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#129015 06/23/04 06:54 AM
Joined: Dec 2001
Posts: 201
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ayrton Offline OP
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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%

#129016 06/23/04 09:23 AM
Joined: Sep 2003
Posts: 650
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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

#129017 06/23/04 10:36 AM
Joined: Dec 2001
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ayrton Offline OP
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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).]

#129018 06/23/04 11:27 AM
Joined: Sep 2003
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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

#129019 06/23/04 12:11 PM
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ayrton Offline OP
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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??

#129020 06/23/04 02:07 PM
Joined: Sep 2003
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Actually, a delta connection simply has your three secondary coils in a triangle (hopefully phased correctly [Linked Image]. 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

#129021 06/23/04 02:27 PM
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ayrton Offline OP
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It is a high leg system.

So it seems to me the transformer is not necessary

#129022 06/23/04 04:35 PM
Joined: Apr 2004
Posts: 349
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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


There are 10 types of people. Those who know binary, and those who don't.
#129023 06/23/04 07:20 PM
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ayrton Offline OP
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What is the difference between 3wire delta and 4wire high leg. I thought it was the same thing.

#129024 06/23/04 09:23 PM
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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

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