# overcurrent protection of transformer



## tates1882 (Sep 3, 2010)

hardworkingstiff said:


> I have to provide power for a 230-volt (11-amp), single-phase piece of equipment and my building power is 208Y/120 3-phase. The customer has a SQD transformer (CAT# 1.5S46F, http://ecatalog.squared.com/fulldetail.cfm?partnumber=1.5S46F ).
> 
> I would like to wire this in as a boost transformer and use a 2-pole 20A breaker to protect the transformer and the machine. The machine calls for a 20A circuit.
> 
> ...


Depending in the equipment it may tolerate 208v.


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## hardworkingstiff (Jan 22, 2007)

tates1882 said:


> Depending in the equipment it may tolerate 208v.


It doesn't matter in this jurisdiction, if it's a 230V rated load (instead of a 208-230V) then the AHJ will not pass the job wiring the 230 load with 208.


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## stlouis (Aug 25, 2012)

hardworkingstiff said:


> I have to provide power for a 230-volt (11-amp), single-phase piece of equipment and my building power is 208Y/120 3-phase. The customer has a SQD transformer (CAT# 1.5S46F, http://ecatalog.squared.com/fulldetail.cfm?partnumber=1.5S46F ).
> 
> I would like to wire this in as a boost transformer and use a 2-pole 20A breaker to protect the transformer and the machine. The machine calls for a 20A circuit.
> 
> ...


_
Amps = KVA ÷ Volts ÷ 1.732 x 1000
_


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## hardworkingstiff (Jan 22, 2007)

stlouis said:


> _
> Amps = KVA ÷ Volts ÷ 1.732 x 1000
> _


Thank you, but now how does that apply to the overcurrent protection sizing for a 1.5kVA single-phase transformer being wired as an autotransformer?


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## hardworkingstiff (Jan 22, 2007)

Let me see if I can explain my concerns a little better.

The transformer is rated 1.5kVA, 120/240 primary x 16/32 secondary.

So, the primary amperage rating is 1500/240=6.25 amps, the secondary is 1500/32=46.88 amps. 

Since the secondary windings will be wired in series with the load to boost the voltage, the new capacity of the transformer will be 230*46.88=10.8kVA.

The primary windings will only see the amperage that is the "boost" part of the load. The rest of the load actually goes around the primary although all of the load goes through the secondary. The 11-amp load on the secondary will induce a 1.3-amp load (plus transformer loss) on the primary winding. 

I need to provide over current protection for the equipment (manufacturer recommends a 20-amp circuit). So, if I install a 20-amp 2-pole breaker in the panel and feed the transformer (wired as an autotransformer) it seems to me if there is a fault in the transformer primary, it will burn up. If I try to protect it at a lower rating, the load will trip the breaker.


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## Executive (Aug 11, 2012)

The manufacturer provides diagrams and ratings of transformers used as autotransformers in a "buck-boost" application. When used in this way, there is very little strain put on the transformer. If the transformer is sized and wired correctly based on the load, simply limit your OCP on the primary side to less than 125% of the full load input current as listed by the manufacturer. This is all 450.4(A) is stating.

Chris


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## hardworkingstiff (Jan 22, 2007)

Executive said:


> The manufacturer provides diagrams and ratings of transformers used as autotransformers in a "buck-boost" application. When used in this way, there is very little strain put on the transformer. If the transformer is sized and wired correctly based on the load, simply limit your OCP on the primary side to less than 125% of the full load input current as listed by the manufacturer. This is all 450.4(A) is stating.
> 
> Chris


OK, so basically if the primary part of the transformer should have a problem, it's just too bad because the rated load of this 1.5kVA transformer goes to 11kVA when used in the scenario I posted. If I provide 50-amp OCP and the primary should have a fault, it's going to be damaged before the OCP might trip.

I guess if it has a fault, it doesn't really matter.


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