4 wire delta

[peetey]

I was wondering if customers ever actually use the 208 voltage (Phase to neutral) or is it just a mathematical voltage but not one actually used as a matter of practice? Any help would be appreciated. Voltages are 120/240/208.

peetey

 

[ccjersey]

I have used the 208 L-N circuit for control wiring on a 3 phase starter with a 208 VAC coil. Coil is single voltage and duty is continuous, so I didn't want to take the chance with the ~250 VAC we have line-line here. No reason for using it otherwise.

 

[waross]

The typical loads are three phase 240 Volts delta or single phase 120:240 Volts. Single phase 240 volt loads may be connected to the wild leg and to one other leg. There are sometimes capacity issues, if the wild leg is supplied from a smaller transformer in open delta for a relatively small motor load. You may have a 50 KVA transformer feeding a 200 Amp service with only a 10 KVA or 15 KVA transformer feeding the wild leg for one or two small motors.
A 4 wire delta service is not a good place to make assumptions. If you don't check the system before adding loads you may get bit in the butt.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

120/208V is used and not just mathematical. It's a 4-wire 120V to neutral. Utilization voltage is 120V, all loads connected line to neutral.

 

[davidbeach]

burnt2x, wrong system, the question is about a delta system not a wye system.

Using the 208 connection involves current in two windings, possibly all three in a closed delta. If the winding with the center tap is fully loaded, there is no capacity available on the wild leg as it needs capacity on the fully loaded winding. Better to use a 240:208 transformer if you really need 208 single phase and all you have is this delta system.

 

[waross]

An example to illustrate Davids post.
A customer has a 200 amp service fed by a 50 KVA transformer. He wishes to run a 25HP 3 phase motor. A 15 KVA transformer is added in open delta.
The motor current is 54 Amps and the transformer capacity is 62 amps. You don't want to add any more load to this transformer. Also, during motor starts the voltage drop on the wild leg will be much more than the voltage drop on the 50 KVA transformer.
4 wire delta is often used as an economical solution to unusual load combinations. The available capacity is often unsymmetrical. Don't assume.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[peetey]

I'm giving a metering class on 4 wire delta and one of the possible individual loads that may exists is a 208v load connected wild leg to neutral which yields a 208 volt 2 wire load. I just don't know if electricians wire in the breaker box for this load. It would be nice to say that it is used or not. We will be building vectors and I don't know whether to include it as a loading possibility.

peetey

 

[dpc]

Does the NEC even allow this? I've never seen it done, but I don't have the energy to look it up.

 

[PHovnanian]

peety, it appears that it is used (whether legal, proper practice, or not). So give it as an example and investigate what its impact (good or bad) is.

I've run across some pretty strange connections in my time. They are out there and having seen them in a classroom will help students recognize them in the future.

 

[davidbeach]

I'm sure that somebody somewhere uses it. In my younger days I certainly considered it before I knew better. I'd suggest mentioning it and discussing why it is 208V and not 240V or some other voltage, but not present it as a viable connection.

 

[waross]

Hi peetey;
For a metering course I would cover the wild leg-to-neutral loads, with the caveat that most of the students will never encounter this, but if someone does connect a load from the wild leg to neutral, it must be metered accurately. Accurate metering of any possible combination of loads should be demonstrated even though some connections are rarely encountered and may even be illegal in some instances.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

1. Yes it is a mathematical but real voltage available.
2. NEC does not seem to prevent it from using it (not that I know of, I haven't checked in detail). It does require to identify the high leg so that people do not connect a 120V load between the high leg and the so called "neutral".

3. It is rarely if ever used, because the whole premise of having a 4-wire delta system is that majority of the load is 3 phase, 240V and a small part is 120/240V single phase(such as a workshop). So the single phase equipment rating expected to be used is either (110-120V) or 220-240V.

If you do connect a 208V load between the high leg and N, it will work, but should not be encouraged as it is an oddity. As usggested earlier, a proper way to use a 208V load on 240V system would be to use a 240:208V transfomer, even a buck-boost type. Just the way you would do if you had a 240/120V single phase, 3 wire system.

 

[waross]

Hi Rafiq;
I have seen quite a few systems such as you describe. I also see 4 wire delta used when the majority of the load is single phase and one or two three phase motors are needed. In this case it is often open delta.
But the point is, the three phase loads and the single phase loads are generally unequal.
I saw quite a few legacy 4 wire delta systems in Central America. A large home would have possibly a 200 amp service with a single phase 50 KVA transformer feeding it. A three phase panel would be used and a smaller transformer connected in open delta to supply three phase air conditioning units.
With the advent of larger single phase condensors, most of these systems have had the teaser transformer removed and are now single phase only.
Another issue is that it is wasteful of panel spaces.
Except for one or two three phase breakers, the center pole spaces are usually unused.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Bill:

That is interesting, but as you noted the 3 phase and 1 phase loads are very unequal on such systems otherwise one or more transformers of the " 3 phase bank" will remain grossly under utilized.

 

[waross]

Hi Rafiq;
In the part of Central America that I visit, 4 wire delta is common for industrial plants. A typical installation will be three large equal transformers in delta to supply the 240 Volt motor loads with 120:240 single phase taken off one transformer for lighting and office loads. The center point is grounded and so we have the advantage of a grounded system also.
In Canada I see an open delta teaser transformer added to a single phase transformer usually to supply one or two motors.
The second transformer is typically much smaller than the main transformer. In rural areas we used to see two primary conductors run out to a farming area. There would be the normal distribution transformer for the farm house and a small transformer added in open delta to supply a three phase irrigation pump.
This arrangement saved the cost of running miles of the third phase conductor.
Almost always, though, a 4 wire delta system has an unequal balance of three phase and single phase loads.
Although delta utilization systems were more common years ago, I believe that most new installations are going wye.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[busbar]

There are thousands of utility-owned 240/120V open-delta transformer banks Way Out West. They are typically used for small dairy barns, irrigation pumps or convenience stores; i.e., limited to loads of ±20hp where two 15kVA 1ø transformers (likely smallest in a utility’s inventory) are installed. By local preference, they can be served open-delta or open-wye on the primary side. [El Paso Electric used to have good engineering/construction/loading drawings on line, but I can’t find them now.]

An important NEC/UL white-book requirement is that if a single-pole breaker on “Bø” is used for overcurrent protection, it must be rated 240 volt, and not 120V or 120/240V.