High-Power Transformer with Parallel Secondary Protection

I do not know about N.American codes but hear in UK [Europe!] there would be no problem, just treat them as two separate loads, the fact that they are identical is not relevant.

Presumably you would have a main bus bar [transformer output] and then distribute through a fuse to each loads dedicated bus bar. There should be no complaints as it would be a better protected system although not the simplest - ie one bus bar with fused supplies to each load direct, but if each load has its own bus bar you have inherited that system. This is normal cascade protection, a tree.

The only point being if you had a fault in one system would you want the other to shut down as well for safety reasons, if so put them on the same protection.

Sorry but - reading your question again, if you are suggesting can I have two half size / power bus bars rather than one main bus bar I would say NO, that's got inherent dangers of; in the future someone trying to take full power from one of the bus bars. You may get away with that on a low power private domestic setup but not at the level you are talking of, I would do it properly: main bus bar & secondary bus bars etc.

I may stand corrected but ...

Thank you all for your responses. I might not have been clear enough on the transformer secondary connections. I intend to mount the two fuses per phase right at the transformer connection taps, and then feed one isolated bus per fuse. This would prevent anybody from drawing more than 1500A on a bus.

Another detail that I omitted is that the transformer, its protections and the two loads are all located in the same large control panel. The loads are large AC/DC/AC converters used to produce ozone intended for water purification. It is unlikely that anybody would use this dedicated product as a power source for anything else but it doesn't hurt to be protected if this happen.

Based on your description, I believe this equipment would fall under Article 409, Industrial Control Panels. It appears to meet the critical requirements, with one exception: you have not said anything about the incoming source (transformer primary supply). Overcurrent protection on the supply must meet the requirements of Art. 240, Part IX.

Art. 409 has several miscellaneous requirements regarding enclosure size and marking which should be thoroughly reviewed for compliance.

Your design MAY be acceptable under NEC Article 240.21(C), Transformer Secondary Conductors. There are several caveats regarding the size of the loads and the location and length of the secondary conductors between the transformer and the fuse. Without more information, I can't give a definitive answer.

Additionally, your description seems to indicate that the output of the 2 loads is paralleled. If one load trips off, will the other load try to supply the total output? If so, it will try to pull 2X its normal load current through the fuse & secondary conductors, probably well above your 1.5KA fuse.

The two loads are electrically independant (isolated). If one load fails, the other one will continue undisturbed. There is no obligation to have both loads operating simultaneously other that it is more efficient to produce the ozone.

In general your layout is fine. There are some details that can make a difference.

Correct me if I am wrong guys but doesn't this fall in the six disconnect rule. SInce this is the secondary of a transformer you can have up to six mains. Since these are above 150 Volts to ground,, above 1000Amps ,you will need GF on each switch. ( Asuming you have a 3phase, 4-wire secondary - if your secondary is ungrounded you should at least have a grounding resistor). Each switch should be service entrance rated.

If your transformer is a substation type, and you are close coupling the 2- switches,the through bus will still be rated 3000 amps.