Feeders & tables

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rcarroll

rcarroll

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Location
Fort Collins, Colorado
I have an EE who wants to use 310.15(B)(7) for a multi-family building, instead of 310.15(B)(16)
Other than the code violation, what problems could arise?

Thank you for your help!
 
rcarroll said:
I have an EE who wants to use 310.15(B)(7) for a multi-family building, instead of 310.15(B)(16)
Other than the code violation, what problems could arise?

Thank you for your help!
Click to expand...

A little more detail of specifically he wants to use it for would help. He can use this table for feeders to individual dwelling units if they are 120/240 single phase feeders.
 
Fire :D
depending on the type protetion provided on thr service condcutors (individual vs MLO), you could overload the cables to a critical point,
Note that if all the required criteria is met, it might be possible to use (B)(7) for the individual unit feeders (as kwired noted as I was slow typing) )
 
rcarroll said:
Sorry, I should have added 120/208V service instead of 120/240.
Click to expand...

Whether it is 120/208 single phase or 120/240 single phase, I would think you would still be able to use table 310.15(B)(7) for the feeders to the individual dwelling units, but as others have stated, it can't be used from the transformer to the building.

Although the code does specifically mention 120/240 and does not say 208! hehe:happyno:
 
rcarroll said:
Sorry, I should have added 120/208V service instead of 120/240.
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Unless there was a change in '11 (no copy available at the moment),,,the 120/208 won;t allow (B)(7) for the individual feeders either.
 
augie47 said:
Unless there was a change in '11 (no copy available at the moment),,,the 120/208 won;t allow (B)(7) for the individual feeders either.
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No change in the '11 code. The EE is pretty much asking for special permission to use smaller feeders to each dwelling units he designs in my town.
I would like to know if there are any inherent dangers when I plead my case to the AHJ. Other than code says no.
 
I am still wondering why it would make a difference with 120/208 vs. 120/240 if one wanted to use table 310.15(B)(7). Is it because the CMP was worried that 230 V or 240 V loads would be used at 208 and result in more current on the dwelling feeder? And this extra current works against the "diversified" load of the dwelling?:huh:
 
wirenut1980 said:
I am still wondering why it would make a difference with 120/208 vs. 120/240 if one wanted to use table 310.15(B)(7). Is it because the CMP was worried that 230 V or 240 V loads would be used at 208 and result in more current on the dwelling feeder? And this extra current works against the "diversified" load of the dwelling?:huh:
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Since most residential equipment is single phase I wonder whether they are worried about an imbalance esp. with reduce size from the table.
 
wirenut1980 said:
But wouldn't that loading unbalance only be an issue from the transformer to tbe building, which does not qualify to use the reduced sizes?
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I would think this is an issue to protect the poco lines and their transformer. Not sure. But it would also affect the service wires.
 
Not sure why 120/208 is excluded, but consider the fact that the neutral of a 120/208 is a current carrying conductor when only two phase conductors are used.
 
kwired said:
Not sure why 120/208 is excluded, but consider the fact that the neutral of a 120/208 is a current carrying conductor when only two phase conductors are used.
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I have always assumed that is the exact reason 208 is excluded.

We have large areas of the city that have 208 distribution but each home or smaller multifamily only gets two legs of the three.
 
kwired said:
Not sure why 120/208 is excluded, but consider the fact that the neutral of a 120/208 is a current carrying conductor when only two phase conductors are used.
Click to expand...
I have to believe that a multi gang meter stack supplied by a 3ph. system will utilize all phases. Unit 1 a-b, unit 2 b-c, unit 3 c-a, etc. To me, the neutral wouldn't be overloaded that much, yes?
 
rcarroll said:
I have to believe that a multi gang meter stack supplied by a 3ph. system will utilize all phases. Unit 1 a-b, unit 2 b-c, unit 3 c-a, etc. To me, the neutral wouldn't be overloaded that much, yes?
Click to expand...

Neutral is not overloaded with exception of harmonics, but that is a different discussion, and usually not too significant in dwellings.

The problem is you have 3 current carrying conductors instead of 2 for single phase feeders therefore more heat is generated within raceways and cables containing the feeder.

With a three phase service the neutral only carries imbalance of three phase conductors. It is also not permitted to size the three phase service with table 310.15(B)(7). The single phase feeders to each dwelling are three current carrying conductors if 120/208 but are only two current carrying conductors if 120/240
 
rcarroll said:
I have to believe that a multi gang meter stack supplied by a 3ph. system will utilize all phases. Unit 1 a-b, unit 2 b-c, unit 3 c-a, etc. To me, the neutral wouldn't be overloaded that much, yes?
Click to expand...

I would tend to agree. If the engineer spec'd a full sized neutral (at least in this context, full sized to the phase conductors) then in the worst case phases A and B could be max'ed out with zero load on C - but the neutral would still be protected at ampacity so it wouldn't be overloaded.

The catch is that they undoubtedly would like to use SE-R between the service and the unit panelboards, which almost always has an undersized neutral - and that neutral could easily be overloaded without the third phase cancelling some of that neutral current. One way around that would be oversized conductors compared to the load, which defeats their end game, using 310.15(B)(7).

Unless there is some really pressing circumstance they really should play by the rules and use 310.15(B)(16). One official CMP reason for disallowing 120/208 for Table 310.15(B)(7) is neutral current.

Tell them to blow off the project for three years, we'll see if they accept my proposal to delete 310.15(B)(7) altogether. :D
 
kwired said:
Not sure why 120/208 is excluded, but consider the fact that the neutral of a 120/208 is a current carrying conductor when only two phase conductors are used.
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Yes, that is it. Thanks! Can't believe I overlooked that in 120/208 single phase feeder, the two phases are not 180 degrees apart, and as such, the neutral current is not simply the cancelled out phase currents.
 
George Stolz said:
The catch is that they undoubtedly would like to use SE-R between the service and the unit panelboards, which almost always has an undersized neutral - and that neutral could easily be overloaded without the third phase cancelling some of that neutral current. One way around that would be oversized conductors compared to the load, which defeats their end game, using 310.15(B)(7).
Click to expand...

George, I have never seen any SER cable with a reduced neutral. The bare EGC is usually smaller but not the insulated white wire.
 
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