I am tapping out on understanding tapping taps...

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cottora

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First, a great forum! I have lurked around and learned so much.

I am working with an Electrical Engineer for a design that will essentially have multiple bays with each bay containing three 200-amp panel boards. The question is around what is a tap and if we can split vs taking each panel board back to the transformer (or back to another OCPD since the transformer will be very close to the "distribution block").

The idea is to go from the secondary side of a 240/120 transformer with 15ft of 600 amp conductor - into a Power Distribution Block (600 amp) - and then inside via three separate 8ft 200 amp conductors which terminate into a 200 amp Panel Board (with 200 amp OCPD). This will be done ten times (which is why we would like to split the feeder since the transformer will run out of room)

I do not think (but I am not sure) that the above is within NEC guidelines, but I believe it would be ok if we ran each 200 amp conductor back to the transformer.

Any thoughts/guidance would be greatly appreciated!

Raine
 
The basic requirement is that every “conductor” (and I would lump transformer windings, panelboards, and your power distribution block into that term) must be protected against overcurrent. The Overcurrent Protective Device (OCPD, for example a fuse or circuit breaker) must be located where the conductor gets its power, unless you meet one of a number of rules. The rule that applies to transformers is that you put the OCPD immediately after the secondary terminations, or you follow one of several options. The options involve such choices as distance from the transformer to the OCPD, ampacity of the secondary conductors, and location (i.e., in an industrial facility or perhaps outdoors). But all of those options require an OCPD at the end of whatever conductor you connect to the transformer secondary terminations.

Your description did not include an OCPD until after the 600 amp conductor is split into several 200 amp cables (i.e., the first OCPD is the main breaker on the downstream panel). That would not be allowed. If you put a fused disconnect or an enclosed circuit breaker close to the transformer (i.e., within 10 feet), that would be a good start. If you keep the 200 amp panels within 25 feet of the power distribution block, that should be all that you need.

The relevant rules are NEC article 240.21(C) and article 240.21(B)(2).

Yes you can run all the 200 amp conductors back to the transformer. But you will not be able to land that many conductors on the transformer secondary terminals. You would need a separate enclosure to house the terminations, and you will need an OCPD upstream of that enclosure. So I don't think this would work out for you.
 
Thank you for the response! You mentioned, "If you keep the 200 amp panels within 25 feet of the power distribution block, that should be all that you need." I assume that this is in conjunction with a circuit breaker at the transformer (because the panels will indeed be within such distance)? Also, can the built-in transformer fuses be used as the transformer OCPD? Thanks again
 
Thanks! Is there any allowance for situations where the transformer terminals need expansion? I.e. can we go into an enclosure mounted beside the transformer to pull our secondary conductors (or is it the case that as soon as we tap the transformer we have created a conductor that must end in a single OCPD)? It seems that the ten-foot rule would allow this but I am not sure.
 
cottora said:
(or is it the case that as soon as we tap the transformer we have created a conductor that must end in a single OCPD)?
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Yes, you've got it.

Now if you want to confuse things it is possible to run multiple sets of conductors off of a special set of secondary lugs, but you would need a big enough space in the transformer for all the conductors and then each one of those sets would have to end in a single overcurrent close to the transformer and your gonna spend gobs more money than you have to just to be such a rebel.
 
That is unfortunate because we will have to spend far more money protecting a piece of equipment than what the equipment is even worth. I think another possibility may be to utilize multiple smaller transformers. Thanks again!
 
After some reading, it looks like another option is maintaining the same size/amperage conductor and use splices for the runs to the panels. Is it correct that splices are not considered Taps? Thanks
 
In simplest terms, when you connect a wire to the transformer secondary terminals, the other end of that wire must connect to an OCPD.
 
cottora said:
We are classified as industrial by most all definitions (not sure about the specific NEC definition).
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That won't come into play for your situation. The "special allowance" rule that involves industrial facilities applies when the transformer supplies power to a switchgear or switchboard.

You need an OCPD at the downstream end of your 600 amp conductors. But you can tap a set of 200 amp conductors to the 600 amp conductors, as long as the 200 amp conductors are less than 25 feet long.
 
WIth the addition of the 600 amp OCPD, Option B becomes totally unnecessary as far as the tap rule is concerned.

JAP>
 
If you find yourself using the word "tap" twice in one sentence it should give you a guilty NEC code violating feeling. :)

JAP>
 
Does the 'six throws of the band's come into play here at all? In other words would three 200A breakers be acceptable if they were all in a single panel at the end of the 600A conductor?

Thanks
Jon
 
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