Overcurrent protection for transformer

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Damien

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Hey all this is a first post for me. I just walked onto a job with a 480v 4 wire service. The tenant space I’ve been asked to work in has a three phase 480-240 delta delta 30kva transformer in it with no neutral coming in on the primary side of the transformer. Currently there is a 100 amp fused disconnect on the primary side. On the secondary side of the transformer are two panels, a single phase three wire panel with a 100main breaker AND a three phase 225 amp panel with no neutral in it. Both of these panels and the transformer were recently installed by a different electrician prior to me and currently have no loads on them. The tenants do not need three phase for any of their equipment which is all 240v or 120v.
My questions regards the ocpd in relation to the transformer. Since there is both primary and secondary ocpds it’s my understanding based on table 450.3(b) that the primary ocpd can be 250% of Ip or

36.1*2.5=90.25a.

can this be rounded up to 100a fuses?

on the secondary side I would be allowed 125% of 72.2 amps also equaling 90.25a.

Can this be rounded up to a 100 amp breaker?

This leads me to believe that the second 225amp panel exceeds the overcurrent protection for the secondary side of the transformer correct?

also, since no neutral is brought from the main service there is a neutral produced from the center tapped leg of the delta on the secondary side. This leaves me with some weird voltages to ground. 70 volts neutral to ground, 128v L1 to ground and 138v L2 to ground and 148v L3 to ground. What is the best remedy for this?

thanks everyone for the help!
 
If you use the 250% primary protection rule, the TOTAL of your secondary protection can not exceed the value specified for a single overcurrent device (next size up allowed)
Your voltage rereading indicate that the secondary neutral has not been bonded to ground.
 
Damien said:
Hey all this is a first post for me. I just walked onto a job with a 480v 4 wire service. The tenant space I’ve been asked to work in has a three phase 480-240 delta delta 30kva transformer in it with no neutral coming in on the primary side of the transformer. Currently there is a 100 amp fused disconnect on the primary side. On the secondary side of the transformer are two panels, a single phase three wire panel with a 100main breaker AND a three phase 225 amp panel with no neutral in it. Both of these panels and the transformer were recently installed by a different electrician prior to me and currently have no loads on them. The tenants do not need three phase for any of their equipment which is all 240v or 120v.
My questions regards the ocpd in relation to the transformer. Since there is both primary and secondary ocpds it’s my understanding based on table 450.3(b) that the primary ocpd can be 250% of Ip or

36.1*2.5=90.25a.

can this be rounded up to 100a fuses?

on the secondary side I would be allowed 125% of 72.2 amps also equaling 90.25a.

Can this be rounded up to a 100 amp breaker?

This leads me to believe that the second 225amp panel exceeds the overcurrent protection for the secondary side of the transformer correct?

also, since no neutral is brought from the main service there is a neutral produced from the center tapped leg of the delta on the secondary side. This leaves me with some weird voltages to ground. 70 volts neutral to ground, 128v L1 to ground and 138v L2 to ground and 148v L3 to ground. What is the best remedy for this?

thanks everyone for the help!
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You cannot round up the 250% on the primary.
You cannot round up the 125% on the secondary, if you are greater than 125% on the primary.

You must bond your secondary neutral point to ground, just like you would with any other transformer. This should fix your secondary L-G voltages.
Your B phase leg should be the one with the high voltage to ground (nominally 208V).
 
Thanks guys! Appreciate your responses. Do either of you have a code reference to clarify whether or not I can round up?
 
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