Boosting 208V to 240V

LarryFine said:
Then use N' instead of N. Perhaps clearly label the outlet, or build it into or onto the load/equipment.
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I wouldn't wire a receptacle with that, as the N terminal is expected to be at ground voltage. But for hardwired equipment, where you've carefully checked that it's OK for the N terminal to be at 60V to ground, you could do it.

What you need to do this exactly with a single transformer is a 104x208V : 16x32V unit. Then you can wire the coils as 16V, 104V, 104V, and 16V in series. A 120x240V : 18x36V transformer would be close.

Cheers, Wayne
 
If 230V is within acceptable voltage range for 3 wire single phase equipment would it make sense to just boost to 115/230?
EDIT or I guess that would would be more like 220/127?
 
Little Bill said:
Sorry, posted in a hurry and forgot to say it is single phase. So now, how to wire the neutral?
The load is a grinder pump for a septic,or sewer system.
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Well the pump won't need a neutral, so what does? Is this that septic deal with high water alarm circuit? If so, I fail to see the issue. Deliver legs from xfmr to pump, then just use an untapped leg and neutral for your 120V loads. Am I missing something here? Circuit diagrams sure help.
 
fastline said:
Well the pump won't need a neutral, so what does? Is this that septic deal with high water alarm circuit? If so, I fail to see the issue. Deliver legs from xfmr to pump, then just use an untapped leg and neutral for your 120V loads. Am I missing something here? Circuit diagrams sure help.
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As I mentioned in my OP, I'm just helping a friend and just by phone. I haven't seen the pump, or diagrams. I'm assuming the neutral is for a control panel.
 
LarryFine said:
Not the way I'm talking about. I'm saying to wire each secondary to opposite ends of the primary.

Each 104v half gets one half of the boost, maintaining the secondary's center tap centered-ness.
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The more I think about it no it will be the mentioned ~134. Draw it out to scale- two lines of 120 units intersecting at a 120 degree angle. Now add 16 more units to them and they are both 136 units long. Is 240 between the open ends though.

To get 120 to the common point you need a 180 degree angle and 120 length to each side, just like a true single phase winding.
 
kwired said:
The more I think about it no it will be the mentioned ~134. Draw it out to scale- two lines of 120 units intersecting at a 120 degree angle. Now add 16 more units to them and they are both 136 units long. Is 240 between the open ends though.

To get 120 to the common point you need a 180 degree angle and 120 length to each side, just like a true single phase winding.
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I'm lost in the conversation but what in the world does 120* have to do with anything here? Load is single phase. Any single phase, whether in a 1P or 3P service is only a single sine wave. There is no phase offset. but if you split the phase, it would be 180*.

If someone actually had even a pencil to show how a load was configured, it might be solvable. Otherwise, this is like a "how much does the moon weigh?" sort of question. 120V is already present between neutral and any selected phase currently. So there are ways to work out the potential issue.
 
kwired said:
The more I think about it no it will be the mentioned ~134. Draw it out to scale- two lines of 120 units intersecting at a 120 degree angle. Now add 16 more units to them and they are both 136 units long. Is 240 between the open ends though.
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Your description corresponds to using two separate single coil boost autotransformers to boost L1 relative to N and separately L2 relative to N to get 2 of 3 legs of a 240Y/138V system.

If you use a single coil boost autotransformer connected to L1 and L2 from a 208Y/120 system, the voltages you get will all lie on a line that goes through both L1 and L2 in voltage phase space. That line does not pass through the 208Y/120 neutral (and its closest point is the midpoint of L1-L2, which is 60V away from that neutral).

kwired said:
If not connecting the center tap to source neutral, you end up with three ungrounded conductors on your "outputs". Doesn't that go against what it says in 210.9 that was mentioned in post 8?
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In post 8, I commented that Exception 1 would apply, as the Exception is not limited to 2-wire circuits.

Cheers, Wayne
 
kwired said:
The more I think about it no it will be the mentioned ~134. Draw it out to scale- two lines of 120 units intersecting at a 120 degree angle. Now add 16 more units to them and they are both 136 units long. Is 240 between the open ends though.

To get 120 to the common point you need a 180 degree angle and 120 length to each side, just like a true single phase winding.
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We're looking at 1ph 208v with two lines and no neutral, it would be 180 degrees, not 120.

Remember, we're talking about one transformer; an open delta B-B uses two transformers.
 
kwired said:
If not connecting the center tap to source neutral, you end up with three ungrounded conductors on your "outputs". Doesn't that go against what it says in 210.9 that was mentioned in post 8?
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Exception No. 1: An autotransformer shall be permitted without the connection to a grounded conductor where transforming from a nominal 208 volts to a nominal 240-volt supply or similarly from 240 volts to 208 volts.
 
LarryFine said:
We're looking at 1ph 208v with two lines and no neutral, it would be 180 degrees, not 120.
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No.
You cannot have 120/208 without having a 120° phase shift, if it didn't it would be 120/240

There is no clean/easy way to deal with the neutral of a 120/208 not being at the same potential as that of a 120/240 when using buck-boost transformers.
 
Little Bill said:
As I mentioned in my OP, I'm just helping a friend and just by phone. I haven't seen the pump, or diagrams. I'm assuming the neutral is for a control panel.
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Ok. Forget all the complicated clever stuff. You have several complicated choices:
Use a normal boost arrangement to take 208 to 240, but this gives you one leg at 120V and the other leg 149V to neutral. The complication is that now you have to absolutely guarantee that you don't feed the 120V portion of the load with 149V.
The more balanced boost approach that gives you 240V line-line, but about 135V L-N on both legs. If your 120V loads can tolerate 135V....
The clever center tapped autotransformer arrangement, which gives you true 120:0:120, but relative to a 'neutral' that is floating at 60V relative to ground. Most 'neutral' loads are assumed to be connected to a grounded conductor.

But you are helping a friend over the phone, not there in person to make sure that all the complications and the implications of those complications are understood. Miss one of the subtle details (oh, my control board was on the wrong leg, oh the signal circuit from someplace else is connected to the neutral bus....)

In this case, simplicity trumps elegance. You need to use a true 208V:120/240V transformer, not a boost arrangement. This is a physically bigger transformer, and less efficient than a buck boost arrangement. But it isn't complicated. The other simple approach is to first modify the load so it only uses 240V L-L (say by running whatever 120V loads via a control transformer) and then use an ordinary boost arrangement to get 240V.
 
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