HVAC: Not Just Your Average Load
The topic of heating, ventilation, and air conditioning (HVAC) presents a target-rich environment with regard to the topic of safety. As installers, we may only be concerned with the tools, personal protective equipment, liquids and chemicals, and electrical hazards from a safety perspective. We have to also consider that HVAC equipment could play an important role throughout its life for the contents (including people and goods) of the structure or area it serves. The fact that HVAC systems account for 39 percent of the energy used in commercial buildings in the United States means that we probably see a lot of these types of applications.
For many reasons, the most important day in the life of HVAC equipment is the day it is installed. The selection and installation of HVAC equipment is surrounded with parameters that must be considered that go beyond heating and cooling to the electrical supply equipment. The system must perform to the expectations of the design and do so for a long time without experiencing issues outside of normal maintenance concerns. The use of listed products for not only the HVAC equipment itself but for all of those components used in the installation help the system perform safely. It’s important that the equipment is installed per their listing requirements/ manufacturer instructions. The installer can be presented with a challenge when trying to determine the proper size of overcurrent protective devices and/or proper short circuit current ratings for this equipment. It’s the installer, for the most part, that must deal with the details.
An important section of the UL White Book (www.ul.com/whitebook) is in Appendix A, “UL Marking Guide,” which includes a marking guide for “Electric Heating and Cooling Equipment.” This marking guide can be useful to the installer as it describes all of the marks required by UL for the equipment. This reference helps us understand nameplates for this equipment, which can be confusing. For example HVAC equipment can be quite complex with voltage ratings that vary. Voltage ratings can be a single nominal value such as 230V or a ranges expressed as “220 – 240.” Standard voltage ranges include110—120, 200—208, 220—240,254—277, 440—480, and 550—600. A single unit may have more than one voltage rating because more than one supply circuit can be connected. A unit with more than one supply circuit will probably have different load ratings for each circuit. This is important for sizing overcurrent protective devices, conductors, and even generators should one be used to supply this load along with others.
Per UL 1995, the HVAC nameplate can specify the type of overcurrent protective device that must be used. When the nameplate specifies “Maximum Overcurrent Protective Device,” then either a circuit breaker or fuse is permitted. If the nameplate is marked “Maximum Fuse_____,” then fuse protection must be provided in accordance with the label. If the nameplate is marked “Maximum Circuit Breaker_____,” a circuit breaker must be provided in accordance with the label. Sometimes the equipment makes life easy for the installer/inspector.
Reading nameplates can be challenging. The UL Marking guide can help interpret what you are seeing. As always there is the call to the manufacturer that should be made if needed.
HVAC equipment must get power from some electrical source and how that occurs is directly impacted by the National Electrical Code® (NEC®). The following are some key articles important to the successful application of HVAC equipment:
- Article 110, “Requirements for Electrical Installations”
- Article 210, “Branch Circuits”
- Article 220, “Branch-Circuit, Feeder, and Service Calculations”
- Article 240, “Overcurrent Protection”
- Article 422, “Appliances”
- Article 424, “Fixed Electric Space-Heating Equipment”
- Article 426, “Fixed Outdoor Electric Deicing and Snow-Melting Equipment”
- Article 427, “Fixed Electric Heating Equipment for Pipelines and Vessels”
- Article 430, “Motors, Motor Circuits, and Controllers”
- Article 440, “Air Conditioning and Refrigerating Equipment”
The following are those special locations and specific applications that may impact your HVAC installation:
- Article 502, “Class II Locations”
- Article 517, “Health Care Facilities”
- Article 550, “Mobile Homes, Manufactured Homes, and Mobile Home Parks”
- Article 552, “Park Trailers”
- Article 620, “Elevators, Dumbwaiters, Escalators, Moving Walks, Platform & Stairway Chairlifts”
- Article 645, “Information Technology Equipment”
- Article 646, “Modular Data Centers”
- Article 665, “Induction and Dielectric Heating Equipment”
- Article 670, “Industrial Machinery”
- Article 680, “Swimming Pools, Fountains, and Similar Installations”
The following are two important topics with regard to the electrical portion of the HVAC installation. Branch circuit protection and short-circuit current rating (SCCR) are two topics we’ll explore in this article.
Branch Circuit Protection HVAC-
Individual Motor-Compressor(s) and HVAC Equipment Having Motor- Compressor(s) and Other Loads (Such as fan motors, electric heaters, coils, etc.).
NEC Section 440.22(A) for individual motor-compressor applications tells us that the overcurrent protective device (OCPD) sized for branch circuit protection only must not exceed 175 percent of the hermetic motor-compressor rated load current or branch circuit selection current (whichever is larger). If this size OCPD cannot withstand the motor starting current, a higher amp rating is permitted, but in no case can the OCPD size exceed 225 percent.
Always refer to the nameplate on the equipment as the sizing (amp rating) for overcurrent protection may have been determined by the manufacturer of the equipment. Should that be the case, it is not necessary to apply any further multipliers to arrive at the proper size.
The marked protective device rating is the maximum protective device rating for which the equipment has been investigated and found acceptable by nationally recognized testing laboratories. When the nameplate states the maximum size fuse or circuit breaker, it is critical that this direction must be followed without exception to be in compliance with 110.3(B) of the NEC. Remember, NEC 110.3(B) requires that listed or labeled equipment must be installed in accordance with any instructions included in the listing or labeling.
Marked Short-Circuit Current Rating –
New Air Conditioning and Refrigeration Equipment with Multimotor and Combination-Loads.
NEC Section 440.4(B) requires the nameplate of this equipment to be marked with its SCCR. There are exceptions for which this requirement does not apply to this equipment:
- One- and two-family dwellings
- Cord and attachment-plug connected equipment
- Equipment on a 60A or less branch circuit
So for most commercial and industrial applications, air conditioning and refrigeration equipment with multimotor and combination loads must have the SCCR marked on the nameplate. This facilitates the inspection and approval process. Inspectors need this information to ensure that NEC® 110.10 is met. A potential hazard exists where the available short-circuit current exceeds the SCCR.
HVAC equipment installed in locations where the available fault current exceeds the listed short-circuit current levels could present a real hazard to property as well as personnel. The SCCR of the HVAC unit, which is on the nameplate, shall be equal to or greater than the available short-circuit current at the terminals of the HVAC unit. If the HVAC unit nameplate specifies a specific type and size overcurrent protective device (not supplied integral with the unit), then that specific size (as a maximum) and type overcurrent protective device shall be in the building distribution system that supplies the unit.
HVAC equipment can be very important for safety, and care during installation can help ensure a long future for the equipment being installed. HVAC systems that do not function properly is one of the most common problems that impact workplace indoor environmental quality. The HVAC is responsible for ventilating, heating, and cooling an area or a structure. They not only heat and cool the air, they distribute and even remove air pollutants. In hot climates their cooling work can make a structure habitable, and in cold climates their heating work can make a structure habitable. When they don’t function properly, safety becomes a concern. The installation of HVAC equipment may not be as simple as some other aspects of the electrical distribution system. Care should be taken when selecting, installing, and inspecting this equipment. Remember, the devil is in the details.
As always, keep safety at the top of your list and ensure you and those around you live to see another day.
Thomas Domitrovich, P.E., is a National Application Engineer with IEC Platinum Partner Eaton Corporation in Pittsburgh, Pennsylvania. He has more than 20 years of experience as an Electrical Engineer and is a LEED Accredited Professional. Domitrovich is active in various trade organizations on various levels with IEC, International Association of Electrical Inspectors, Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturer’s Association (NEMA), and the National Fire Protection Association (NFPA). He is involved with and chairs various committees for NEMA and IEEE and is an alternate member on NFPA 73. He is very active in the state-by-state adoption process of NFPA 70, working closely with review committees and other key organizations.