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Secondary of transformers

Posted By: SJT

Secondary of transformers - 09/23/09 02:06 AM

I was looking at a job that had a 225KVA transformer suspended from the ceiling in a large utility room of Building. Below the transformer there are 2 - 400amp disconnects. The secondary conductors go to these two switches. Is this allowed, or do you need one single disconnect? I believe the transformer is 3 phase 480V to 208V. There is a breaker on the Primary side also. Is there a section in the code about the number of disconnects allowed on the secondary side?
Posted By: Yoopersup

Re: Secondary of transformers - 09/23/09 02:26 AM

240.21 (C) Each set of conductors feeding seperate loads Shall be permitted to connected to a transformer secondary. (Notice EACH)Devices must be grouped to meet 450.3 (B) note 2
NEC 2005 Handbook Details this clearly on page 170.
Posted By: JBD

Re: Secondary of transformers - 09/23/09 02:27 AM

The transformer is protected under article 450.3. If the primary disconnect is sized at not more than 125% of the primary FLA, a single disconnect is not required on the secondary.

The secondary conductors are protected by the tap rules in 240.21(C). There are situations where a set of tap conductors can feed multiple devices.
Posted By: SJT

Re: Secondary of transformers - 09/23/09 03:00 AM

I checked that 240.21 (C) and you were right on with that. We may leave the 2 Disconnects alone and change a distribution panel downstream from those.
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