Step Up Transformer Disconnect need it?

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Charlypt

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Electrician
We have a Step Up 75KVA transformer 208 to 480V, configured Delta-Wye, feeded from a 150A sub-feed 3-phase breaker in a panel 42 ft away.
The secondary load is an equipment panel with a 100A 3-phase main disconnect, 10ft away from the transformer. No neutral.
Do we need a disconnect for the transformer?
 
Charlypt said:
We have a Step Up 75KVA transformer 208 to 480V, configured Delta-Wye, feeded from a 150A sub-feed 3-phase breaker in a panel 42 ft away.
The secondary load is an equipment panel with a 100A 3-phase main disconnect, 10ft away from the transformer. No neutral.
Do we need a disconnect for the transformer?
Click to expand...
You always need a disconnect for a transformer. But it does not have to be anything other than the feeder breaker in most cases.
 
450.14 Disconnecting Means. Transformers, other than Class 2 or Class 3 transformers, shall have a disconnecting means located either in sight of the transformer or in a remote location. Where located in a remote location, the disconnecting means shall be lockable in accordance with 110.25, and its location shall be field marked on the transformer.
 
My confusion is about 240.21(C)(1):

(1) Protection by Primary Overcurrent Device. Conductors
supplied by the secondary side of a single-phase transformer having
a 2-wire (single-voltage) secondary, or a three-phase, delta-delta
connected transformer having a 3-wire (single-voltage) second-ary,
shall be permitted to be protected by overcurrent protection
provided on the primary (supply) side of the transformer, provided
this protection is in accordance with 450.3 and does not exceed
the value determined by multiplying the secondary conductor
ampacity by the secondary-to-primary transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase (other than
delta-delta, 3-wire) transformer secondary conductors are not
considered to be protected by the primary overcurrent protective
device.
 
It seems that a wye-wye transformer would be likewise protected as virtually three two-wire 1ph units.

Is it not because secondary line-to-line loads can somehow cause higher primary line currents?
 
Charlypt said:
My confusion is about 240.21(C)(1):

(1) Protection by Primary Overcurrent Device. Conductors
supplied by the secondary side of a single-phase transformer having
a 2-wire (single-voltage) secondary, or a three-phase, delta-delta
connected transformer having a 3-wire (single-voltage) second-ary,
shall be permitted to be protected by overcurrent protection
provided on the primary (supply) side of the transformer, provided
this protection is in accordance with 450.3 and does not exceed
the value determined by multiplying the secondary conductor
ampacity by the secondary-to-primary transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase (other than
delta-delta, 3-wire) transformer secondary conductors are not
considered to be protected by the primary overcurrent protective
device.
Click to expand...
It seems your question is about the secondary conductors and not the transformer. Look at the other paragraphs in 240.21(C). Your installation is allowed.
 
petersonra said:
Do you find it curious that a local transformer disconnect is not explicitly required to be lockable?
Click to expand...
NFPA 70E and agencies like OSHA cover LOTO.
My opinion is the NEC locking provisions go beyond its intended scope of not duplicating other standards. But I appreciate how having fixed in place provisions installed makes complying with other standards easier.
 
LarryFine said:
It seems that a wye-wye transformer would be likewise protected as virtually three two-wire 1ph units.
Click to expand...
Agreed, assuming you mean the secondary conductors supplied by a wye-wye transformer would be likewise protected by the primary OCPD.

LarryFine said:
Is it not because secondary line-to-line loads can somehow cause higher primary line currents?
Click to expand...
As far as I can see it's either an oversight, or an omission for simplicity on the basis that wye-wye transformers are seldom used for NEC applications.

Cheers, Wayne
 
The Brit here again. My field is industrial power electronics. We rarely, if ever, use step-up transformers. Often we 11 kV down to 700 DC. Specials for us. Regular systems are just 400Vac.
 
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