I have a building with two services. One 3 ph 240v Delta, the other single phase.
The single phase is 400a (all irrelavent to this topic) which has two 200a panels which are FULLLLLLL!
Three phase service is mostly 2pole and three pole breakers in a 200a panel.
Trailer was attached to building in rear for more space. Has its own panel.
No room from the single phase service to feed so the sparky at the time installed a 240/480 hi to 120/240 lo transformer.
The nameplate doesnt say it, but I assume it is delta to wye, because hi side has no neutral tap. 2 legs with chassis ground run. Lo side has neutral tap which goes to the trailer.
How much neutral though do we actually have? It cant be 100%

If the primary of the transformer is fed with two phases, then it _should_ be a _single_ phase transformer.

The neutral on the secondary is derived at the secondary. The rating of the secondary neutral terminal should be the same as the secondary phase terminals; it is connected to the same secondary coil. The conductor between the transformer and the panel might be reduced in size...

-Jon

Yes I know it is single phase. I_ dont_ recall_ stating_ it_ wasnt.

Why even use the transformer, when the Delta service has a grounded conductor??

[This message has been edited by ayrton (edited 06-23-2004).]

I may need a bit of educating on this topic, but I've never seen a single phase transformer with a delta primary. How do you have a delta primary with only two phase inputs?

However I'm one to talk; I regularly work with the '17 phase analog to the delta connection', so I'm willing to be open to a single phase connection that could be reasonably called 'delta'; could you explain?

-Jon

You are right, I should have though before I typed. Delta is three coils in a triangl with a ground tap. You are right. It is only single phase. The primary is fed from a delta service panel.
With a delta service, what is the % useage of the neutral.
I am not understanding the purpose of installing a transformer for the same voltage, unless therte was no neutral on the primary service.
Does that make sense??

Actually, a delta connection simply has your three secondary coils in a triangle (hopefully phased correctly . There is _no_ ground connection required for a delta.

There are three common possibilities.

1) Corner grounded delta, where one of the phase conductors is at ground potential.

2) High leg delta, where one of the secondary coils is center tapped, and the center tap grounded.

3) Ungrounded delta. No ground connection at all. Phase to ground faults don't produce significant current flow, but failures can lead to extremely high voltages across insulation.

In the case of 1 or 3 the transformer is necessary to provide the grounded neutral for single phase loads. In the case of 2 I don't know why the transformer would be required.

-Jon

It is a high leg system.

So it seems to me the transformer is not necessary

Lemme see if I'm following this. The 3 phase service is 120/240V 3 phase 4-Wire (hi leg) Delta? Most times I see two services, the 3 phase service is a straight 3-wire delta, not a 4-wire hi leg.

Nevertheless, perhaps the 120V portion of the 3-phase service (that would be phases A to C around here) is max'd out also. So, as an alternative, you could take phases A & B -or- B & C to a 240V primary and derive another 120/240V single phase source out of it.

That's about the only reason I can think of for installing that transformer. The other part of your question regarding delta to wye does not really apply to a single phase transformer. The primary side is 2-wire (2 hots), the secondary is 3-wire (2 hots + a neutral). The neutral has the same rating as either of the two hots. Remember, the current in the neutral is equal to the difference in the currents in the 2 hot wires. So, IIF the hot sides were rated at, say, 50 amps, the neutral is also. If both hots are carrying 50 amps, the neutral is coasting near 0. If one hot is carrying 50 amps and the other hot is carrying only 10 amps (badly unballanced), the neutral is carrying 40 amps.

Hope this helps some,

Radar

What is the difference between 3wire delta and 4wire high leg. I thought it was the same thing.

4 wire high leg has one of the transformer secondaries center tapped, with the center tap grounded. 3 wire delta does not have the center tapped secondary.

If you have 240V service, then 4 wire delta gives you 240V phase to phase, 120V from the A and C phase terminals to the grounded center tap, and 208V from phase B (the high leg) to the grounded center tap.

With 240V service, 3 wire delta gives you 240V from phase to phase, with no other voltages available.

-Jon

To summarize, there are 2 flavors of a 240V Delta:

240V 3-Phase 3-Wire (Straight delta)

120/240V 3-Phase, 4-Wire (Hi-Leg Delta)

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The 208 Volts that coincidentally exists between phase B and neutral (the ki-leg) is NEVER usable. No 1P CB's on phase B.

Radar

Other comments from E-C.net members are welcomed if you see this as inappropriate advice...

Given no grounded-circuit conductor from the serving transformer connected to a neutral bus in the three-phase panel, it is not usable in 4-wire delta service, and not intended to serve 120V branch circuits. In basic terms, a 240V-primary transformer is needed to furnish 120/240V 1ø 3w (connected to two phases in the 3ø panel.)




[This message has been edited by Bjarney (edited 06-24-2004).]

That makes sense BJarney. A grounded conductor was pulled with the three phases.
Phase to phase is 240v Cphase to ground is in the 208 neighborhood.
I was confused with the xfmr installation in the beginning because I wasnt 100% on the Delta. It is clear now the other sparky wasnt either.
Most installations I see are 4wire w/ high leg.
Some reason though they only pulled a #2 with the 500's for Grounded conductor. Really limits the single phase circuits unless size is upgraded.