2P breakers on 208Y/120 panelboard?

[biofueler]

just curious, I have some used 208/120 (and 480/277) panelboards with 2 pole breakers.

other than for loads you would want to turn on simultaneously (lighting loads for example), I'm trying to think what sort of equipment, if any, would use that sort of power.

 

[wareagle]

Any 208 volt load such as a water heater, electric range,
dryer, 208 single phase motor and on and on.

 

[biofueler]

I see a lot of 240V equipment (water heaters, well pumps, AC, etc, but have never seen 208V single phase equipment. does most 240v equipment run on this stuff?

 

[biofueler]

ok, after a google search, I guess there are 208V clothes dryers commonly available. I was searching hard for a 208V well pump, but never could find one.

on a panelboard diagram, would you calculate the kva per leg as follows:

say a 208V single-phase dryer @ 20A

it would be 20A x 277V/1.73 = 2.4KVA per leg

or alternatively 20A x 120V = 2.4KVA per leg

 

[rbulsara]

No and yes.

Where does 277V comes from? That equation is incorrect and the answer is coincidental.

The second part is correct. I am not sure you caught on it or not, the actual kVA of the 208V machine is 208*20A=4.16kVA. or 2.08 kVA per phase. (and not 2.4+2.4=4.8). 208V for single phase machine is still a single phase and the equation for single phase holds true.

But the 120V volts L-N voltage is 30 degrees out of phase with 208V line to line voltage in a 208Y/120V system. The effective output will be 0.866 times the apparent kVA. Or in other words to provide 2.08kVA between lines, the per phase kvA, in this (wye) configuration,will have to be 2.08/0.866= 2.4kVA and hence your original answer 120*20=2.4kVA per phase is correct.

This , two phases of a wye system, behaves just as a 3 phase open delta connection would behave.

 

[biofueler]

Oops, that was a typo had 277 on my mind, I meant 208/1.73*20=2.4kva

thanks for the confirmation!

 

[alehman]

A lot of 240V single-phase equipment will work on 208V. Some 480V single phase equipment includes lighting, heaters, welders.

 

[waross]

The apartment building that I am presently living in is supplied with 120/208 volts. Tens of thousands of other apartment buildings in Canada have a similar supply.
240 volt heaters, electric ranges, clothes driers and other 240 volt equipment is often used on the 208 volt circuits.
The electric stove is a little slower heating than the stove in my mothers house or my cabin, both of which are supplied with 120/240 volts.
Many clothes driers use 120 volts for the motor so the motor gets 120 volts regardless of the system.
Depending on the type of heat control on some electric ranges, the elements may be fed 120 volts at some lower heat settings. The voltage that is applied to the element is the voltage that must be used in the KVA/KW calculations.
You won't often find motors rated for 240 volts or 208 volts. The common motor ratings are 230 volts or 200 volts for use on 240 volts and 208 volts respectively.

This , two phases of a wye system, behaves just as a 3 phase open delta connection would behave.

Would you care to correct this statement?

respectfully

 

[rbulsara]

waross:

Open delta system transformers also have two voltages applied that are 120 degree out of phase. Therefore their capacity is also limited to 0.866 times their nameplate rating. The phase currents are out of phase with line currents there.

If a single phase load is applied across open terminals of a open delta system, it will have the same effect as mentioned in this discussion. Draw a diagram and you will see.

 

[rbulsara]

Or other way to explain, as evident by the example in this post, is that to serve a 4.16kVA of load (208*20A) from two phases of a wye system you will need the two phase sources (transforners) rated 2.4kVA each. Totaling to 4.8kVA.

4.8*0.866 is 4.16, same as it would in a open delta system.

 

[rbulsara]

To further, clairfy 4.8*0.866 is 4.16, same as it would if it were a 208V open delta system.

 

[waross]

With two legs of wye system you may have voltages of 120V, 120V, and 208V. The included angles of the diagram will be 30deg., 30deg., and 120deg.
With the same transformers connected in open delta, you will have voltages of 120V, 120V, and 120V. The included angles of the diagram will be 60deg., 60deg., and 60deg.
For open delta service, the transformers are usually reconnected for 240 volts secondary to yield 240V, 240V, and 240V.
"Draw a diagram and you will see."
With two phases (and of course three phases) of a wye system, like terminals of the transformer secondaries will be connected together (X2 and X2). The vector diagram will show an included angle of 120 degrees.
If these transformers are re-connected in open delta, the polarity of one transformer will be reversed, so that X2 of one transformer is now connected to X1 of the other transformer. The included angle is now 60 degrees.

respectfully.

 

[rbulsara]

waross:

Just take a deep breath. If you try to feed a single phase load with two transformers that are part of a three phase system, the results will be the same, whether the original 3 phase source consisted of a wye connected bank or a delta.

Think of this source as a black box, containing three transformers of a 3 phase system. You connect a single phase load to two of the terminals. Now this black box could have a wye-connected bank inside or a delta. In either case, you take one transformer away to which the load is not connected, the end result will be the same. In each case both remaining transfromer will assume the extra load by the same percentage.

 

[rbulsara]

waross:

The key difference: I did not say the same transformers!!!

 

[waross]

OK
respectfully

 

[waross]

I am confused, though.
I considered a 2.08 kva load, on 208 volts.
I calculated 10 amps.
If I am getting the 208 volts from two transformers of a wye system, the transformer voltages will be 120 volts.
The kva load of each transformer will be 1.2 kva.
That gives me a ratio of 2.08/2.4=.867
So far so good.
To get 208 volts from an open delta bank, we will have to use 208 volt transformers.
The kva load of each transformer will be 10 amps x 208 volts or 2.08 kva.
That gives me a ratio of 2.08/(2.08+2.08)=.5
Part of the discrepancy is the fact that this is a single phase load connected across an open delta.
The accepted figure (58%) for the reduction of capacity of a three phase bank used on open delta is based on a balanced three phase load and a loss of one third of the transformer capacity.
Thus a 150kva bank on open delta will have a capacity of 58%x150kva or 86.6kva. (Three 50kva transformers with one transformer removed).
A two transformer bank consisting of two 50kva transformers in open delta will have a capacity of 86.6% of 100kva or 86.6kva.
THESE FACTORS ARE ONLY VALID FOR BALANCED THREE PHASE LOADS.
Single phase loads, particularly loads across the open delta, must be calculated individually.
In your example of the black box, the transformer currents do not change when the unused transformer is removed from a wye system.
In a delta system, all three transformers contribute to the support of a single phase load and the transformer currents
do change when one transformer is removed.
respectfully

 

[rbulsara]

Waross:

I may get back when I have more time. The original load mentioned by OP was 208V Single phase dryer at 20A. So it is 4.16kVA.

You are correct about no change in two wye xfmrs if a third one is removed, but it still does not change the way loads will behave.