tap connection rules

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Davids1964

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We are installing a 75Kva transformer tapping a single set of 3/0's off parrallel set of 500's 3 phase. I say the tapp conductors need to tapp each set of parrallel 500's. He says only one set. Where in the code can I find the answer to this question? The enginered drawings did not specify.
 
Chris, that brought a question to mind.
It would seem that would be violation of 240.21: "Conductors supplied under the provisions of 240.21(A) through (H) shall not supply another conductor except through an overcurrent protective device meeting the requirements of 240.4."
If his parallel 500s are a transformer secondary per 240.21(C), would it not be a violation of 240.21 to tap them in any manner.?
(if they were transformer secondaries it seems it would be easier just to connect directly to the transformer anyway)


Davids1964: What do your 500s feed ?
 
augie47 said:
If his parallel 500s are a transformer secondary per 240.21(C), would it not be a violation of 240.21 to tap them in any manner.?
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Yes.
(if they were transformer secondaries it seems it would be easier just to connect directly to the transformer anyway)
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And legal that way, as long as the GEC is sized correctly. But the 500s may be already properly protected.
 
600 amp breaker at switchgear. parrallel 500's going to switch fused at 400amp. tap box was just installed just before switch. Parrallel 3/0 now supply the switch. a tap of 3/0 will supply trans. Its the 3/0 to the trans that we have only taped to one set of 500's.
 
Davids1964 said:
600 amp breaker at switchgear. parrallel 500's going to switch fused at 400amp. tap box was just installed just before switch. Parrallel 3/0 now supply the switch. a tap of 3/0 will supply trans. Its the 3/0 to the trans that we have only taped to one set of 500's.
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I'm totally confused. Are you no longer feeding the original switch:confused: with 500's? Is there now a second switch for the transformer being fed from the 3/0's?
 
Davids1964 said:
600 amp breaker at switchgear. parrallel 500's going to switch fused at 400amp. tap box was just installed just before switch. Parrallel 3/0 now supply the switch. a tap of 3/0 will supply trans. Its the 3/0 to the trans that we have only taped to one set of 500's.
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Assuming 500s in separate conduits, 380 x 2 = 720, ok at 600a breaker/load.
If 500s in single conduit, 380 x 2 = 720, 720 x .8 = 576, ok at 600 a OCP, 576a load.

Tap to the switch from both 500s with parallel 3/0s to the switch must comply with 240.21 (B).

Tap to the transformer from both 500s need to terminate in the proper OCP, or comply with 240.21(B)(3).

But as Chris pointed out, both 500s need to be terminated in the same manner.
 
I misread the OP and thought you were tapping off transformer secondary.
If I now read correctly you are tapping off a set of parallel 500s fed by a 600 amp breaker.
As others have stated the taps would need to feed from both 500s.
If you only need the ampacity of the single 3/0, the most common method I have seen would be to install a distributing block or "polaris" connectors in your tap box with both 500's terminating on the block and then tap off with parallel or individual taps as desired.

sidenote: You mentioned your 3/0 going to a 75 kva transformer, hopefully they go to an OCP device and then to the transformer and not directly to the transformer.
 
augie47 said:
sidenote: You mentioned your 3/0 going to a 75 kva transformer, hopefully they go to an OCP device and then to the transformer and not directly to the transformer.
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Yeah. I started to write that, then remembered 240.21(B)(3). But I think that you are right, because while the conductors may be allowed, the transformer still needs primary protection.

Assuming 480 volt 3 phase, primary current 90 amps, T450.3(B) allows 250%, or 225 amp OCP. Standard size. If we can comply with the thermal overload and impedance of Note 3 to T450.3(B), we can protect at 600%, or 540 amps. IMO we can't use the next higher rating for the protection of the transformer, as Note 1 is not referenced for 250%. 600 amps is too large.

Good catch.
 
and, for reference, the only dry transformers I have seen that met Note 3 were some fan cooled ones (GE I think)
 
Davids1964 said:
. No ocp required after the taps.
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So he is saying that it is ok to protect 3/0 conductor with a 600a OCPD ?

Also, by tapping one 500mcm once and the other 500mcm twice, it leaves the potential of

overloading the conductor (500mcm=380amps). I would have your j-man read this thread.
 
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