TRANSFORMER HELP 240.21(C)1

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SIRSPARKSALOT

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I am currently working on a project for a customer that manufactures equipment for domestic and foreign use. The building votage is 240V Delta. They have 300kVA transfomers throughout the place for testing their specific machinery prior to shipment. These transfomers have a primary voltage of 240V and a secondary of 575V/480V/380V. They adjust the taps at the transfomer depending on the equipment they are testing. Unfortunately, over 15 years, they did their own wiring and the methods and sizing of this wiring is completely non-compliant. I have been asked to submit a plan to bring them into compliance. The questions I have pertain to primary and secondary protection. Would someone please explain 240.21(c)1 and how it would pertain to this situation. I believe, based on reading it, that I am not required to have secondary protection, but I am confused about the language referring to "secondary-to-primary ratio". Let's assume:
300kVA transformer operating one machine with a 260kVa load.
260,000/(240*1.732)=626*1.25=783A ---- Next higher fuse size 800A --- Feeder [2 Sets of (3) 500kcmil & (1) 1/0]
Secondary voltage 575V
260,000/(575*1.732)=261*1.25=326A --- Is protection needed? --- Feeder [2 Set of (3) 2/0 & (1)

Any and all help is appreciated. I wonder if I should walk away...
 
SIRSPARKSALOT said:
300kVA transformer operating one machine with a 260kVa load.
260,000/(240*1.732)=626*1.25=783A ---- Next higher fuse size 800A --- Feeder [2 Sets of (3) 500kcmil & (1) 1/0]
Secondary voltage 575V
260,000/(575*1.732)=261*1.25=326A --- Is protection needed? --- Feeder [2 Set of (3) 2/0 & (1)
Click to expand...

With an 800A primary OCPD, the 2 sets of 2/0 would be properly protected by 240.21(C)(1) when the transformer is at the 575V secondary tap. The secondary conductors have an ampacity of 350 Amps:

.............350A x (575/240) = 838A. The 800A primary OCPD does not exceed 838A, so the secondary is protected.

However, the 2 sets of 2/0 would not be protected at the 480V or 380V transformer tap.


.............350A x (480/240) = 700A.

.............350A x (380/240) = 554A.

The 800A primary OCPD exceeds both of those values.

If the secondaries were two sets of 250MCM (with an ampacity of 510) then the primary OCPD would protect the secondary conductors at all three of the taps.
 
SIRSPARKSALOT said:
I am currently working on a project for a customer that manufactures equipment for domestic and foreign use. The building votage is 240V Delta. They have 300kVA transfomers throughout the place for testing their specific machinery prior to shipment. These transfomers have a primary voltage of 240V and a secondary of 575V/480V/380V. They adjust the taps at the transfomer depending on the equipment they are testing. Unfortunately, over 15 years, they did their own wiring and the methods and sizing of this wiring is completely non-compliant. I have been asked to submit a plan to bring them into compliance. The questions I have pertain to primary and secondary protection. Would someone please explain 240.21(c)1 and how it would pertain to this situation. I believe, based on reading it, that I am not required to have secondary protection, but I am confused about the language referring to "secondary-to-primary ratio". Let's assume:
300kVA transformer operating one machine with a 260kVa load.
260,000/(240*1.732)=626*1.25=783A ---- Next higher fuse size 800A --- Feeder [2 Sets of (3) 500kcmil & (1) 1/0]
Secondary voltage 575V
260,000/(575*1.732)=261*1.25=326A --- Is protection needed? --- Feeder [2 Set of (3) 2/0 & (1)

Any and all help is appreciated. I wonder if I should walk away...
Click to expand...

From what is see here is that the taps are on the secondary and the primary is 240v. You omitted stating if the secondary was a delta or wye but I my answer is based upod it being a delta secondary and as such you need only primary protection. The question is are you protecting the secondary? You should be if the secondary conductors are sized sufficiently to carry the maximum current at the lowest voltage tap. The primary protection as provided would be based upon the current at the lowest voltage tap.
When you use a high voltage tap the current goes down which means that you certainly won't be exceeding tha ampacity of the wire for so you are set there.
As far as the pri OCPD rating? That is a non issue also because if remains a ratio issue which will not change no matter what the secondary voltage tap is.
Remember that an OCPD protects cable which is sized to carry the load, the transformer is sized for the load, the load on the secondary conductors is based upon the current at the lowest voltage should a single conductor be used for all tap voltages so your covered there. should a single conductor be brought out from each individual tap each of those conductors should be appropriately sized based upon the maximum current at that voltage and as such would still be protected by the pri OCPD bases upon the ratio. Remember kva in = kva out with 240v in and no matter what tap ypu are using on the secondary. Lower voltage taps= higher current and larger conductors and the higher voltage taps= lower current with smaller conductors all which would be protected be an appropriately sized pri. OCPD.
Remember that only one tap of the transformer should be used at a time unless the transformer has been designed to do otherwise.
 
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