2014 NEC 310.15 (B) (7) Clarification

[Daniel Biggar]

I have been asked to post a question to what I believe has an obvious answer. This code article is in entitled "120/240-Volt, Single-Phase Dwelling Services and Feeders." By the title I would naturally assume this section does not apply to dwelling units supplied by a 208 volt single phase (3W) system derived from a 208/3/4 service. The building is mixed-use retail below apartment building.

My (2) questions are:

1 - May I use this section for 208/1/3 feeders to the individual apartment dwelling units and if so, where in the code would it allow that?

2 - If this is not allowed, would someone explain the reason why 240/1/3 can be sized in this manner and NOT 208/1/3 feeders?

 

[Dennis Alwon]

I am not certain why but no you cannot use it and I have never seen 208 single phase service although I realize the feeder may be 208 single phase. I guess it may be because there is no control over the phase balancing

 

[jumper]

The basis of the table and section was historical usage data provided by POCCs for 120/240V SFDs. 208/120V services were not included, so120/208V feeders are omitted also.

 

[iwire]

I suspect it is because when you use just two hots of a 208 volt system the neutral carries as much as the ungrounded conductors meaning you have three current carrying conductors vs two with a 240 volt supply.

 

[don_resqcapt19]

An accepted PI for the 2017 code removes the 120/240 volt language and will make the section usable for single phase 120/208 volt systems.

Committee Statement:
The Canadian Electrical Code allows reduced conductor sizes for single dwellings, apartments and similar buildings for both 120/240 and 120/208 V feeder conductors. This revised language will contribute to harmonization of the NEC and CE Code requirements.
The load diversity in residential applications is similar whether the residence is fed with 120/240 or 120/208; therefore, 120/208 will be added to 310.15(B)(7).

Negative with Comment
Huddleston, Jr., Robert L.
1. Section 220.61(C)(1), which (4) references in the FR, states: Prohibited Reductions. There shall be no reduction of the neutral or grounded conductor capacity applied to the amount in 220.61(C)(1) or portion of the amount in (C)(2) from that determined by the basic calculation: (1) any portion of the 3-wire circuit consisting of 2 ungrounded conductors and the neutral conductor of a 4-wire, wye-connected 3-phase system. 2. Section 310.15(B)(5)(b) states: In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(3)(a). 3. In addition, the ampacity adjustment for the grounded conductor allowed in single-phase 120/240 volt systems is based on the fact that when the two phase conductor currents are balanced, there is no current at all in the grounded conductor. If the two phase conductor currents are not identical, the grounded conductor only carries the imbalance, which is generally a very low amount of current. Because of this, the neutral is not really considered a current-carrying conductor and it does not produce heat in the raceway or cabling encasing the feeder. However, if the grounded conductor conducts significant current, as it would in a 3-wire 208/120 volt system, heat would be generated in this conductor. This should eliminate the reduction in sizing that FR 1504 allows.

Kent, Gerald W.
No technical substantiation was provide to allow for this change. In a 120/240v systems, heat is generated in effectively two conductors under full load. In a fully loaded 120/208v system, heat would be generated in all three conductors. Time honored tradition has shown the deductions work for 120/240v systems I believe due to the 'zero' effect of the neutral load that would not be present in 120/208v system

 

[iwire]

An accepted PI for the 2017 code removes the 120/240 volt language and will make the section usable for single phase 120/208 volt systems.

Intresting that they are still playing with this section.

Just fix article 220 and have us all use the normal wire size tables.