Air-Conditioning and Refrigeration Equipment

Sep 1, 2009 12:00 PM, By Mike Holt, NEC Consultant

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 (A/C) and refrigeration equipment, they also fall under Art. 440. For these motors, the rules in Art. 440 amend the rules in Art. 430 and other Arts. [440.3(A)].

Fig. 1. The disconnecting means must be located within sight from the A/C equipment.

Article 440 exists because of the difference in cooling characteristics between motors that are hermetic motors and those that aren't. Its primary goal is to keep hermetic motors within a tighter than normal temperature band by limiting excess current. Let’s begin with some background on hermetic motors so you can understand why they have special requirements in the NEC.

Down to basics. The motors in A/C and refrigeration equipment are not your ordinary motors — they’re hermetic. Referring to the environment in which the motors are installed, the word "hermetic" means "sealed," as in air-tight. Hermetic motor windings are actually in the refrigerant, inside a sealed casing.

Because of their location in the equipment and 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. Custom-made for specific equipment, hermetic motors are built to an equipment manufacturer’s precise standards.

Fig. 2. Although you can mount the disconnecting means on or within the A/C equipment, you can’t locate it in access panels — or where it will obscure the equipment nameplate.

For example, let’s say 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. 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? Because the system must meet certain performance objectives for a particular market application, the motors must be 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/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 Arts. 430 and 440.

You won't find hermetic motors in all equipment that's classified as A/C and refrigeration. Heaters, for example, fall under that classification but don't have hermetic motors (with the exception of heat pumps). See Arts. 422 and 424, as appropriate.

Disconnecting means. Locate the disconnecting means within sight from — and readily accessible from — the A/C equipment [440.14]. "Within sight" means visible and not more than 50 ft from the equipment [Art. 100] (Fig. 1).

You can mount the disconnecting means on or within the A/C equipment, but you can't locate it in access panels — or where it will obscure the equipment nameplate (Fig. 2).

Fig. 3. Sample calculation on how to size conductor and short circuit and ground fault protection device for a single 18A motor compressor.

Two exceptions to this rule exist. One is that an accessible attachment plug and receptacle can serve as the disconnecting means. The receptacle for the attachment plug doesn't have to be readily accessible [440.14 Ex 2].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 [440.14 Ex 1]:

• 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.

440.14 Ex 1 is for special conditions such as very large process refrigeration equipment and is rarely applicable.

Overcurrent protection and conductor size. 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.

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

The branch circuit short circuit and ground fault protection device is sized at no more than 175% of the motor compressor current rating [440.22(A)]. If the protection device sized at 175% 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% of the motor compressor current rating.

Let's work a problem for practice: What size conductor and short circuit and ground fault protection device are required for a single 18A motor compressor (Fig. 3)?

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

18A x 1.25 = 22.50A

A 12 AWG conductor is rated 25A at 75°C [110.14(C) and Table 310.16].

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

18A x 1.75 = 31.50A

The next size down protective device is rated 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% of the equipment load current rating.

18A x 2.25 = 40.50A

The next size down protective device is rated 40A.

A 30A or 40A overcurrent device is permitted to protect a 12 AWG against short circuits and ground faults for an A/C circuit as per 240.4(G).

Hermetic refrigerant motor compressor and other loads — Multimotor and combination-load equipment must have a visible nameplate containing the maker's name, rating in volts, number of phases, minimum conductor ampacity, maximum rating of the branch circuit short circuit and ground fault protective device [440.4(B)].

Let's work another problem. What size conductor and short circuit and ground fault protection device are required for a multimotor and combination-load equipment where the nameplate specifies minimum circuit ampacity 24A and maximum circuit protection 30A?

Step 1: Size the circuit conductors to the nameplate using Table 310.16 [110.14(C)]. In this case, a 12 AWG conductor is rated 25A at 75°C.

Step 2: Size the circuit protection device as per the nameplate and 240.6(A). In this case, a 30A protection device would be used.

Note: Conductors must be protected in accordance with their ampacity, but this “general” requirement does not apply to A/C and refrigeration equipment [240.4(G)].

Now you understand how to apply those requirements when hermetic refrigerant motor-compressor equipment is involved.