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Load Calculations

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rhpope

Mechanical
Mar 3, 2006
16
When calculating the load for a building, how do you spread the single and three phase loads over the incoming three phase lines? I belive the correct way for 120/208 Wye is that single phase 208 or 240 loads spread equally across two transformers (A&B, B&C, or C&A) whereas for 120/240 Delta, 208 or 240 single phase load is applied to only one transformer (A, B, or C)/phase (just as the 120 loads are only on the A & C phases). The three phase loads are spread equally across all three phases.
 
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Hello rhpope
I think you have it. Your 240 volt loads only receive 208 volts. Many small motors are now rated for 230 or 208 volts.
yours
 
In a 208y/120volt, 3-phase, 4 wire system, 120 volt loads are on one of the three transformer legs. This is a phase to neutral load. Any of the three phases can have single phase load. A 208/1-ph load is connected between two phase points so the load is distributed on two of the transformers. Three phase laods are distributed on all three phase transformers. The single phase loads should be alternated between phases so that the overall system loads are balanced.

In 120/240V/1ph/3wr system( now common in some residential and industrial distribution sectors), 120v loads are connected between one of the phases and the neutral point, 240v, loads between the two outboard phase points. In 240v/3ph services there are no 120v. loads, unless a center earth point is established.
 
Hello EEJaime
I took the original post to refer to an open delta service, or 4 wire delta.
This is a basic 120/240 volt service with a second transformer added in open delta to support thre phase loads. In some localities a third transformer is added and the service is three phase delta with one winding center tapped and grounded to support the 120 240 volt loads. Caution must be excersised as there is a "Wild" leg or phase with 208 volts to ground. This is not used for 120 volt loads and often not used for 240 volt loads on open delta services.
yours
 
waross,

I was just talking to some people at my office today about the Dela High-Leg configuration, and there are a few things I don't understand.

-If 240V is obtained between the high leg and another phase, then why would it not be used? I was originally under the impression that the high leg was only utilized for three phase loads, but one of the engineers explained to me that the high leg can also used for 240V single phase loads.
-When a winding is center tapped in a Delta configuration, aren't the 120V loads limited to 5 or 10 percent of the transformer's rating? I recall reading something along those lines in SquareD's catalog, although it could have only applied to a specific transformer. I don't even remember.

Thanks,

--
 
There are several applications of 4-wire center tapped delta.
Two of them are:
1> There is a large single-phase load and a small three-phase load must also be supplied. A small transformer is connected open delta to supply the three-phase load.
The single-phase load may be a normal 120/240 volt local distribution serving an area of residential and commercial customers. For some reason a customer requires three-phase.
It is not uncommon to see a 50 KVA or a 75 KVA or a 100 KVA transformer with a 15 KVA transformer sitting beside it to feed the open delta.
One example I saw years ago was a bowling alley. They were on a 120/240 Volt distribution system supplied with pole mounted, utility owned transformers.
They bought new automated pin-setting machinery and then discovered that the special motors were all three phase. One possible solution was for the utility to add a transformer in open delta and supply them with 4-wire delta.
A common rural installation is a three-phase irrigation pump. If the pump is in a remote location served by a single phase primary, only one additional primary line need be run.
In this case it is common practice to avoid using the "Wild" leg for any but three phase loads. It can be used for 240 volt loads but you must be aware of the loading on the smaller transformer.

2> In a three transformer closed delta system; the "Wild" leg may be used for 240 volt loads. It should not be used for line to neutral loads. The voltage is 208 Volts rather than 120 Volts.
Note; RUS standards warn that in a three transformer Wye-Delta transformer bank the primary neutral must be left floating or transformer burnout may result.
I know of no load limit on the 120/240 Volt loading. Your information may have been for a special application or may have been misunderstood.
In a full delta bank the single-phase load can safely exceed the rating of the center-tapped transformer if the three-phase load is low.
In fact if there is no three-phase load, the single-phase load on a delta bank with equal sized transformers may be double the capacity of one transformer.
That is; A bank of three 100 KVA transformers in delta.
The single-phase load on the 120/240 Volt center tapped transformer may safely be 200 KVA. Any three-phase load and any single-phase load connected to the other transformers will of course reduce the 200% figure.
Before there is a howl of protest over this statement and the 200% load figure, spend a little time researching the conversion factors for supplying single-phase power from a three-phase generator with particular attention to the double delta connection.
respectfully
 
The Square D transformer referenced is a delta transformer that happens to have a center tap on one winding; designed for three phase loads with a bit of single phase. The type of connection waross is referring to is composed of two, sometimes three, individual transformers; one of which is designed to supply to up 100% single phase load. Two very different ways of getting to something that looks the same to a voltmeter.
 
Now I understand for sure how the loads are calulated for the 120/208 wye service. Now if I were to choose the 120/240 CT Closed Delta service, then how do I calculate the loads for this as far as dividing the single phase loads across the incoming legs/phases?
 
You don't always get to make the choice. The utility may only supply one configuration,with utility owned transformers. With customer owned transformers they may only be willing to meter one configuration.
The 4 wire delta is common in many areas, in other areas it is never seen.
I am wondering why you would choose a delta connection?
What are your loads?
Basically combine your single phase loads so as to be equivalent to one three phase load. Add this to the normal three phase loads.
Subtract this from the capacity of the three phase bank.
Example;
Three 50 KVA transformers
Three phase loads = 75 kva.
Single phase loads combined as one three phase load = 30 KVA
Total three phase load = 105 KVA
Excess capacity = 45 KVA. or 15 KVA per transformer.
The center tapped transformer has 15 KVA of capacity available for single phase loads. The other two transformers will contribute an additional 15 KVA of capacity to the third transformer.
The unbalanced single phase loads in this example may be 30 KVA without over loading the transformer bank. The transformer bank capacity with this combined loading will be;
105 KVA three phase plus 30 KVA unbalanced single phase = 135 KVA on the 150 KVA bank.
A factor that I never see mentioned in load calculations is the power factor of the individual loads. If the loads have widely varying power factors the actual current will be less than indicated by the simple sum. Rather than calculate this, I suggest that you be aware of it and consider it an error on the safe side.
For widely differing loading or differing transformer sizes, do not allow the single phase load to exceed twice the available capacity of the smallest transformer or overloading of the smallest transformer may result.
I prefer Wye.
davidbeach, I spent some time lost in the Square D web site, trying to research your comments. I will be pleased if you can expand on your statments and or give a link where I can research the information. I remember hearing something similar years ago but thought I had misunderstood it, or that it had been mis-stated. Now I am curious. Am I correct in assuming that this is not a factor in building services?
Thank you davidbeach.
respectfully
 
Ran into the 5% limit on 120V loads when we bought a 208Y/240D 3 phase step up transformer for a wye connected backup generator on a 3 phase 4 wire 120/240 delta service. We connected the single phase load center directly off the generator output (using 2 transfer switches). It quickly became overly complicated. Would have been much better to have the correct generator in the first place.

I believe the limit is on 3phase unit transformers not the transformer banks.
 
Hello ccjersey
If you have a 12 lead generator you can reconnect it for 4 wire delta. No performance problems. Your KVA rating will jump up about 15% as well.
If you have a 10 lead generator, you're out of luck.
yours
 
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