Transformers - Primary & Secondary Protection, Articles 450 & 240, etc.

[Anthony Giordano]

Hey all, I know that this topic has probably been discussed here on this forum hundreds of times, but I need some clarification on a few things. Regardless of how much reading and "googling" that I do on this subject, there are still some items that are confusing to me. As I state in my thread title, I have read/continue to read Articles 450 and 240 to try and fully understand this subject.

Here is my specific situation on a Data Center project in Newark, NJ. I have a 480V, 600A Distribution Panelboard that is feeding a 75 KVA transformer, from a 125A/3P circuit breaker. The DP will be located in a newly constructed satellite electric closet, and the 75 KVA transformer will be located in the adjacent room, the 3rd Floor Main Electric Room. The lateral distance from the DP to the transformer is approximately 14', and the distance of the primary feeder counting "ups & downs" is approximately 20'. I have my primary side feeder to the 75 KVA transformer set at 3#2 & 1#6G. in 1 1/4"C.

Question #1: Since the DP is in the other room, is extra protection required before the primary feeder hits the primary side of the transformer? I am thinking that it is not required, but I just want to make sure, because I need all necessary scope accounted for.

The low voltage panelboard that the 75 KVA transformer is feeding is a 208/120V, 225A MLO panelboard, and is located right next to the 480V DP in the satellite electric closet. The lateral distance from the transformer to the LV panel is approximately 17', and the distance of the secondary feeder counting "ups & downs" is approximately 22', thus within 25'. I have my secondary side feeder to the 225A panelboard set at 4#250 KCMIL & 1#2G. in 2 1/2"C.

Question #2: Since the LV panelboard is in the other room, is extra protection (i.e. either a 225A enclosed circuit breaker or a 400A disconnect switch fused @ 225A) required for the feeder after coming out of the secondary side of the transformer, before connecting to the LV panelboard? As in Question #1, I am thinking that it is not required because the feeder run is less than 25', but again I just want to be sure.

I would greatly appreciate all responses and guidance that you can give me for this design example and topic in general. Thanks very much and look forward to your responses!

Regards,

Anthony Giordano

 

[jim dungar]

First you need to properly protect your panelboard.
Second you need to properly protect your transformer secondary conductors. You may be able to use any protection from step 1.
Third you need to protect the transformer primary.
Fourth you can evaluate your panel and secondary protection, to see if you can 'oversize' the transformer primary protection.
Fifth select primary side conductors, based on the choice made in step 4.
Sixth you need to check if a local 'in sight of' primary side disconnect is required.

 

[electrofelon]

Think of it this way: your conductors, your transformer and your panelboard all need protection (450.3, 408.36, 240.21). Also remember the 450.3 protection is just for the transformer and may not cover your conductor protection.

 

[don_resqcapt19]

The 125 amp breaker and the #2s on the primary are compliant with the rules in the NEC. The breaker is not within sight from the transformer so you will need sign on the transformer stating the location of the breaker. (450.14) A local disconnect is not required at the transformer.

The transformer secondary conductors are less than 25' long so overcurrent protection is not required at the transformer. Both the transformer secondary conductors and the panel need overcurrent protection. The easiest way to do that is to use a 225 amp main breaker in the panel.

 

[Anthony Giordano]

The 125 amp breaker and the #2s on the primary are compliant with the rules in the NEC. The breaker is not within sight from the transformer so you will need sign on the transformer stating the location of the breaker. (450.14) A local disconnect is not required at the transformer.

The transformer secondary conductors are less than 25' long so overcurrent protection is not required at the transformer. Both the transformer secondary conductors and the panel need overcurrent protection. The easiest way to do that is to use a 225 amp main breaker in the panel.

Don...Thank You VERY MUCH for your great insight and assistance!