-- Mike Holt
Multifamily service calculations (Figure 10-1) can seem confusing, especially since you have a choice of methods-standard (Article 220, Part II) and optional (Article 220, Part III). Breaking down each method into its component steps clears up the calculation confusion. But, which of the two methods should you use?
On electrical exams, use the standard method unless the question specifies the optional method. In the field, the optional method is usually preferable-if the dwelling qualifies for it. The optional method has fewer steps and results in a smaller service. In either case, the calculations differ somewhat from the single-family dwelling calculations.
This is a six step process.
Step 1: Determine the general load. This consists of the general lighting and receptacles (3VA per sq ft), small appliance (3,000VA) and the laundry (1,500VA) circuit loads of all dwelling units [Table 220.11]. You can omit the laundry load (1,500VA), if a central laundry facility is available to all building occupants [210.52(F) Ex. 1].
Because these circuits will not all be on (loaded) simultaneously, 220.16 permits you to apply Table 220.11 demand factors to the total connected load as follows:
Step 2: Determine air-conditioning versus heat [220.15, 220.21]. Because the air-conditioning and heating loads dont run simultaneously, you can omit the smaller of the two loads. Observe these two requirements:
Step 3: Determine the appliance demand load [220.17]. You can use a demand factor of 75% for four or more appliances fastened in place. These are such appliances as a dishwasher, waste disposal, trash compactor, or water heater-not space-heating equipment [220.15], clothes dryers [220.18], cooking appliances [220.19] or air-conditioning equipment.
Step 4: Determine the clothes dryers load [220.18]. Use 5,000W or the nameplate rating-whichever is greater. Then, adjust that per the demand factors in Table 220.18. You can omit the dryer load if the dwelling units do not have hook-ups for electric dryers. Dont calculate laundry room dryers per this method.
Step 5: Determine the cooking equipment loads [220.19]. For household cooking appliances rated over 13/4kW, calculate feeder and service loads per the demand factors of 220.19, Table and Notes.
Step 6: Size the feeder and service conductors. First, add up all of your loads. How large the conductors must be depends on whether the load is greater than 400A. If its 400A or less, size the ungrounded conductors per Table 310.15(B)(6) for 120/240V, 1Ø systems. If its over 400A, size the ungrounded conductors per Table 310.16 to the calculated demand load.
Applying the steps
To see how these steps work, lets do a sample calculation.
Step 1: Determine the general load.
What is the demand load for a 20-unit apartment building? Each apartment is 840 sq ft., and there is a central laundry facility provided on the premises for all tenants [210.52(F)].
(a) 5,200VA (b) 40,590VA (c) 110,400VA (d) none of these
Answer: (b) 40,590VA
How did we get that answer? We add up the loads and apply the demand factor, this way:
Total connected load for one unit: 5,520VA
Demand Factor per Table 220.11: 5,520VA x 20 units = 110,400VA
Total Demand Load = 3,000VA + 37,590VA = 40,590VA
Step 2: Determine air-conditioning versus heat.
What is the net computed air-conditioning versus heat load for a 40-unit multifamily building with A/C (3-hp, 230V) and two baseboard heaters (3kW) in each unit?
(a) 160kW (b) 240kW (c) 60kW (d) 50kW
Answer: (b) 240kW
We can omit the A/C, because its smaller than the heat.
Step 3: Determine the appliance demand load
What is the appliance demand load for a 20-unit multifamily building where each unit contains a 940VA waste disposal, a 1,250VA dishwasher, and a 4,500VA water heater?
(a) 100kVA (b) 134kVA (c) 7kVA (d) 5kVA
Answer: (a) 100kVA
Here are the numbers:
Total Demand Load = 690VA x 20 units x 0.75 = 100,350VA
Step 4: Determine the clothes dryers load
Each unit of a 10-unit multifamily dwelling building contains a 4.5kW electric clothes dryer. What is the feeder and service dryer demand load for the building?
(a) 5kW (b) 25kW (c) 60kW (d) none of these
Answer: (b) 25kW
Demand Load =
Step 5: Determine the cooking equipment loads
What is the feeder and service demand load for three ranges rated 15.6kW each?
(a) 15kW (b) 14kW (c) 17kW (d) 21kW
Answer: (c) 17kW (closest answer)
Refer to Table 220.19 Note 1-Over 12kW
(a) Column C demand load = 14kW (3 units).
(b) The average range (15.6kW) exceeds 12kW by 3.6kW. Increase Column C demand load (14kW) by 20%. Thus: 14kW x 1.2 = 16.8kW.
Step 6: Size the feeder and service conductors
What is the minimum size service conductors for a multifamily building that has a total demand load of 270kVA for a 120/208V, 3Ø system, if the service conductors are run in parallel?
(a) Two 300 kcmil per phase (b) Two 350 kcmil per phase
Answer: (c) Two 500 kcmil per phase
To get this answer, refer to Table 310.15(B)(6)
I =VA/(E x 1.732)
I = 270,000VA/(208V x 1.732) = 750A
Amperes per parallel set: 750A/2 raceways = 375A per phase.
500 kcmil conductor has an ampacity of 380A [Table 310.16 at 75ºC].
You can use an 800A protection device for two sets of 500 kcmil conductors (380A x 2) [240.4(B)].
The optional method
This is a three-step process. To use this method, the dwelling must meet three conditions:
Step 1: Determine the total connected load. You need to add up the following loads:
Step 2: Determine demand load.
You determine the net computed demand load by applying the demand factor from Table 220.32 to the total connected load (Step 1). You can covert the net computed demand load (kVA) to amperes by:
Step 3: Determine feeder and service conductor size. As with the standard method, how you do this depends on whether the load is greater than 400A. If its 400A or less, size the ungrounded conductors per Table 310.15(B)(6) for 120/240V, 1Ø systems up to 400A. If its over 400A, size the ungrounded conductors per Table 310.16-based on the calculated demand load.
As you can see, these calculations arent hard if you take them one step at a time. Should you do these if you are using stamped drawings? Yes. Even stamped drawings can contain errors-doing these calculations is a good way to ensure the conductor sizes meet or exceed NEC requirements. If your drawings use the optional method, you should also ensure the dwelling qualifies for that method.
Now that youve seen how these calculations work, you can head off service conductor sizing mistakes before construction and inspection. That eliminates delays and expensive rework. More importantly, you will have done your work with the sure knowledge the service conductors are safe for every family in that multifamily dwelling.
Copyright © 2003 Mike Holt Enterprises,Inc.