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5-Family Load Calcs #165464 06/28/07 10:06 AM
Joined: Jun 2002
Posts: 206
Happi_Man Offline OP
Wow. I haven't been on for awhile...the site looks amazing.

At any rate, I'm estimating a job for a 5-family apartment building. Each unit will have its own independant electrical system, service panel, meterbase, disconnect etc. Except obviously it will be fed from one primary service by the poco.

The engineers' calculations show a load of 155 AMPS. Now I know that its a different method of calculating because its a multi-family unit...but I'm baffled by the thought of putting a 200 AMP service in each of these 975 sq ft units. And that's total sq ft not just living space. I did a calculation for each unit and came up with 89.88 AMPS per unit.

I've asked one local inspector but I keep getting referred to the code and unfortunately I do not have access to a code book at the moment. (Because I had to go get a real job - yep I wasn't surviving amoungst the blood thirsty home builders and GCs - I ain't got that much blood).

Do I multiply the 89.88 by 5 (the number of units) and take 43% of that. I'm trying to remember what the factor in the table in 220.84 but I dont even know if the unit "qualifies" because code book in hand.

Thanks in advance for the input.
Mike Donley

Last edited by Happi_Man; 06/28/07 10:09 AM.
2017 / 2014 NEC & Related Books and Study Guides
Re: 5-Family Load Calcs [Re: Happi_Man] #165471 06/28/07 11:27 AM
Joined: Jul 2004
Posts: 9,571
gfretwell Offline
You are probably looking for this

220.32 Optional Calculation — Multifamily Dwelling.
(A) Feeder or Service Load. It shall be permissible to compute the load of a feeder or service that supplies more than two dwelling units of a multifamily dwelling in accordance with Table 220.32 instead of Part II of this article where 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.
The method of load calculation under 220.32 is optional and applies only where one service or feeder supplies the entire load of a dwelling unit. If all the stated conditions prevail, the optional calculations in 220.32 may be used instead of those in Part II of Article 220.
Exception: When the computed load for multifamily dwellings without electric cooking in Part II of this article exceeds that computed under Part III 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.
Section 220.32(A)(2) requires that each dwelling unit be equipped with electric cooking equipment in order to use the load calculation method found in 220.32(A). The exception to 220.32(A)(2) permits load calculation for dwelling units that do not have electric cooking equipment by calculating a simulated electric cooking equipment load of 8 kW per unit and selecting the lesser of the two loads.
(3) Each dwelling unit is equipped with either electric space heating, air conditioning, or both. Feeders and service conductors whose demand load is determined by this optional calculation shall be permitted to have the neutral load determined by 220.22.
(B) House Loads. House loads shall be computed in accordance with Part II of this article and shall be in addition to the dwelling unit loads computed in accordance with Table 220.32.

Table 220.32 Optional Calculations — Demand Factors for Three or More Multifamily Dwelling Units
Number of Dwelling Units Demand Factor (Percent)
3–5 45

(C) Connected Loads. The computed load to which the demand factors of Table 220.32 apply shall include the following:
(1) 1500 volt-amperes for each 2-wire, 20-ampere small-appliance branch circuit and each laundry branch circuit specified in 220.16.
(2) 33 volt-amperes/m2 or 3 volt-amperes/ft2 for general lighting and general-use receptacles.
(3) The nameplate rating of all appliances that are fastened in place, permanently connected or located to be on a specific circuit, ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, water heaters, and space heaters. If water heater elements are interlocked so that all elements cannot be used at the same time, the maximum possible load shall be considered the nameplate load.
(4) The nameplate ampere or kilovolt-ampere rating of all motors and of all low-power-factor loads.
(5) The larger of the air-conditioning load or the space-heating load.
220.33 Optional Calculation — Two Dwelling Units.
Where two dwelling units are supplied by a single feeder and the computed load under Part II of this article exceeds that for three identical units computed under 220.32, the lesser of the two loads shall be permitted to be used.

Last edited by gfretwell; 06/28/07 11:28 AM.

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