Transformer Primary Overcurrent Protection

[muddyboots]

I am a little confused on Primary OCPD for transformers, and I tend to second guess myself when questioning an engineered set of transformer/panel schedules. My understanding is Primary OCPD must be sized according to 125% of FLA of your transformer. I believe in certain cases you can or maybe even are required to round down to the next breaker size. But regardless that would still be based off of 125% of Primary FLA according to the nameplate of the transformer, correct?

In my case I have a 45kVA 480 - 208Y/120 xfmr with a FLA on the nameplate of 54A, call it T1. According to the drawings they want to feed with a 50A breaker, 3-#6 and #10 ground (Cu). According to the secondary side load calc I was provided, there will only be about 15A per phase, so I understand the load on this panel is low but something still feels off. Secondary side is feeding a 150A MCB panel, with 4-2/0 and #2 SSBJ (AL).

Not too sure why they called for aluminum on every secondary side xfmr, when the whole building is fed off copper, but that is beside the point I suppose.

What is throwing me for a loop is the panel that is feeding T1 is also feeding a 30kVA 480 - 208Y/120 transformer in the adjacent room, call it T2, with a Primary OCPD of 70A. Fed by 3 - #4 and #8 ground (Cu). The secondary side of xfmr T2 is feeding a 100A MCB panel with 4 - #1 and #4 ground (Al). Calculated load for secondary side of T2 is only 5A per phase. It's a new construction manufacturing facility with planned provisions for future additions and manufacturing lines.

My gut is telling me the breakers need to be swapped.
T1 - 54 x 1.25 = 67.5 --> 70A Primary OCPD.
T2 - 36 x 1.25 = 45A --> 50A Primary OCPD.

My boss has insisted I install it as written on the plans so I am kind of in a pickle as he is seemingly trying to avoid sending an RFI as I suggested. Part me thinks I am making a mountain out of a mole hill, but it has been bugging me for weeks now and the other part is telling me not to energize until I have an answer. Anyways I apologize if any of this is confusing or poorly worded. I respect the heck outta this forum and I appreciate any and all feedback. Thanks for your time.

 

[augie47]

Transformer OCP is shown in Table 450.3
125% (next size up) is common although with proper secondary protection 250% is allowable.
The conductors should be sized base on the overcurent device protecting them noting on secondary conductors you can not use next size up.

A 50 amp breaker on T1 would most likely cause trip due to inrush.

Look like they got their T1 & T2 labels crossed.

 

[Space]

I agree with augie47.

Since at least one side of your transformer is WYE you need primary and secondary overcurrent protection. See 240.21(C)(1).
Table 450.3(B) give you max of 250% FLA on primary side and maximum of 125% on secondary (using the currents of 9 amperes or more on that table).

It is likely something got mixed up. It it still protected and code compliant based on what I see. I googled a typical thermal/magnetic circuit breaker curve and the typical inrush current as percentage of full load current for a transformer. It looks well within the realm of reality to cause the primary ocpd on T1 to trip.

It also looks like the conductors for the transformer primaries were crossed as well (#4AWG should stay with the 70A breaker, and #6 with the 50A breaker).

Well done on your part on using your critical thinking skills and pointing this out to your boss .

 

[Elect117]

You are correct. Tell your boss to send the RFI. That is such a simple fix.

I believe in certain cases you can or maybe even are required to round down to the next breaker size.

This comes from 240.21 and depends on the circuit's configuration. Having a feeder tap means you can not round up. The conductors on the secondary side must be protected AT their ampacity (so no rounding up).

The transformer's protection is in 450 but your OCPD must still comply with 240 to also protect the conductor, and in particular, 240.21(B) & (C). Usually if you do FLA of the transformer and protect the conductor at their ampacity you will be fine.

 

[muddyboots]

Thank you all for the clarification and references. I knew it was something fairly cut and dry, and the rfi has now been submitted after another conversation thankfully. Y'all gave me exactly what I was looking for with all references and extra info which helped in my favor. Thanks for the encouragement as well!