boost xfmr wire size

I am new to the use of buck-boost transformers but after consulting the TEMCO selection chart and talking to their personnel, I selected 2- 1.5kva 120/240-16/32v isolation transformers to boost my 208v, 3 phase, 63kva, 93amp load to 222volt. I understand, in theory, that these transformers are not handling the entire load, but when I look at the connection diagram (#8 on the TEMCO website) I can see that phase B bypasses the transformers but phase A and phase C each connect to a transformer through two #12 wires. I have a hard time connecting my 93A load to these two #12 wires. Can anyone help me understand this and justify that connection?
 

Ingenieur

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GoldDigger

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How about a little more help finding the diagram on the TEMCO website?

It is perfectly reasonable to connect the neutral side of each transformer with #12 wire, since no direct load current will be flowing through that wire, only the primary current corresponding to the 93A on the secondary side.
I would expect one terminal of the autotransformer to be directly connected to the phase conductor though, and that will carry all of the load current plus some of the primary current.
 
How about a little more help finding the diagram on the TEMCO website?

It is perfectly reasonable to connect the neutral side of each transformer with #12 wire, since no direct load current will be flowing through that wire, only the primary current corresponding to the 93A on the secondary side.
I would expect one terminal of the autotransformer to be directly connected to the phase conductor though, and that will carry all of the load current plus some of the primary current.
There is no neutral as all the loads are motors. here is the wiring diagram for the two 1.5 kva transformers
http://attachments.temcoindustrialpower.com/Wiring_diagram/buckboost_wiring_8.pdf
 
That is boosted load rating
basically 14 v x 93 A

edit not sure you have the right transformer
Group 2 3 phase 208/222 multiple choices based on kva
http://www.temcoindustrialpower.com...?p=buck_boost_transformer_selection_guide#III

http://attachments.temcoindustrialpower.com/Wiring_diagram/buckboost_wiring_8.pdf
The transformer size would normally bother me, but the literature states that it only handles a small portion of the load current. Unfortunately the wiring diagram looks as if two of the x windings on each transformer handle the full load current.
 

Ingenieur

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The transformer size would normally bother me, but the literature states that it only handles a small portion of the load current. Unfortunately the wiring diagram looks as if two of the x windings on each transformer handle the full load current.
I was wrong
I was thinking it was an autotransformer type
I believe yours should be rated fr the full load kva ~ 35 kva or so
I would go for 40

edit:
OK I see now
your 1.5 rating is not from the chart ( on the chart it is 36)
when you click the link for the associated transformer the data shert comes up
1.5 kva http://www.temcoindustrialpower.com/products/Transformers_-_General/FT0167.html
 
Last edited:

topgone

Senior Member
I am new to the use of buck-boost transformers but after consulting the TEMCO selection chart and talking to their personnel, I selected 2- 1.5kva 120/240-16/32v isolation transformers to boost my 208v, 3 phase, 63kva, 93amp load to 222volt. I understand, in theory, that these transformers are not handling the entire load, but when I look at the connection diagram (#8 on the TEMCO website) I can see that phase B bypasses the transformers but phase A and phase C each connect to a transformer through two #12 wires. I have a hard time connecting my 93A load to these two #12 wires. Can anyone help me understand this and justify that connection?
You need to have a 222V, 3-phase from 208V, 3-phase?
Connect the two transformers in open-delta. The common winding should be wired 240V (series 120V windings) while the series windings connected 16V (parallel 16V).
dbst1a.jpg
 
You need to have a 222V, 3-phase from 208V, 3-phase?
Connect the two transformers in open-delta. The common winding should be wired 240V (series 120V windings) while the series windings connected 16V (parallel 16V).
View attachment 15134
Your winding wiring diagram looks like the one I have, but I don't understand how the x windings of the 1.5 kva transformers can carry 93 amps. What am I missing?
 

Ingenieur

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Location
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Your winding wiring diagram looks like the one I have, but I don't understand how the x windings of the 1.5 kva transformers can carry 93 amps. What am I missing?
without getting into phasors and such
there are 2 coils in parallel
each carry 1/2 or 47 A
each coil contributes 14 V
2 x 14 x 47 ~ 1.3 kva

rated at 16 V per the data sheet
close to 1.5 kva
 

topgone

Senior Member
Your winding wiring diagram looks like the one I have, but I don't understand how the x windings of the 1.5 kva transformers can carry 93 amps. What am I missing?
93A (technically 1500VA/16V = 93.75A) is the rated full-load current of your 16V secondary winding (2 windings in parallel, each 16V winding capable of providing 93.75/2 = 46.875A).

Take note that the paralleled 16V-windings are connected in series with the common winding (2-120V windings wired in series)! That would mean that the secondary current will have to be limited to the secondary current-carrying capacity of 93A. So the line current of your 222V output should be this value = 93A!
If you might wonder, your kVA capacity is not 63kVA, as you posted! Why? It's because 93 x 1.732 x 222 = 36,048 VA ~36kVA! You should have chosen a 2 x 3kVA boost transformer.
 

Ingenieur

Senior Member
Location
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93A (technically 1500VA/16V = 93.75A) is the rated full-load current of your 16V secondary winding (2 windings in parallel, each 16V winding capable of providing 93.75/2 = 46.875A).

Take note that the paralleled 16V-windings are connected in series with the common winding (2-120V windings wired in series)! That would mean that the secondary current will have to be limited to the secondary current-carrying capacity of 93A. So the line current of your 222V output should be this value = 93A!
If you might wonder, your kVA capacity is not 63kVA, as you posted! Why? It's because 93 x 1.732 x 222 = 36,048 VA ~36kVA! You should have chosen a 2 x 3kVA boost transformer.
In their selection guide
208-222
36 kva load
2 x 1.5 kva xfmrs are selected
 
without getting into phasors and such
there are 2 coils in parallel
each carry 1/2 or 47 A
each coil contributes 14 V
2 x 14 x 47 ~ 1.3 kva

rated at 16 V per the data sheet
close to 1.5 kva
Does the phasor relationship make the 93 amps appear to be less as it passes through the transformer? It seems strange to connect a #2 wire to 2#12 wires on the primary and 2# 10 wires to a #2 on the secondary.
 

Ingenieur

Senior Member
Location
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Does the phasor relationship make the 93 amps appear to be less as it passes through the transformer? It seems strange to connect a #2 wire to 2#12 wires on the primary and 2# 10 wires to a #2 on the secondary.
Nope
pretty much the 93 or 45 each
the make #12 rated for >55A for applications like this
 

Smart $

Esteemed Member
Location
Ohio
Does the phasor relationship make the 93 amps appear to be less as it passes through the transformer? It seems strange to connect a #2 wire to 2#12 wires on the primary and 2# 10 wires to a #2 on the secondary.
Sorry I'm a bit late to this discussion, but let me jump in here...

Where are you getting the idea there are #12 and #10 wires involved here?

Assuming a full load of ~94A is flowing through the secondaries, the equivalent is 94A*222V/208V=105A on the 208V primary conductors at 208V. Under the NEC, the conductors must be rated for this current, basically. However, we're talking motor loads, so there's a possibility the conductor ampacities can be lower. Missing details. How many motors and the ratings of each? Any short-circuit and ground-fault protection or controller(s) after the transformer?

While these are constructed as isolation transformers, they are being connected as autotransformers, so the autotransformer rules apply...
 
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