ECN Forum Forums Code Related Discussion NEC & other Code issues Reverse wiring a transformer 2014

I have a 120/240 3 ph sevice and need to wire a 100 amp 480v 3ph chiller I will be installing a 112 kva transformer under the new code can we reverse wire the transformer 240 pri 480 sec or does the secondary need to be 480/277 to bond the neutral. I can't find it in the code but I though 2014 code now requires a neutral on the secondary side.

Read 450.11 2014 NEC .
Be curious to see what you came up with for breakers
on primary & secondary??
Also if you don't have a Neutral on the secondary
(480v) how you going to ground it????
Yoopersup

I think the more challenging part will be finding a suitable transformer to reverse wire.
480/277 Y to 120/240 Delta aren't common since most transformers with a 480V primary are set up for a Delta connection.
I'm sure they're out there but don't fall into the trap of ending up with a Delta/Delta connection that will overheat from the circulating currents.

Transformers can be supplied at the secondary voltage (reversed wired) only in accordance with the manufacturer’s instructions
450.11(B) Transformers (New)2014 NEC

Yoopersup

I found a 240 primary 480/277 secondary but at one time were you allowed to go 240 primary 480 secondary no neutral for one piece of equipment?

Without a Neutral on the Secondary ,you won't have a grounded system???
Again I say 450.11 comes into play!

Transformers can be supplied at the secondary voltage (reversed wired) only in accordance with the manufacturer’s instructions
450.11(B) Transformers (New)2014 NEC



yoopersup

Probably the best tale of reverse wiring transformers comes from the radio show of Art Bell.

Back in 1996, Art received a call from "Mad man Markum," and the story was confirmed by the man's probation officer. Simply put, "Markum" claimed to have made a time machine by stealing some PoCo transformers and setting them up on his front porch.

He succeeded in making a Jacob's ladder about 4-ft. across. When asked if he had 'tested' it for time travel, he said 'yes;' he had tossed some change into the arc. He said there was a very big flash and the coins disappeared. He was waiting for them to reappear, so he could 'calibrate' the device.

I think he will have a long wait.

A police visit to a neighbor led to the accidental discovery of his contraption, and his prosecution. Time served, he went to the PoCo and bought the transformers this time, and continued with his experiments.

I shudder to think of the voltage required to make an arc of that size. AFAIK, his neighbors haven't complained, and the NEC has no rules regarding such 'appliances.' I will admit that a 4-ft. arc does call into question the usual clearance rules

Over a period of years, I personally heard the many chats Art had with this man.

Please note that a secondary's 'neutral' need not have ANY connection with Mother Earth, the equipment ground, the neutral of the incoming supply, or anything else. The term 'grounded' is somewhat misleading here!

Indeed, unless you make a deliberate effort and install a jumper, your usual transformer 'neutral' connection connects only to the center taps of the windings.

If you think that big, bent copper strap that connects the coil assembly to the case bonds the neutral tap to the case / equipment ground, you are mistaken.

I have found little clarification in the instructions, the NEC, or even in other publications, such as Soares and Stallcup's. Various contractors I have worked for have had wildly differing practices, to the point where I'm not sure what's the "correct" way to hook up the usual 480 x 208 three-phase transformer.

Contact a Square D rep. He'll tell you what you need.
yoopersup

Question

Can UL Listed (Certified) dry type transformers be reversed wired? Does that affect the Listing (Certification)?

Answer

UL Certified (Listed) dry type transformers can only be reverse wired if marked for such use or the installation instructions detail the practice. Dry type power transformers are UL Certified (Listed) under the product category Power and General-Purpose Transformers, Dry Type (XQNX), located on page 567 in the 2014 UL White Book and also in UL’s Online Certifications Directory at www.ul.com/database (enter XQNX at the category code search field).

The Guide Information for XQNX details the required electrical rating markings including the primary voltage and secondary voltage. When reverse-wired, a transformer may experience problems with nuisance tripping of the branch circuit protection due to inrush current. However, users can reverse wire the transformer if it is marked for reverse wiring or the installation instructions provided with the Certified (Listed) transformer indicate that it can be reverse wired per NEC 110.3(B). The Guide Information for XQNX has recently been updated to state: Where the manufacturer permits the transformer be supplied from the marked secondary and loaded from the marked primary, i.e., “reverse-fed” or “bi-directional,” installation instructions shall be provided by the manufacturer detailing how this connection shall be made and specifying any concerns or precautions that should be taken by the installer. The suitability of the installation should be determined by the Authority Having Jurisdiction. The Guide Information for XQNX can be viewed on the UL’s Online Certifications Directory as detailed above.
This is consistent with a new requirement in the 2014 NEC, Section 450.11(B) that states “Source Marking. A transformer shall be permitted to be supplied at the marked secondary voltage, provided that the installation is in accordance with the manufacturer’s instructions.”