208v to 240v Buck Boost Transformer Calculations

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Jmays

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Oceanside, CA, USA
Hello all, I have read numerous posts on the subject, but wanted to get specific verification on our calculations for a 208 to 240 boost, since I have not worked heavily with autotransformers. Thanks in advance.

This is a light industrial warehouse. This unit is fed by 208/120 WYE 3 phase 4 wire service (line/line/line/neutral/ground). We need a circuit dedicated for a single phase 240v welder.

We will be using a Acme T113074 buck booster. Primary coming from a double pole 40amp breaker runs to the BB, secondary will be a 240v receptacle. So primary OCDP only.

The BB boosting the needed 32v to achieve 240v is rated at 11.25 Kva/46.90 Amps. Which is plenty for the required load of 40amps. 8awg from the panel to the BB and from the BB to the receptacle.

I don't think I'm missing any pertinent info. Please let me known if I did.
 
T

topgone

Senior Member
Yep, you are correct. A 1.5 kVA autotransformer can supply that 240V welder unit rated 11.25 kVA. Maximum current for the secondary winding at 1.5 kVA, 32V is 46.875A. Your autotransformer current drawn at 208V source will be 40.7A.
 
J

Jmays

Member
Location
Oceanside, CA, USA
topgone said:
Yep, you are correct. A 1.5 kVA autotransformer can supply that 240V welder unit rated 11.25 kVA. Maximum current for the secondary winding at 1.5 kVA, 32V is 46.875A. Your autotransformer current drawn at 208V source will be 40.7A.
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Follow up question:

The BB has two windings as you know, four leads for each winding. However one set of leads are 12awg and one set 14awg. How is that allowable with the given current? Shouldn't 8awg be the minimum? The wire rating is CL1251, @ 125 C. So is the lower awg allowable given the higher temp rating? Just trying to understand the seemingly under sized leads.
 
T

topgone

Senior Member
Jmays said:
Follow up question:

The BB has two windings as you know, four leads for each winding. However one set of leads are 12awg and one set 14awg. How is that allowable with the given current? Shouldn't 8awg be the minimum? The wire rating is CL1251, @ 125 C. So is the lower awg allowable given the higher temp rating? Just trying to understand the seemingly under sized leads.
Click to expand...

Correct. 40.7A with a wire #8AWG and 46.9A should have a wire the size of which is #6AWG.

Somebody missed something there, somehow! Honestly, we can't answer for Acme. Why not give them a call?
 
J

JFletcher

Senior Member
Location
Williamsburg, VA
Jmays said:
Follow up question:

The BB has two windings as you know, four leads for each winding. However one set of leads are 12awg and one set 14awg. How is that allowable with the given current? Shouldn't 8awg be the minimum? The wire rating is CL1251, @ 125 C. So is the lower awg allowable given the higher temp rating? Just trying to understand the seemingly under sized leads.
Click to expand...

The 125*C insulation.

I'm missing something tho; this xfmr has a 16/32V output, yes? I get that 208V+32V = 240V. How is a 1.5kVA xfmr doing all of this or does the welder get the 32V ~40A "boost" from it, and the remaining power come from another 208V source?
 
J

Jmays

Member
Location
Oceanside, CA, USA
JFletcher said:
The 125*C insulation.

I'm missing something tho; this xfmr has a 16/32V output, yes? I get that 208V+32V = 240V. How is a 1.5kVA xfmr doing all of this or does the welder get the 32V ~40A "boost" from it, and the remaining power come from another 208V source?
Click to expand...

The 1.5kVA is a misnomer. The nameplate only is literal when it acts as an insulating transformer (bucking). When hooked up as noted by Acme, it boosts the 32v like you mentioned and passes through the 208v for a total of 240v which runs at 11.25kVA/46.90amps (max).

As explained by Acme, the nameplate would be too complex to list all the possible kVA possibilities and therefore lists as a standard the kVA related to the insulating transformer.

Go to the online catalogue:
http://www.acmetransformer.com/en/products/buck-boost and look up the section (I believe it's sections 7) on buvl boost transformers. It has a few pages that explain all the details on that along with a Q/A
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Jmays said:
The 1.5kVA is a misnomer. The nameplate only is literal when it acts as an insulating transformer (bucking). When hooked up as noted by Acme, it boosts the 32v like you mentioned and passes through the 208v for a total of 240v which runs at 11.25kVA/46.90amps (max).
Click to expand...

Even though the load is 11.25 the transformer is only being loaded under its 1.5 rating.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Jmays said:
The BB has two windings as you know, four leads for each winding. However one set of leads are 12awg and one set 14awg. How is that allowable with the given current? Shouldn't 8awg be the minimum? The wire rating is CL1251, @ 125 C. So is the lower awg allowable given the higher temp rating? Just trying to understand the seemingly under sized leads.
Click to expand...

The small leads will be fine, it is the normal thing with buck boosts.
 
Sahib

Sahib

Senior Member
Location
India
Jmays:
For any transformer, the output is almost equal to input. So if you have 11.25 KVA on the output side of your autotransformer, almost the same 11.25 KVA on the input side.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
Sahib said:
Jmays:
For any transformer, the output is almost equal to input. So if you have 11.25 KVA on the output side of your autotransformer, almost the same 11.25 KVA on the input side.
Click to expand...

Yet the transformer is still loaded under 1.5 kVA or the smoke comes out.
 
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