240 Volt, single-phase service. Normally this is
what is delivered to a typical residential service. Now measuring across
either of the other two transformers you will also get 240 Volt. While
using all three transformers you get the three-phase delta service,
providing 240 Volt phase to phase. The National Electrical Code requires
that all panels with this type of service have the high phase marked either
like you indicated "High Leg" or colored orange or similar readily
identifiable mark. Between either of the other two phases (A and C) and the
neutral of the high leg transformer you will get 120 x 1.73 or 208 Volts.
Quite often you will have a separate panel for the 3-phase loads and then
have a separate panel for the single-phase loads.
Most new facilities will now use what is called a 208Y/120 Volt, 3-Phase,
4-Wire service and if it is a larger service it will be a 480Y/277Volt,
3-Phase, 4-Wire service. The advantage for these types of services is that
all voltages are equal between neutral and any of the three phases.
Bob, hopefully you have been able to follow this. If you have any
questions, send me some mail direct and I will be glad to help.
73,
Roger Huntley PE, W7VV
-----Original Message-----
From: owner-amps@contesting.com [mailto:owner-amps@contesting.com]On Behalf
Of Bob Duckworth
Sent: Monday, April 09, 2001 2:25 PM
To: amps@contesting.com
Subject: Re: [AMPS] Re:230 /115 volts QUIZ.
Who knows about a "high leg" on a three phase system?
Have this at my shop.
Three phase (4 wire plus ground).
One of three phase is marked "high leg".
Between the other two phases and neutral, I see 120VAC.
Anyone know what I should be seeing between the "high leg" phase and the
other wires?
-bob
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