Transformer fused disconnect needed?

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Barbqranch

Senior Member
Location
Arcata, CA
Occupation
Plant maintenance electrician Semi-retired
I am involved with planning the moving of a piece of equipment in our plant. Our power is 208Y/120, and there is a transformer that raises this to 480 V 3p for this one cutting machine. Machine name plate calls for 50 A (41.5 kva) and the transformer is 45 kva. Machine draws full power only for a couple of minutes every 10 minutes or so. All this was installed before I was hired, and there is much evidence around here that things were just done, not planned.

Transformer currently is on a 100 A 3pole breaker, not in line of sight, with no other disconnects. At the very least, I need to install a lock on the breaker. Does this transformer require a fused disconnect on the output, and could you please direct me to the area of the NEC (2011) where I can learn more about this. Should I be concerned about a fuse blowing and dropping a phase out to the machine, if I were to install 50 A fuses? (Machine actually draws a peak of 42 amps, at least when I checked.)

To all of you who know so much more than I do, I owe a lot. It has been an education being on here. Thanks.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The 100 amp breaker is well within the 450.3 requirements for transformer protection. The 2011 Code does require a permanent means of locking it out if it is not in sight of the transformer.
As far a secondary protection, the story gets a bit more interesting.
If you transformer is 208 primary with a 480 3 wire secondary, your primary protection (100 amp) may well serve as your secondary protection. See 240.21(C) for the particulars. It might be worth nothing that if the secondary is ungrounded, 250.21(B) requires a ground detection system.
If your transformer is 480Y/277 secondary then 240.21(C) does require secondary overcurrent protection.
 

Barbqranch

Senior Member
Location
Arcata, CA
Occupation
Plant maintenance electrician Semi-retired
First off, I made a mistake. Primary protection is 125 amp, not 100. Also, I don't know yet if it is wired delta or Y or if it is grounded. I haven't opened any covers to access the voltage at this point.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
same answers for a 125 amp breaker, That breaker is still withing the allowable range to provide transformer protection without secondary protection as far as transformer protection is concerned.
To see if the 125 amp breaker will provide protection for the secondary conductors, see 240.21(C) for details.
With a 480 delta secondary it is likely you will not need secondary OCP.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
same answers for a 125 amp breaker, That breaker is still withing the allowable range to provide transformer protection without secondary protection as far as transformer protection is concerned.
To see if the 125 amp breaker will provide protection for the secondary conductors, see 240.21(C) for details.
With a 480 delta secondary it is likely you will not need secondary OCP.

Enlighten me. Why does a wye need secondary OCP while a delta does not?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Enlighten me. Why does a wye need secondary OCP while a delta does not?
Because 240.21(C) says so :D
sorry, devil made me.... plus I can't say I am qualified to give you the answer. Paraphrasing what I have been told it has to do with a secondary load, other than delta-delta loading the transformer in an unbalanced manner where as a delta to delta (or two-wire to two-wire) causes a directly proportional load between the primary & secondary.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
A good way to see the reasoning behind the wye limitation is to look at a 240V primary and a center tapped 120/240 secondary.
The primary protection must allow full load current at 240V, but that same primary protection would then allow twice the rated current through a single120V load, overloading the winding and burning out the transformer.
It is harder to give a concrete example with a wye secondary, but the same principle applies.
 
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