Table 310.15(B)(6)

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Ragin Cajun said:
This table refers to 120/240V 1 phase services. Why not also for 120/208V 1 phase??? What am I missing?


Thanks

RG
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Because the neutral conductor in these systems (120/208) carries neutral current even when the load on the phases is balanced.
 
websparky said:
208/120 is a 3 phase system and does not supply residential.
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Dave, actually it can and does. I know here in the south some high rise Condo's are supplied with 120/208 wye systems which are split at the multi-gang meter equipment to single phase feeders to the individual units.

Roger
 
I agree, as Don stated many apartment buildings use a three phase, 4 wire service with single phase feeders to each apartment.
 
As Charlie once posted, the study that was conducted by the EEI which produced Table 310.15(B)(6) never submitted data on 208/120 volt systems. It appears they were not that common at the time.



Here is a proposal that was rejected for the 2008 NEC:


6-63 Log #1174 NEC-P06 Final Action: Reject
(310.15(B)(6))
____________________________________________________________
Submitter: Daniel Leaf, Seneca, SC
Recommendation: Add:
“or 120/208-volt” after “120/240-volt”.
Substantiation: Edit. This section should be applicable where dwelling units
in apartment buildings are supplied with 120/208-volt systems.
Panel Meeting Action: Reject
Panel Statement: The neutral conductor of a 120/208 volt, 3 wire system does
carry significant load. In a 120/240 volt system the load on the neutral
conductor is reduced. No substantiation was provided to justify applying the
Table to 120/208-volt service conductors.
Number Eligible to Vote: 11
Ballot Results: Affirmative: 11
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I don't see what Table 310.15(b)(6) has to do with neutral currents. It is an allowance to use a difference size conductor for all the conductors vs. what Table 310.16 would give.
 
paul32 said:
I don't see what Table 310.15(b)(6) has to do with neutral currents. It is an allowance to use a difference size conductor for all the conductors vs. what Table 310.16 would give.
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Gotta agree with you here. Either way we have to account for neutral load, so why the difference? Just between us, though, a #2 copper (115A) can be protected at 125A, next standard trip rating, which happens to be the same as the table. Ditto for other sized conductors. Ohhhhh welllll. RG
 
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