Service entrance cable sizing

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102 Inspector

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N/E Indiana
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Inspector- All facets
Currently using 2008 NEC. Table 310.15(B)(6) indicates for a 300 amp service, 250 Awg Copper or 350 Awg Alum is needed. This is a triplex apartment building, but since this Table does not indicate it is for single family, are those conductor sizing correct. In this small town, nobody does load calculations so there would be no further adjustment. Originally I thought 350 Awg Copper or 500 Awg Alum until I remembered Table 310-15. Am I correct in my interpretation for service entrance conductor sizing. Thank you.
 
In the 2008 edition, Table 310.15(B)(6) allows for the adjustment factor. Section 310.15 (B) (6) states "For individual dwelling units of one-family, two-family and multifamily dwellings, conductors, as listed...….." This tells me that 250Awg copper is an acceptable size for a 300 amp meter base. Table 310.16 is telling me 350Awg copper but does not allow for an adjustment factor. That is why I am checking with the experts here to confirm that 250 Awg is code compliant. Am I missing something?
 
102 Inspector said:
In the 2008 edition, Table 310.15(B)(6) allows for the adjustment factor. Section 310.15 (B) (6) states "For individual dwelling units of one-family, two-family and multifamily dwellings, conductors, as listed...….." This tells me that 250Awg copper is an acceptable size for a 300 amp meter base. Table 310.16 is telling me 350Awg copper but does not allow for an adjustment factor. That is why I am checking with the experts here to confirm that 250 Awg is code compliant. Am I missing something?
Click to expand...

the key is in red. You get the size reduction only for the service or feeder to a single dwelling unit, not the entire building.
 
Article 220.84

Article 220.84

220.84 Multifamily Dwelling. 220.84(A) Feeder or Service Load.

It shall be permissible to calculate the load of a feeder or service that supplies three or more dwelling units of a multifamily dwelling in accordance with Table 220.84 instead of Part III of this article if all the following conditions are met:
  • (1)
    No dwelling unit is supplied by more than one feeder.
  • (2)

    Each dwelling unit is equipped with electric cooking equipment. Exception: When the calculated load for multifamily dwellings without electric cooking in Part III of this article exceeds that calculated under Part IV for the identical load plus electric cooking (based on 8 kW per unit), the lesser of the two loads shall be permitted to be used.
  • (3)
    Each dwelling unit is equipped with either electric space heating or air conditioning, or both. Feeders and service conductors whose calculated load is determined by this optional calculation shall be permitted to have the neutral load determined by 220.61.



This calculation method is only permitted for dwelling units with electric cooking equipment. The exception permits load calculation for dwelling units that do not have electric cooking equipment by adding an artificial electric cooking equipment load of 8 kW per unit to the load calculated in accordance with Part IV and comparing this calculated load to the load for the dwellings without electric cooking equipment calculated in accordance with Part II of Article 220 (commonly referred to as the “standard calculation”).



220.84(B) House Loads.
House loads shall be calculated in accordance with Part III of this article and shall be in addition to the dwelling unit loads calculated in accordance with Table 220.84.
Number of
Dwelling Units
Demand Factor
(%)
3–5
45
6–7
44
8–10
43
11
42
12–13
41
14–15
40
16–17
39
18–20
38
21
37
22–23
36
24–25
35
26–27
34
28–30
33
31
32
32–33
31
34–36
30
37–38
29
39–42
28
43–45
27
46–50
26
51–55
25
56–61
24
62 and over
23



220.84(C) Calculated Loads.

The calculated load to which the demand factors of Table 220.84 apply shall include the following:
  • (1)
    33 volt-amperes/m2 or 3 volt-amperes/ft2 for general lighting and general-use receptacles
  • (2)
    1500 volt-amperes for each 2-wire, 20-ampere small-appliance branch circuit and each laundry branch circuit covered in 210.11(C)(1) and (C)(2)
  • (3)

    The nameplate rating of the following:
    • a.
      All appliances that are fastened in place, permanently connected, or located to be on a specific circuit
    • b.
      Ranges, wall-mounted ovens, counter-mounted cooking units
    • c.
      Clothes dryers that are not connected to the laundry branch circuit specified in item (2)
    • d.
      Water heaters
  • (4)
    The nameplate ampere or kVA rating of all permanently connected motors not included in item (3)
  • (5)
    The larger of the air-conditioning load or the fixed electric space-heating load
 
That is why I ask the experts. Totally missed the first four words and read what I thought it said. Looks like it is 350 Awg copper. Thanks for all the replies.
 
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