Washing Away Electrical Costs

At our company, we believe that it is incumbent to implement new technology and good design practice to provide more value and efficiency to the owner and end user.

One such technology that's becoming more prevalent in multi-unit high-rise residential projects: the ventless combination washer and dryer. This appliance uses a 120-volt/20-amp circuit with plumbing requirements similar to that of a common dishwasher. Because of its reduced size, the ventless combination unit can be placed in a number of locations, depending on the floor plan of the condo and the needs of the resident (see "All in One," below). We've actually just successfully completed the design phase of a large high-rise residential project that is intending to incorporate the ventless combo units.

But before exploring the implications of this new technology, let's consider more standard practices. National Electrical Code-required calculations for multi-family dwelling units can be complex, and the appliance packages installed in the individual condos/apartments play a major role, as they determine the size of individual unit electrical panels, the meter center serving multi-unit panels, the 120/208-volt step-down transformers feeding the meter centers, the main electrical service and the electrical utility transformers.

So what about electrical load? According to NEC Section 220.18, Electrical Clothes Dryer, the load for electric dryers in a dwelling unit shall not be less than 5,000 watts for each unit. However, if the nameplate rating exceeds 5,000 watts, the electrical design must utilize that rating as the load. You can omit this load if the condo unit has no electric dryer provision. However, it is not uncommon to provide both gas and electric sources for the dryer hookup. If the plumbing plans indicate a gas connection for the dryer, the electrical designer should verify that an electrical connection should be omitted from the plans prior to completing the electrical design.

On the other hand, the ventless combo appliances do not use the same technology or power requirements as a conventional dryer unit and, again, are more like dishwashers with respect to operation and power requirements, with a total power requirement of approximately 1,500 VA in lieu of the 5,000-VA, 208-volt, 30-amp circuit required for a conventional dryer and 1,500-VA, 20-amp, 120-volt circuit required for a typical washer. As such, when they're in place, the 5,000 VA required for a conventional dryer should be replaced with 1,500 VA in the load calculation.

Both NEC Section 220.30, Optional Calculations for Dwelling Units, Section (B) General Loads, and Section 220.32 (C) Optional Calculations for Multifamily Dwellings General Loads, indicate the following loads shall be used in the load calculations for a condo unit electrical panel:

(1) Laundry Circuit as specified in 220.16: 2-wire, 120-volt, 1,500-VA.

(3) 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 and water heaters.

The ventless combination units we've reviewed have nameplate ratings between 1,200 VA and 1,500 VA, for both the dryer and the washing machine. Therefore, the 1,500-VA nameplate should satisfy both (1) and (3). There is no need for an additional 1,500 VA, because the washer is part of the combination unit load.

Unlike the example of a 30-amp, 240-volt combination stack unit utilizing conventional technology, sometimes found in small homes or apartments, combination units are served from a 120-volt, single-phase, 20-amp outlet, so the requirement of a 20-amp, 120-volt, single-phase branch circuit required in the laundry area per the NEC, Section 220.16 (B) is satisfied.

In addition, NEC Section 220.32 (C) (3) allows you to utilize the nameplate rating of the combination washer and dryer (1,500 VA) for the meter center, the transformer and the service load calculations. This section of the NEC also indicates that there shall be 1,500 watts allocated for the laundry circuit. As with the calculations for the condo unit electrical panel detailed above, the electrical designer can argue that the laundry circuit is not required for the meter centers, step-down transformers and main electrical services because the combination washer and dryer unit includes the washing machine.

In reference to 220.32 (C), a total of 5,000 VA can be removed from the condo unit panel calculations. With conventional technology, 5,000 VA is required for the dryer and 1,500 VA for the laundry circuit. With the ventless combination units, only a total of 1,500 VA is required for both the washer and dryer, so the net difference is 5,000 VA.

In addition, for the feeder calculations for the meter centers, the 120/208-volt step-down transformers and the main electrical service calculations, 5,000 VA can be reduced for each unit.

I have detailed an actual calculation below, which assumes that all of the condominium units use a ventless combination appliance, to give you an idea of the full impact that the use of these appliances can have on the electrical distribution system.

In this example each 600-amp meter center will feed 24 condo units, and each 120/208-volt transformer will feed six meter centers. In addition, the main electrical service will feed a total of two step-down 120/208-volt transformers. These transformers will feed 144 condo units (6 meter centers × 24 condo units per meter center), and the main electrical service will feed a total of 288 condo units (2 transformers × 144 condo units).

Per the diversities allowed in NEC Section 220–30, the reduction allowed per the ventless combination unit would reduce the electrical load in the condo panels by approximately 10 amps.

NEC 220.30 (B) indicates, "The general calculated load shall be 100% of the first 10 kVA plus 40% of the remainder of the following." One of the required loads is the nameplate data for the fastened-in-place clothes dryer and another is the 1,500 VA for the laundry circuit. Based on this portion of the code, the following is the calculation to determine the savings in the condo panel over utilizing a conventional dryer:

5,000 VA × 40% = 2,000

VA 2,000 VA/208 volts = 9.6 amps

(Load Reduction in the Condo Unit Electrical Panel)

All of the calculations below will utilize a dwelling unit connected load of the following and also consider the aforementioned requirements of NEC Section 220.32 (C) (1) and (3):

21 kVA - utilizing the combination ventless washer and dryer unit

26 kVA - utilizing a conventional washer and dryer

The actual appliances utilized in addition to the washer and dryer and the square footage of the units will dictate the actual connected load of the dwelling units. The calculations below are only an example, but do represent the net difference between the conventional washer and dryer and the combination ventless units (5,000 VA):

21 kVA (per condo unit) × 24 (units served from a meter center) × 35% (per table 220–32) = 176.4 kVA = 490 amps @ 208 volt, 3 phase — utilizing a 600-amp meter center

26 kVA (per condo unit) × 24 (units served from a meter center) × 35% (per table 220–32) = 218.4 kVA = 606 amps @ 208 volt, 3 phase — utilizing an 800-amp meter center

5,000 VA × 24 condo units = 120,000 VA × 35% (per NEC Table 220–32) = 42,000 VA = 116.6 amps @ 208 volt, 3 phase (load reduction in the meter centers)

The end result is that a 600-amp meter center can be utilized in the electrical distribution system in lieu of 800-amp meter centers. In this example, there are 12 meter centers in the project. The savings also include the tap from the bus and the feeders to the meter centers.

The savings on the 120/208-volt step-down transformer would be:

21 kVA (per condo unit) × 144 (dwelling units served from a step-down transformer) × 23% (per table 220–32) = 696 kVA - Utilize a 750-kVA transformer with 2,500-amp bus

26 kVA (per condo unit) × 144 (dwelling units served from a step-down transformer) × 23% (per table 220–32) = 861 kVA - Utilize a 1,000-kVA transformer with 3,000-amp bus

5,000 VA × 144 condo units = 720,000 VA × 23% (per NEC Table 220–32) = 165,600 VA = 165.6 kVA (load reduction in the 120/208-volt step-down transformer)

The end result is that a 750-kVA transformer can be utilized in lieu of a 1,000-kVA transformer. In this example, there are two 120/208-volt step-down transformers in the electrical distribution system.

The savings in the main electrical service feeding the residential units would equate to the following:

21 kVA (per condo unit) × 288 (dwelling units served from a meter center) × 23% (per table 220–32) = 1,391 kVA = 1,674 amps @ 480 volt, 3 phase - utilizing a 2,000-amp residential service

26 kVA (per condo unit) × 288 (dwelling units served from a meter center) × 23% (per table 220–32) = 1,722 kVA = 2,072 amps @ 480 volt, 3 phase - utilizing a 2,500-amp residential service

5000 VA × 288 condo units = 1,440,000 VA × 23% (per NEC Table 220–32) = 331,200 VA = 398.6 amps @ 480 volt, 3 phase (load reduction in the main electrical residential service)

As you can see from the calculations above, there can be a significant amount of reduction in the electrical infrastructure in the condo unit electrical panel, at the meter center, at the 120/208-volt step-down transformer and at the main residential electrical service. Additionally, the required transformers from the serving electrical utility may also see a reduction in size based on the reduced electrical load. As a collateral effect, space required for the utility transformer building vault could potentially be reduced.

In addition, associated non-electrical changes can be achieved. No venting is required with these combination ventless units. Also, less space will need to be allocated for the laundry area, which can allow for more living space. The architect or space planner also has more options when locating these units because of the small total footprint.

Another benefit of the ventless units is the possibility of LEED points towards project accreditation. Additional LEED points can be obtained through improved energy efficiency. Energy and Atmosphere (EA) credits 1.1 through 1.5 can garner up to 10 points toward LEED certification. Minimum LEED certification is obtained with a total of 26 points.

There is no clear direction given in the NEC on how to deal with the new ventless combination washer/dryer units. Therefore, any assumptions and interpretations from the code must be pre-approved by the authority having jurisdiction. Additionally, if reductions in the electrical distribution system are designed and built, there will probably be no room to change back to the conventional, less energy-efficient technology. This new technology can represent electrical energy and space savings, remove some venting requirements and can save on some of the electrical system infrastructure, but must be clearly communicated with the entire design team prior to implementation. 
 

National Electrical Code — Table 220–32 Optional Calculations — Demand Calculations for Three or More Multi-family Dwelling Units