Transformer - fuse size

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wolfman56

Senior Member
Thank's in advance, this forum is always so much help!!
I'm installing a 30-kva 3 phase transformer. To feed a printing press. The press engineers want the xformer to change the feed from Delta to Wye. (Makes no sense to me 'cause there's no neutral to the press.And voltage they want is 230.) In taking advantage of existing circuits and parts, I'm setting it up this way; The input voltage is 235 volts. A 100 amp. OCPD feeding the primary, secondary out to a service rated disc. loaded with 100 amp fuses, to a 125 amp.rated sub. that has a 3 pole 50, and a 2 pole 30. To feed the press. With a total FLA at 125% of about 73 amps.
When I use the nameplate spec. of 240v. I calc that I need a primary OCPD size of 90 amp. (90.3) But if I use the actual input voltage I calc. to 92 amps. and can use the next highest OCPD at 100 amps.
Does anyone think the inspector will make me down size the primary OCPD to 90?
Or did I look at this incorrectly?
Rick
 

don_resqcapt19

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Location
Illinois
Occupation
retired electrician
Re: Transformer - fuse size

Rick,
Even at 240 volts the 125% OCPD size is 90.32 amps. It is above 90 so you are permitted by Note 1 to Table 450.3(B) to round the size of the primary OCPD up to 100 amps.
How are you getting 230 volts out of a 3 phase wye secondary? The standard secondary voltage would be 208 volts.
Don
 

jim dungar

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Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: Transformer - fuse size

First delta to wye transformers are very common with printing presses and other large variable speed "drive" installations with SCR power supplies. A generic term of this style of transformer is "drive isolation" and they have several specific differences from a standard transformer. The isolated windings of the transformer help to keep the drive from being affected by the rest of the facility (as well as to keep the facility from being affected by the press). The wye connection allows for a new ground reference point for the machine.

Now, to your question.

When determing the ampacity of an item, you need to use the nameplate nominal voltage. If nameplate voltage is not used, you need to use the nominal system voltage (120, 208, 240, 480, 600). Never use the actual voltage (you never know when it will change). Of course motor ampacity is taken from NEC tables but you still use the nameplate voltage level.
 

wolfman56

Senior Member
Re: Transformer - fuse size

Thanks, Don and Jim, you've been most helpful.
Also we got the voltage up to about 227, by changing the input taps on the windings. I see it makes sense to always use nominal voltage.
Rick
 
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