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Home My WebLink About914 W LAKE ST - CORRESPONDENCE - 12/20/2011Adonai Professional Services, Inc. (APS, Inc.) Electrical Engineers March 15, 2012 NEC 310.15(B)(7) Equivalent Objective Approval Mr. Mike Gebo Building Services City of Fort Collins RE: Ram's Crossing K2 West lake Street Fort Collins, CO 80521 Dear Mr. Gebo: This content of this Letter is being issued at you request to document the applicability of the 2011 National Electrical Code (NEC) 310.15(B)(7) to 120/208-Volt, 3-Wire, Single -Phase Dwelling Services and Feeders for the above mentioned project. Please return a signed copy of this Equivalent Objective Approval as a record of the City's concurrence with the analysis below. NEC 310.15(B)(7) 120,240-1,oh, 3-1f'h e, and Feeders. Fur indivfdua/ direllin,; units ofone-family. Ili o-lamih, and nnedriJatnily dwellings, conductors, as listed in Table 310.1 SIBI('). +hall he permitted as 120,240-voh, 3-u'ire, single-phase sc wire -entrance conductors, sen'ice-lateral cotuluctors, ernd feeder conduc-tors that-scrcc us the main pmr�err feeder to each dwelling unit and are installed in racewznv or cable v,ilh (it - it ithow an eyuipmenl gronndbng cnnduclor. For application of this section, /he main poirer feeder shall he the feeder between the nwin disc onaect and the panel hoard that supplies, either by hi anch circuits or by feeder+', or both, oll loads Ihat are part ur as5uciated with the dwelling unil. The Iceder conductors to a chcelling unit shall not be required to have an allouvhle cnnpacill raling greater /hall their set i ice -entrance conductors. 7'he grounded c'oncluctor Shull he permitted to he smaller than the ungrounded conductors, provided the requirenwnts of 215.2, 220.C1. mod 130.47 are met. T,t BLE 310.15(B)(7) Service or Feeder Rating (Autpere�,;) • 100 #2 Aluminum • 110 #1 Aluminum • 125 #1/OAluminum • 150 #2/0Aluminum • 175 #3/OAluminum • 200 #4/OAluminum • 225 #250Aluminum ti Page 2 Code -Making Panel 6 (CMP-6) made the following statement: • Panel Statement: The original data that was used to establish 310.15(B)(6), formally Note 3, was actual utility company data for 1201240 volt 3-wire single phase systems. It established that the conductors specified in the table could be used on a calculated dwelling unit load as shown. The submitter has not provided any technical data to show that this is true for 1201208 4-wire three phase systems. Thus the reduction of wire size allowed per this code section was based on actual load profile data for dwelling units and is thus independent of voltage. I believe it is easy to understand that the KW usage for dwelling unit will remain the same regardless of what voltage it is fed with. It only makes logical sense that the demand factor remain the same for both 120/240 and 120/208 Volt, 3-Wire, Single -Phase Dwelling services and feeders. Consider the following example: • Apartment has a 35KW "calculated" load. At 120/240V the service needs to be sized at 150AMP5 (35,000/240=146) o At 120/208V the service needs to be sized at 175AMPS (35,000/208=168) 1s you can see the service size to the Apartmentis different for the two different voltages The Service conductor size without any detnaud reduction for the 120.240V system per table 310. I5(B)(16} would be? (iU r1N'G Copper or 43i0 .AWG Aluminum. Per table 310.15(13X 16) # 1 /0 AWG copper is rated for I50ANIPS at 7> degrees C and #,:0 AWG alumintnn is rated for I55Ainps at 75 dagraes C. 1f table 3,10.15(1(7) is applied to the 120/240V system the service conductor can be sized to = I AW6 cuppet or rt20 AWG :Aluminum. Per table 310.15(B)(I6) tit AWE; copper is ruled for 130AMPS at 75 degrees C and iQ 0 AWG aluminum is rated for 135Atnps at 75 degree; C. As you can see fixun CMP-6s panel statement this reduction is allowed because the "actual" measured load was shuimi to be loss than the calculated load ftre dwelling units. Phis equates to what is essentialk a reduction in wire size base on the calculated load ( BO 150 or 135:155). If you applied a I R,,, reduction to the 35KW `calculated" load that would equal 30.45KW • Apartment has a 30.45KW "calculated" load. At 120/240V the service needs to be sized at 150AMPS (30,450/240=127) At 120/208V the service needs to be sized at 150AMPS (30,450/208=147) The Service conductor size for the 120:240V system per table 310.15(B)(16) based on the reduced load would be #1 AWG Copper or #2.:0 AWG Aluminum. Obviously this matches what is allowed pet 310.15(BX7). If we size the service conductor for the 120'208V s •stem on this reduced load per table 310.15(B)(16) we would need #1!0 AWG Copper or #3.0 AWG Aluminum. Per table 310.15(B)(16) #1'0 AWG copper is rated for ISOAMPS at 75 degrees C and 0 0 AWG alumntutn is rated for 155Araps at 7-, degrees C. Ll Page 3 If we go back to the original 3%W load and 175Amp service based on a [20,'20W wr%ice and applyTable 310.15(131(7) the service feeders a ould be required to be 1-0 AWG Copper or 43'0 AWG aluminum. As you can see this matches what we determined ahove. Thus from this perspective Table 310.15(B)(7) should be viable for both 120/240 and 120/208, 3-wire, Single -Phase Dwelling services and feeders. The caveat to this deduction is NEC 220.61(C)(1) which prohibits the reduction of the neutral feeder of any portion of a 3-wire circuit consisting of 2 ungrounded conductors and the neutral conductor of a 4-wire, 3-phase, wye- connected system. Typically when dwelling units are fed with 120/208V, 3-Wire, Single -Phase services they are "networked" to a 4-wire, 3-phase, wye-connected system. Per NEC 310.15(B)(5)(b) 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. Thus is it not desirable to have a reduced neutral conductor for 120/208 1-phase feeders The load profile for line -to -neutral load currents can vary from dwelling unit to dwelling unit. In the example above 27,500Watt of the calculated 35,OOOWatt overall load was from line -to -line (208V) connected equipment (Range, Dryer, HVAC) that would only leave 8,OOOWatts for the line -to -neutral loads. Even if you put all those loads on the same phase you would only have a maximum of 67Amp on the neutral (8,000/120 = 67). Although you have no way of controlling how much of the dwelling unit equipment is line -to -line or line -to -neutral it seems reasonable that under no circumstance would the line -to -neutral currents exceed the current on the phase conductors to a dwelling unit. In conclusion it seems to me that Table 310.15(B)(7) is applicable to 120/208-volt, 3-Wire, Single -Phase dwelling services, however, the neutral conductor must be sized the same as the phase conductor selected. This should not be an issue since most SE cables come this way anyway. Please sign this letter and return it to me if concur with this assessment. Respectfully submitted, C�^1P__ Chuck Poison, President Adonai Professional Services, Inc. Approval/Concurrence Signature Mike Gebo, Building Official City of Fort Collins