240.21(C)(3) What is it good for?

[sandsnow]

This was discussed here: http://forums.mikeholt.com/showthread.php?t=102041&highlight=240.21(C)(3%2

Even if it was permissible to tap 4/0 to a 4/0 to feed multiple disconnects, it does not seem practicable.

Feeding a power panelboard (without a main) is not permitted because of 408.36(D) unless it was a Delta-Delta XFMR, but 240.21(C)(1) allows that anyway.

I suppose you could use it to feed a switchboard without a main as long as the OCPD add up to equal to or less than the conductor ampacity and rated secondary amps.

A lot of Code requirements are written to allow someones idea of good installation to be allowed, but I can't imagine what it would be in this case, except for the switchboard scenario.

Anyone have another take on this? What am I missing?

 

[jim dungar]

Even if it was permissible to tap 4/0 to a 4/0 to feed multiple disconnects, it does not seem practicable.

A connection of 4/0 to 4/0 is not a tap according to the NEC. According to the article 240 definition, a tap involves a conductor that is protected "above" it normal current rating.

 

[sandsnow]

A connection of 4/0 to 4/0 is not a tap according to the NEC. According to the article 240 definition, a tap involves a conductor that is protected "above" it normal current rating.

Ok so do you think this rule was intended for say a 75kVA XFMR to feed 4 100amp disconnects, ech one being fed with 4/0? That would take a special order to the factroy to have it come with lugs to take 4/0. Normally the lugs would take up to about #1.

Just doesn't seem practcable or am I wrong?

 

[e57]

I agree with the statement below - OCP for the conductors at 4/0 no matter how many junctions there might be - is allowed. 4/0 to 4/0 is not a "tap".

A connection of 4/0 to 4/0 is not a tap according to the NEC. According to the article 240 definition, a tap involves a conductor that is protected "above" it normal current rating.

The method above is pretty common in multi-unit metering with no OCP at all. And no so common, but still allowed in feeders with multiple panels, so long as the max load is not exceeded. For that matter - the buss-work of any panel is essentially the same configuration....

Ok so do you think this rule was intended for say a 75kVA XFMR to feed 4 100amp disconnects, ech one being fed with 4/0? That would take a special order to the factroy to have it come with lugs to take 4/0. Normally the lugs would take up to about #1.

Just doesn't seem practcable or am I wrong?

In your questions example there is no indication of load for each - or voltage.... Whats the load on the disco's? Whats the total?

215.2B(2) Feeders Supplying Transformers and Utilization Equipment. The ampacity of feeders supplying a combination of transformers and utilization equipment shall not be less than the sum of the nameplate ratings of the transformers and 125 percent of the designed potential load of the utilization equipment that will be operated simultaneously.

 

[e57]

My apologies. Sorry just got around to reading the convoluted thread you linked to....

My statements above ass-u-me-d that there was OCP on the 4/0 in your example.... and partially that it was feeding the T...

I don't see the word tap in 240.21(C) anywhere. As in my previous post they are not tap conductors. They are secondary conductors. There is no OCP on there supply side of any rating equal to the ampacity of the conductor or not.

RE: http://forums.mikeholt.com/showthread.php?t=102041&highlight=240.21%28C%29%283%252&page=2

Anyway - I still say a lot more info is involved in making the decision - so many variables....

 

[jim dungar]

Ok so do you think this rule was intended for say a 75kVA XFMR to feed 4 100amp disconnects, ech one being fed with 4/0? That would take a special order to the factroy to have it come with lugs to take 4/0. Normally the lugs would take up to about #1.

Just doesn't seem practcable or am I wrong?

You first connection to the transformer becomes your tap. Most 75KVA transformers I have seen, did not come with factory installed lugs so getting the right lug should not be big deal.

Over the years I have seen instances of transformer secondary conductors in a wireway which then were spliced/branched out to several disconnect devices. I used to feel this was acceptable, but I now question this practice

 

[sandsnow]

I want to pursue this discussion. I'l post more tonight. Thanks for the replies

 

[sandsnow]

The 75kVA XFMR has four 60 amp (not 100amp) fusible switch. These have 50 amp fuses each. This protection complies with the section and also 450.3(B)
The conductor required by this section is 4/0 since it cannot be less than the rated amps of the XFMR.
So I have 4/0 copper going to each fusible switch.
The total load is 200 amps. 50 amps on each switch.
The lugs I was refering to were the lugs in the fusible switches, not the XFMR. I believe we're talking special order here.
The above complies with 240.21(C)(3) but is not practicable in my opinion.

We could use two 100 fusible switches with 100 amp fuses, but still I believe we're looking at a special order lug kit for the switches.

So aside from using this rule to supply a switchboard with the proper OCPD's and no main OCPD, we have little practicable use for this rule.

My next step is to research ROP as to what was the submitter thinking.