240 volt 3 phase delta

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lanceleve

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i have a 150kva , 208 volt primary and 240 volt secondary transformer 3 phase
is it ok to ground the mid point tap to create a grounded system?
all loads on 240 volt side are 3p3w
 
lanceleve said:
i have a 150kva , 208 volt primary and 240 volt secondary transformer 3 phase
is it ok to ground the mid point tap to create a grounded system?
all loads on 240 volt side are 3p3w
Click to expand...

Yes it is OK to ground the mid-point tap of a delta secondary and feed only 3 phase loads.

Welcome to the forum.

Chris
 
081106-1424 EST

What is a midpoint tap?

Is this the center tap of a 240 secondary, or is it the center point of Y that was never mentioned?

.
 
gar said:
081106-1424 EST

What is a midpoint tap?

Is this the center tap of a 240 secondary, or is it the center point of Y that was never mentioned?

.
Click to expand...

The mid point tap would be a tap at the mid point of one winding of a 240 volt delta transformer.

Chris
 
If you are so inclined, you may corner ground the system, use full voltage rated 2 pole circuit breakers, and a full voltage rated single phase panelboard. The 2 pole circuit breakers would actually control the two phases while the grounded phase (white conductor) would be the third phase. Your installation would cost less and the results would still be code compliant. :)
 
lanceleve said:
i have a 150kva , 208 volt primary and 240 volt secondary transformer 3 phase
is it ok to ground the mid point tap to create a grounded system?
all loads on 240 volt side are 3p3w
Click to expand...
Obviously, from the above responses, we need a bit more info.

The 208v primary should have a common conductor which should be left floating; i.e., wired as if it were a Delta.

The 240v secondary should be a Delta, which should not have a common conductor. What is this "mid point tap?"
 
Larry,
The midpoint tap is where the grounding and grounded conductors are conected for a high leg system.
 
LarryFine said:
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The 240v secondary should be a Delta, which should not have a common conductor. What is this "mid point tap?"
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It is likely a 240/120 3? 4W secondary (delta high leg system). Typical terminals are X1, X2, X3, and X4, where the latter is the midpoint tap of one winding.

The transformer can be used for a 240 3? 3W system (delta system) by not using the X4 terminal, and code does not require it to be grounded when not used. See 250.20(B)(3).
 
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charlie said:
If you are so inclined, you may corner ground the system, use full voltage rated 2 pole circuit breakers, and a full voltage rated single phase panelboard. The 2 pole circuit breakers would actually control the two phases while the grounded phase (white conductor) would be the third phase. Your installation would cost less and the results would still be code compliant. :)
Click to expand...


i could corner ground, but secondary is already connected to panel with 3 pole 120/240 cb
 
can the homemade 3 light ground detector system for ungrounded delta be on 24/7 without any ill effects on the system?
 
I agree with Charlie (see post 7) but would caution that if this is done, the panel should be clearly marked as to what has been done. It would be too easy for someone who is unaware of the system to assume it is 240V single phase and end up with disaster. I would avoid this type of hookup unless certain of who would be servicing it.
 
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charlie said:
I believe it is permissible to open the grounded conductor with a common trip 3 pole circuit breaker. :smile:
Click to expand...

in the Soares Book on Grounding it says the breakers have to be rated for straight 240 not 120/240 and cannot be installed on a 3 pole cb.
the book shows a diagram of two phases on a 2 pole main and one on the ground/neutral bar.seems kind of crazy but that is how they show it.
the system i have installed is set up with 3 pole cb's.
 
lanceleve said:
in the Soares Book on Grounding it says the breakers have to be rated for straight 240 not 120/240 and cannot be installed on a 3 pole cb.
the book shows a diagram of two phases on a 2 pole main and one on the ground/neutral bar.seems kind of crazy but that is how they show it.
the system i have installed is set up with 3 pole cb's.
Click to expand...

I know of no 3-pole breaker that is not rated for straight 240V. The concern specifically for 2-pole breakers is, the commonly available ones are only rated for 120V to ground.

The Soares book is not showing how it must be done but rather how to do it with 2-pole equipment.

Check with your the manufacturer and see how their equipment was tested, for example Square D QO breakers are only rated for 5KAIC when used on corner grounded systems.
 
jim dungar said:
I know of no 3-pole breaker that is not rated for straight 240V. The concern specifically for 2-pole breakers is, the commonly available ones are only rated for 120V to ground.

The Soares book is not showing how it must be done but rather how to do it with 2-pole equipment.

Check with your the manufacturer and see how their equipment was tested, for example Square D QO breakers are only rated for 5KAIC when used on corner grounded systems.
Click to expand...

so are you saying that you can install all 3phase on a 3 pole cb even if the transformer is corner grounded?
 
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