Help--xformer secondary conductor protection is MLO panel's branch breakers

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donw

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I'm doing the engineering design for a tenant improvement of a warehouse/office that has a 480V-to-208/120V transformer that feeds a main-lug-only panel. It has 15 20-amp single phase breakers (many are the crappy minnie/dual breakers). I have no clue what it's feeder size is, but I could specify it to be sized according to the existing 15 breakers...but it scares me, because someone could install a 30A...or another "dual" breaker. Would you just require the whole panel to be replaced with a main-breaker panel?
 
ron said:
You need a breaker or fused switch between the xfmr and mlo panel.
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I agree. There is also a maximum distance between the xfmr and the breaker/fuse.
 
What is the primary overcurrent protection? If it's less than 125% of the current rating of the transformer couldn't you forego the MCB or CB before the panel per Table 450.3(B)?
 
spsnyder, primary breaker is 20A - this is a 15KVA xfrmer. 450 is for winding protection only. I was looking at 240.21(C)(3) for feeder protection (assuming industrial app.) It would allow you to terminate into branch breakers, but they'd have to add up to protect the feeder conductor. Just seems too risky to me, though, considering branches could be replaced easily. Oh, and the distance between the xfmr and the panel is less than 10'.
 
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spsnyder said:
What is the primary overcurrent protection? If it's less than 125% of the current rating of the transformer couldn't you forego the MCB or CB before the panel per Table 450.3(B)?
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You have to protect (i.e., from overcurrent) the secondary windings, and the feeder conductors to the panel, and the panel itself. You can't do all three with a primary side breaker. The only thing for which the primary breaker can be credited as providing protection is the secondary windings. You still need a breaker or fuse to protect the other two items.

Normally, conductors are supposed to be protected at the point at which they get their power. Under certain conditions, a breaker or fuse at the panel can be used to protect the panel feeder (i.e., the feeder conductors are protected at the far end, not at the point at which they get their power).
 
Thanks, Charlie. That's what I was trying to say about 240.21(C)(3) above. But you said "a" (one) breaker. 240.21(C)(3) allows for multiple breakers, as long as they add up to protect the feeder.
 
donw said:
Thanks, Charlie. That's what I was trying to say about 240.21(C)(3) above. But you said "a" (one) breaker. 240.21(C)(3) allows for multiple breakers, as long as they add up to protect the feeder.
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In your assessment of 240.21(C)(3) you are skipping a major requirement, and that is the very first part of 240.21(C): "Each set of conductors feeding separate loads..."

In your situation you have one set of conductors feeding multiple loads... so 240.21(C)(3) does not apply.

Then when you get into panelboard protection you'll find an OCPD is required, at the very least, at one end or the other of its feeders.
 
Smart, that is a good point. I wish that was made more clear in (C)(3)

Don, it obviously wasn't installed *correctly* under 240.21(C)(2), because (C)(2) says "the" (single?) overcurrent-protective device at the termination of the conductors. It does meet all the other requirements(primary o/c protection size, conductor length), though.
 
donw,
because (C)(2) says "the" (single?) overcurrent-protective device at the termination of the conductors.
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That is not what that rule says. There is no requirement that the tap conductors terminate in a single overcurrent device.
(2) Transformer Secondary Conductors Not Over 3 m (10 ft) Long Where the length of secondary conductor does not exceed 3 m (10 ft) and complies with all of the following:
(1) The ampacity of the secondary conductors is
a. Not less than the combined calculated loads on the circuits supplied by the secondary conductors, and
b. Not less than the rating of the device supplied by the secondary conductors or not less than the rating of the overcurrent-protective device at the termination of the secondary conductors, and
c. Not less than one-tenth of the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary to secondary transformer voltage ratio
(2) The secondary conductors do not extend beyond the switchboard, panelboard, disconnecting means, or control devices they supply.
(3) The secondary conductors are enclosed in a raceway, which shall extend from the transformer to the enclosure of an enclosed switchboard, panelboard, or control devices or to the back of an open switchboard.
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The panel itself is a device.
Don
 
don_resqcapt19 said:
donw,

That is not what that rule says. There is no requirement that the tap conductors terminate in a single overcurrent device.

The panel itself is a device.
Don
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But Don, as I mentioned above, the basic requirement of 240.21(C) is each set of conductors serving separate loads. The set of conductors under question herein serve, or will serve, multiple loads. Therefore 240.21(C) in its entirety does not, or will not, apply.
 
Smart,
But Don, as I mentioned above, the basic requirement of 240.21(C) is each set of conductors serving separate loads. The set of conductors under question herein serve, or will serve, multiple loads. Therefore 240.21(C) in its entirety does not, or will not, apply.
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I don't agree. That wording was placed in the code to permit multiple sets of secondary conductors to originate from a single transformer.
Don
 
don_resqcapt19 said:
Smart,

I don't agree. That wording was placed in the code to permit multiple sets of secondary conductors to originate from a single transformer.
Don
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I'm not saying you can't have multiple sets of conductors (each set feeding a separate load). I'm saying you can't serve multiple loads with one set of conductors (without an OCPD protecting that set at one end or the other or somewhere in between). The prelude to specified criteria states "separate loads" for "each set". [edit]I do see where one can interpret this to mean, non-parallel sets of conductors. So I'll yield on this point and say I am wrong. [end edit, mostly]

[So] rather than arguing that point, perhaps we can agree on another... the latter part of 240.21(C)(1):
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.
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As the OP states, the secondary is 208Y/120.
 
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Smart,
[So] rather than arguing that point, perhaps we can agree on another... the latter part of 240.21(C)(1):
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No, I can't agree there either. We are providing protection for the secondary conductors per the "tap" rules. If you have a two wire to two wire or delta to delta, then the secondary conductors can be protected by the primay OCPD without using the "tap" rules. With this transformer we must comply with one of the rules in 240.21(C).
That being said you have a valid point about 240.21(C)(2). The section is not clear...another oportunity for a 2011 proposal.
Don
 
don_resqcapt19 said:
Smart,

No, I can't agree there either. We are providing protection for the secondary conductors per the "tap" rules. If you have a two wire to two wire or delta to delta, then the secondary conductors can be protected by the primay OCPD without using the "tap" rules. With this transformer we must comply with one of the rules in 240.21(C).
That being said you have a valid point about 240.21(C)(2). The section is not clear...another oportunity for a 2011 proposal.
Don
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Ttypically there are approximately 4000 proposals for an average code cycle...I think that the 2011 cycle will see that many from Don :cool: :grin:
 
don_resqcapt19 said:
Smart,

No, I can't agree there either. We are providing protection for the secondary conductors per the "tap" rules.
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I do not disagree with you here. However, please note this is protection for the secondary conductors. We still have to meet the requirement(s) for protecting the panelboard! Please note the FPN at the end of 240.21(C)(2) that refers one to 408.36 for the protection requirements of panelboards. The 2008 NEC clears up a lot of confusion on panelboard protection. Nevertheless, it is rather clear to me in both 2005 and 2008 cycles. Please refer to 408.36(D) [2005] or 408.36(B) [2008]. The only exception(s) to not having OCP on the secondary side of the transformer FOR THE PANELBOARD is when the conditions specified in 240.21(C)(1) are met, or the panelboard is used as service equipment.
 
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