Air Conditioning and Refrigeration Equipment

Air Conditioning and Refrigeration Equipment

By Mike Holt – a short summary of the requirements contained in the Understanding the 2008 NEC, Volume 1 Textbook.

Keep your cool when working with hermetic motors.

Article 430, the largest Article in the NEC, is commonly understood to be “the motor Article.” But if motors are a part of air-conditioning and refrigeration equipment, they also fall under Article 440. For these motors, the rules in Article 440 amend the rules in Article 430 and other Articles [440.3(A)].

Article 440 exists because of the difference in cooling characteristics between hermetic motors and those that aren’t hermetic. Its primary goal is to keep hermetic motors within a tighter than normal temperature band by limiting excess current.

We began with some background on hermetic motors so you can understand why they have special requirements in the NEC. Now you understand how to apply those requirements.

The motors in air-conditioning and refrigeration equipment are not your ordinary motors. They are “hermetic” motors. The word “hermetic” means “sealed,” as in air tight. This refers to the environment in which the motors are installed. Hermetic motor windings are actually in the refrigerant, inside a sealed casing.

Because of their location in the equipment and because of design factors unique to motors used in such equipment, the cooling characteristics of hermetic motors differ from those of non-hermetic motors. The basic difference is hermetic motors heat up faster.

Non-hermetic motors are typically built to industry standard specifications, and buyers select from standard offerings. The typical non-hermetic motor can satisfy a wide range of purposes and work in a wide range of equipment.

Hermetic motors are built to an equipment manufacturer’s precise standards, and are custom-made for specific equipment.

For example, Trane wants 90-frame hermetic motors for compressor model X. So, Trane contracts with a motor manufacturer to produce X number of 90-frame motors built to work with that compressor. The custom specifications for that work even state which varnish will be used and how much must remain on the windings after dipping (that’s how the varnish is applied) and baking. Carrier may contract that same factory to build its 90-frame motors to a different set of specifications (even using a different varnish).

Why the custom motors? A manufacturer of, say, air conditioning systems, develops a new 30 ton model to satisfy a particular market. The system must meet certain performance objectives, and is designed accordingly. To support the resulting design, the motor that drives the compressor must meet specific design requirements and constraints. Consequently, the manufacturer develops preliminary specifications.

The manufacturer then talks with hermetic motor suppliers. Operations people and engineers look at manufacturability vs. tolerances, production capacity vs. anticipated orders, and other factors. A hermetic supplier may offer a price based on the preliminary specifications, offer a price based on modifications to those specifications, or make suggestions the manufacturer will “take back to the drawing table.”

Once the specifications are finalized, it’s a matter of running a few calculations to come up with the nameplate data required by Articles 430 and 440.

You won’t find hermetic motors in all equipment that’s classified as air-conditioning and refrigeration equipment. Heaters, for example, fall under that classification but don’t have hermetic motors (the exception being heat pumps, which do). See Articles 422 and 424, as appropriate.

Ampacity and rating

Use the rated-load current marked on the nameplate of the equipment to determine the rating of the:

Disconnecting means.
Branch-circuit conductors.
Controller.
Branch-circuit short-circuit and ground-fault protection [440.6(A)].

Exception: If the nameplate provides the branch-circuit selection current, use that instead of the rated-load current.

Disconnecting means

Locate the disconnecting means within sight from, and readily accessible from, the air-conditioning equipment [440.14]. “Within sight” means visible and not more than 50 ft from the equipment [Article 100].

You can mount the disconnecting means on or within the air-conditioning equipment, but you can’t locate it in access panels or where it will obscure the equipment nameplate.

Two exceptions to this rule exist. One is that an accessible attachment plug and receptacle can serve as the disconnecting means. The other is that you don’t need to place a disconnecting means within sight from the equipment, if you meet all of the following conditions:

The disconnecting means is capable of being individually locked in the open position.
The equipment is essential to an industrial process in a facility that has written safety procedures.
The conditions of maintenance and supervision ensure only qualified persons service the equipment.
The provision for locking (or adding a lock to) the disconnecting means is on the switch or circuit breaker and remains in place whether or not the lock is installed.

The receptacle for the attachment plug doesn’t have to be readily accessible.

Overcurrent protection

You must provide protection against short circuits and ground faults. This protection must protect the branch-circuit conductors, control apparatus, and circuits supplying hermetic refrigerant motor-compressors.

You calculate the protection based on either a single motor-compressor configuration or a multi-motor configuration [440.22]. If the equipment nameplate specifies “Maximum Fuse Size,” that size must not be exceeded and a one-time or dual-element fuse must be used.

Single Motor-Compressors. Size the short-circuit and ground-fault protection device at no more than 175 percent of the motor-compressor current rating [440.22(A)]. Compare this to the requirements for non-hermetic motors in Table 430.52, and you’ll see Article 440 is trying to keep hermetic motors within a tighter temperature range.

If the protection device sized at 175 percent isn’t capable of carrying the starting current of the motor-compressor, you can use the next size larger protection device. In no case can the protection device exceed 225 percent of the motor-compressor current rating.

Let’s try out the calculations. Question: What size conductor and short-circuit and ground-fault protection device are required for a 24A motor-compressor connected to a 240V circuit?

(a) 10 AWG, 40A (b) 10 AWG, 60A

Answer: (a) 10 AWG, 40A

Step 1: Determine the branch-circuit conductor [Table 310.16 and 440.32]:

24A x 1.25 = 30A, 10 AWG, rated 30A at 60°C [110.14(C)(a)(a)]

[Table 310.16]

Step 2: Determine the protection device [240.6(A) and 440.22(A)]:

24A x 1.75 = 42A, next size down = 40A

If the 40A short-circuit and ground-fault protection device isn’t capable of carrying the starting current, you can size the protection device up to 225 percent of the equipment load current rating. 24A x 2.25 = 54A, next size down 50A.

Multi-motor compressors. What if the equipment incorporates more than one hermetic refrigerant motor-compressor? Or what if it has a hermetic refrigerant motor-compressor and other loads? Perform multi-motor calculations.

The rating of the branch-circuit short-circuit and ground-fault protection device must not be more than the largest motor-compressor short-circuit ground-fault protection device, plus the sum of the rated-load currents of the other compressors 440.22(B)(1)].

Size the branch-circuit conductors at 125 percent of the larger motor-compressor current, plus the sum of the rated-load currents of the other compressors [440.33]. Conductor sizing

As with branch-circuit protection sizing calculations, base your conductor sizing calculations on either a single motor-compressor configuration or a multi-motor configuration.

Single Motor-Compressors. Branch-circuit conductors must have an ampacity not less than 125 percent of the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater [440.32].

For these same conductors, size the short-circuit and ground-fault protection between 175 percent and 225 percent of the rated-load current [440.22(A)].

Let’s work another problem. Question: What size conductor and short-circuit and ground-fault protection device are required for an 18A motor compressor?

(a) 12 AWG, 30A (b) 10 AWG, 50A

Answer: (a) 12 AWG, 30A

Step 1: Determine the branch-circuit conductor [Table 310.16 and 440.32]:

18A x 1.25 = 22.50A, 12 AWG, rated 25A at 75°C [Table 310.16]

Step 2: Determine the branch-circuit protection [240.6(A) and 440.22(A)]:

18A x 1.75 = 31.50A, next size down = 30A

If the 30A short-circuit and ground-fault protection device isn’t capable of carrying the starting current, you can size the protection device up to 225 percent of the equipment load current rating. 18A x 2.25 = 40.50A, next size down 40A.

A 30A or 40A overcurrent device can protect a 12 AWG conductor against ground-faults and short circuits for an air-conditioning circuit. See 240.4(G) for details.

To purchase Mike Holt’s Understanding the 2008 NEC, Volume 1 textbook, please click here, or call our office at 888-632-2633 for more information.