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Dale001289

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I have an existing installation where a 150AT feeds a 30kVA dry-type transformer (480-208/120V) with #8 Awg cable. However the xfmr primary side has an additional CB at 50AT which should adequately protect the windings and the cable. It seems counterintuitive to have a cable that's only 1/3 ampacity rating of the overcurrent protection. Is this a code violation?
 
Dale001289 said:
I have an existing installation where a 150AT feeds a 30kVA dry-type transformer (480-208/120V) with #8 Awg cable. However the xfmr primary side has an additional CB at 50AT which should adequately protect the windings and the cable. It seems counterintuitive to have a cable that's only 1/3 ampacity rating of the overcurrent protection. Is this a code violation?
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What size conductors come off the 150? It may be a feeder tap situation. If so, maybe they had the 150 and it was easier to do it that way or they needed to stay with that frame size? Getting the parts (bus kit, fingers, strap kit - whatever the manufacturer calls it) to go down a frame size can be expensive after the fact.
 
Not sure I understand the problem or question... what size are the secondary conductors? Or are you concerned about the #8 from the 150A ATS to the 50A breaker on the primary side (if that's what you have).

We did a 45kva 480/208 xfmr a few weeks ago. The factory leads to the lugs on the primary side were #10cu.
 
Only concerned about the #8awg off the 150AT CB upstream at the MCC...but since the primary also has a CB at 50AT I don't believe its an issue

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Dale001289 said:
I have an existing installation where a 150AT feeds a 30kVA dry-type transformer (480-208/120V) with #8 Awg cable. However the xfmr primary side has an additional CB at 50AT which should adequately protect the windings and the cable. It seems counterintuitive to have a cable that's only 1/3 ampacity rating of the overcurrent protection. Is this a code violation?
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The supply side of the 50 amp device is a feeder tap.

If using tap rule that requires 1/3 the ampacity of the 150 amp device an 8 AWG copper @75C is 50 amp conductor and is 1/3 of 150.
 
kwired said:
The supply side of the 50 amp device is a feeder tap.

If using tap rule that requires 1/3 the ampacity of the 150 amp device an 8 AWG copper @75C is 50 amp conductor and is 1/3 of 150.
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But the supply side conductor from the 150AT CB to transformer o/c device at 50AT exceeds 25 feet
 
Just out of curiosity, how was the tap made at the lead end?

JAP>
 
Dale001289 said:
But the supply side conductor from the 150AT CB to transformer o/c device at 50AT exceeds 25 feet
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Now you bring up that detail, I figured since you mentioned one third ampacity in OP you were already applying 25 foot feeder tap rule.

Sounds like you either need to run some 150 amp conductor or move the 50 amp overcurrent protection closer to the 150 amp device - or change the 150 to a 50.
 
Good advice. Are there any circumstances under which the transformer secondary conductors can exceed 25 feet INDOORs?

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Dale001289 said:
Good advice. Are there any circumstances under which the transformer secondary conductors can exceed 25 feet INDOORs?

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You were initially asking about primary side feeder taps. Secondary only taps - no. Those are covered in 240.21(C) Primary plus secondary is covered in (B)(3) but that means if you have 24 feet of secondary tap your primary side tap can only be 1 foot long.

In some industrial settings you could have tap like you described in OP (tap a 150 amp feeder and land in a 50 amp overcurrent device) over 25 feet in accordance with (B)(4). But there are limitations, main being the horizontal distance must still be no more then 25 but could be up to 100 feet long if there is mostly a vertical run. Secondary only tap still is limited to 25 feet
 
I changed horses midstream on this since a new issue arose relative to the initial post. We have a rather difficult construction super who refuses to accept reality. I always like to double check with the gurus on the Forum. Thanks again.

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