When you buy an electric motor, half of the job is the motor itself and the other half is connecting it correctly on site. Failing to close the terminal box properly, choosing the wrong cable gland or making an incorrect link can drag even a brand-new motor into failure within the first few weeks because it takes in moisture, short-circuits or runs at the wrong voltage and burns its winding. As HEM Motor, with our identity as a manufacturer and seller, the feedback we most frequently receive from the field is the sentence "the motor was sound, but a mistake was made in the connection." In this article we take a practical look at the terminal box, choosing the right cable gland (PG/metric), IP55 and IP65 sealing, and star/delta linking, from the perspective of purchasing and installation.

Terminal box and cable gland connection on a cast iron electric motor

What Is a Terminal Box and Why Is It So Important?

The terminal box is the protective enclosure where the ends of the motor's windings come out and the supply cable is connected. Inside it there are six winding ends (U1, V1, W1, U2, V2, W2) and the terminal board that connects them to the supply cable. This box is not just a connection point; it is also the sealing barrier that separates the motor's inner world from the external environment. If the cover, the gasket and the cable entry points of the box are not closed correctly, then even if the motor's frame is IP55, moisture and dust will leak inside through the connection point.

In motors with a cast iron frame, the terminal box is usually cast as well and offers high mechanical strength. In our cast iron body electric motors, the terminal box, with its structure resistant to impact and vibration, can be used safely even on demanding sites. However, just as important as the durability of the box is how the cable entering it is connected and how the entry point is sealed.

The Terminal Board and Winding Ends

In standard three-phase motors, the terminal board has six screws. These six ends allow the motor to be connected in star (Y) or delta (Δ). The position of the linking plates (busbars) determines at which voltage and with which starting method the motor will operate. Incorrect linking can expose the motor to the wrong voltage and cause the winding to burn out instantly. For this reason, the connection diagram on the motor nameplate must always be read before making the connection; our article on avoiding the delivery of the wrong motor: nameplate matching is a useful guide for reading the nameplate and matching one-to-one.

The orientation of the terminal box is also a practical detail. On most industrial motors the terminal box is positioned on top, but it can be rotated around the frame in 90-degree steps. Setting the box orientation correctly according to the direction from which the cable comes prevents the cable from bending unnecessarily and removes the strain on the gland. In narrow machine cabinets or in motors coupled to a reducer, the box orientation is a matter that should be planned before installation. Sharing the answer to the question "which way should the terminal box face?" with us at the ordering stage saves time in the field.

Star or Delta? What Happens if the Linking Is Done Wrong?

The linking on the terminal board of a three-phase motor determines the operating voltage of the motor. In a typical 400/690 V motor, the delta (Δ) connection corresponds to 400 V, while the star (Y) connection corresponds to 690 V. On the standard 400 V grid in Türkiye, such a motor is connected in delta. If the linking is done incorrectly, the motor either remains weak at too low a voltage or burns its winding at excessive voltage.

In star-delta starting applications, all six ends are taken to the panel and the linking is done not in the terminal box but through the contactors. This method is used to reduce the starting current in high-power motors. We detailed which starting method is suitable for you in our article on star-delta or soft starter. And to make sure the linking and commissioning are done in the correct order, the electric motor commissioning and first start-up checklist should always be followed.

Watch the Linking Busbars

If the screws of the linking busbars remain loose, heating and arcing occur at the connection point; over time the terminal board carbonizes and a phase loss occurs. For this reason, all terminal screws should be tightened to the appropriate torque value and rechecked a few weeks after the first start-up. This check is even more important in vibrating applications; a loosened terminal screw is one of the most common and most easily preventable causes of failure.

The cable lugs (terminals) used when making the linking must also be suitable for the cable cross-section. A lug that is too small will not fully grip the cable, while a lug that is too large will sit loosely under the screw. Lugs should be tightened with a suitable crimping tool, and soldered connections should be avoided, because solder can crack over time under vibration. Using a hex socket or torque wrench when tightening the screws on the terminal board prevents both insufficient tightening and over-tightening that strips the thread. This simple installation discipline is also important for keeping the motor within warranty coverage, because winding burns caused by faulty connections are usually outside the warranty.

Cable Gland Selection: PG or Metric?

The cable gland is the part at the point where the cable enters the terminal box that provides both mechanical grip and maintains the sealing. The correct gland selection is essential to preserve the IP protection class of the terminal box. A wrong size or a gland hole left empty becomes the weakest point of the motor.

There are two common thread standards for glands:

  • PG (Panzer Gewinde): An old but still very widespread thread standard. It is referred to by sizes such as PG9, PG11, PG13.5, PG16, PG21 and PG29. Each size suits a specific range of cable outer diameter.
  • Metric (M): The newer and internationally preferred standard. It is referred to by sizes such as M16, M20, M25, M32 and M40, and the number directly gives the thread diameter in millimeters.

Two things are important in gland selection: the thread size of the entry hole in the box and the outer diameter of the cable. The gland must be selected so that it fully tightens around the cable's outer diameter; a gland that is too large will not grip the cable and will not provide sealing, while a gland that is too small will crush the cable. For this reason, sharing your cable cross-section and outer diameter with us at the quotation stage allows us to deliver a motor with the correct gland.

Unused Holes and Blind Plugs

If the terminal box has more than one cable entry and you are not using all of them, the empty holes must be closed with a blind plug appropriate to the IP class. A single hole left open invalidates the entire sealing. On an open site, this detail directly determines the lifespan of the motor; we covered the subject in detail in our article on corrosion protection and outdoor use in cast iron body motors.

Metric and PG cable gland types and terminal box sealing on a motor

Choosing the Gland Size According to the Cable Diameter

The basis of correct gland selection is knowing the outer diameter of the cable. Each gland size is designed for a specific range of cable outer diameter. For example, an M20 gland generally tightens cables with an outer diameter of 7-13 mm, an M25 gland the 9-17 mm range, and an M32 gland the 15-21 mm range. If your cable's outer diameter falls below the lower limit of the gland you choose, the gland seal will not grip the cable sufficiently and both the mechanical grip and the sealing will weaken. If it exceeds the upper limit, the gland will not compress and will not close. For this reason, you need to clarify the cable cross-section foreseen for the motor power rating, the cable's outer diameter and the number of glands to be used before ordering. The power-to-cable-cross-section match is decisive in terms of installation ease as much as the number of terminal box entries.

In high-power motors, the terminal box is larger, the cable cross-section is thicker and therefore the gland size is bigger. In motors of 90 kW and above, parallel cables are often used instead of a single cable and more than one gland entry is required. In such high-power projects, handling the connection and delivery planning together with asynchronous motor pole selection and the power calculation guarantees both the correct cross-section and the correct gland selection.

IP55 and IP65 Sealing: Completing Protection with the Right Gland

At HEM Motor, all motors are produced as standard in the IP55 protection class. IP55 means protected against dust and resistant to water spray coming from all directions; it is more than sufficient for the vast majority of general industrial environments, including dusty workshops. However, the motor being IP55 is not enough for the terminal box to remain IP55 as well; this protection is only carried to the field with the right gland and the right closing. An IP55 motor connected with the wrong gland can drop to the IP20 level at the connection point.

In environments exposed to water, intense moisture or washdown (food plants, outdoor pumps, washing lines, cold storage rooms and exterior fan units), IP65 protection may be requested. IP65 provides full protection against dust and resistance to low-pressure water jets from all directions. In these cases, both the motor and the glands to be used must be of the IP65 class, and additionally the unused holes must be closed with IP65 blind plugs. You can evaluate in which application the difference between IP55 and IP65 is decisive together with our article on mistakes made when buying an electric motor, where we cover the motor selection criteria.

The Right Gland and Cable Direction Outdoors

When making a connection outdoors, not only the class of the gland but also the entry direction of the cable is important. The cable should be routed so that it forms a downward "drip loop" before entering the gland; this way, water accumulating on the cable flows down before reaching the gland. The gasket of the terminal box cover must be intact, and the cover screws must be tightened in a crosswise order with equal torque. These small details are the elements that truly preserve the motor's IP protection outdoors. We listed the protection equipment that should be ordered together with the motor in our article on electric motor protection equipment.

Do Not Skip the Earthing Connection

In the terminal box, next to the three phase ends, there is another connection that is often overlooked but vitally important: earthing. There is a marked earthing screw on the motor frame, inside the terminal box or on the outer surface of the frame. This screw safely transfers to earth any voltage that leaks to the frame in the event of an insulation fault, protecting both personnel and equipment. The earthing conductor must be of the same quality and a suitable cross-section as the phase conductors, and it must be pressed against the screw with a clean metal surface. Earthing made on a painted or oxidized surface is electrically non-functional, even if it appears connected. Measuring the insulation resistance (megger) after connection is recommended to verify both the earthing and the health of the winding.

Plan the Right Motor and Connection Equipment Together

As HEM Motor, we offer motors from 0.55 kW to 355 kW, in IE3 and IE4 efficiency classes, with cast iron and aluminum frame options. We help you select the right motor in terms of the terminal box, the cable gland and the connection diagram. Our standout product groups for a connection solution suited to your site are:

You can review all the options in our efficient electric motors category, and for more content on cast iron motors, take a look at our cast iron body motors blog category. A motor delivered with the correct gland size and the correct linking diagram runs trouble-free and long-lived from day one.

In short, the terminal box and cable connection are the "invisible" but decisive half of a motor investment. The correct gland size, a blind plug appropriate to the IP class, a solid earth, terminal screws tightened to the torque value and the correct linking are the five fundamental elements that allow a brand-new motor to deliver the full lifespan written on its nameplate. As HEM Motor, we plan these details together with you, prepare the motor to suit your site and your cable and deliver it. This way, no missing material appears in the panel on commissioning day, the installation is completed in a single pass and the motor turns trouble-free from the first start-up.

Frequently Asked Questions

Should I use a PG or a metric gland in the terminal box?

Both work correctly; what is decisive is the thread standard of the box's entry hole. In new projects, metric (such as M20, M25) is preferred because it is an international standard and spares are easy to find. If PG glands are widespread in your existing facility, you can choose PG for compatibility. What matters is that the gland is sized to fully tighten your cable's outer diameter; if you tell us your cable cross-section, we deliver it with the correct gland.

My motor is IP55 but it takes in water from the connection, why?

The problem is most likely not in the motor but in the terminal connection. A wrong-size gland, an untightened gland nut, a blind plug hole left open or a loose cover gasket invalidates the IP55 protection at the connection point. Fitting a correct-size gland, closing the empty holes with a blind plug appropriate to the IP class and tightening the cover with equal torque solves the problem.

What happens if I do the star-delta linking wrong?

The linking determines the operating voltage of the motor. If you connect a motor that should be delta-connected on a 400 V grid in star, the motor cannot deliver its power and is strained; conversely, if you connect it in delta when it should be in star, the motor is left at excessive voltage and its winding can burn. Before connecting, always follow the connection diagram on the motor nameplate; if you are unsure, we will share the connection diagram with you.

Get a Quote

For the supply of a motor with the correct terminal connection, the right gland size and the IP protection class suited to your site, the HEM Motor team is by your side. As a manufacturer and seller, we deliver the motor together with the correct connection equipment, quickly and from stock. Call our line on +90 (532) 345 49 86 right away or fill out the form on our contact us page; let us determine the most suitable motor for your cable and your site together.