Connections made within a motor junction box between motor leads and feeder cables are far more prone to failure than those at ordinary loads. This is because a motor circuit, unlike normal loads, is regularly subjected to well-defined transient motor-starting current. For example, the across-the-line starting of an induction motor will result in starting currents that may be up to five or six times normal full load current. Also, the duration of this current is in seconds rather than cycles and is dependent on the type of driven load.
However, this performance characteristic does not rule out the use of aluminum alloy conductors. Tests have proved that 8000 Series aluminum alloy electrical conductors with mechanical connectors perform as well electrically equivalent copper conductors in these applications. (See Sidebar "Connector Tests" below.)
Table. Conductor materials and sizes. "CP" denotes compact strand; "CPR" denotes compressed strand.
(*) Copper conductors of comparable ampacity at 90 [degrees] C from the Table 310-16 of the NEC are selected.
(**) CSA-listed RW90 with aluminum alloy conductor material and 600V insulation thickness and T90 with copper conductor are equivalent products to UL listed XHHW-2 and THHN-2 products, respectively, shown above and evaluated in this project.
([dagger]) Test current is the current, in amperes, required to raise the temperature of the control conductor in a given test loop by 100[degrees] C above ambient air temperature in the Current Cycle Submersion Test.
Additional benefit
An additional benefit derived from the 8000 Series material is its increased flexibility, which assumes great importance in cable installation in tight spaces, such as motor terminal boxes. Mechanical set screw-type connectors have the advantage of only requiring hand tools rather than heavy hydraulic compression tools needed to make crimp connections. This is an advantage when working with motors. Also, mechanical connectors can be removed if required, which is a desired capability since motor leads have to be separated from feeder cables when doing winding testing or for removal or replacement of the motor or driven equipment.
Good installation practices
There are some basic rules of good installation practices you should use when making connections with aluminum conductors in various types of installations. These rules vary per type of connector and termination material.
Using mechanical screw-type connectors.
* Use connectors that are dual rated (AL7CU or AL9CU) and listed by UL for use with aluminum and copper conductors and sized to accept aluminum conductors of the ampacity specified.
* Use a suitable stripping tool to avoid damage to the conductor, to remove insulation from the required length of the conductor.
* Wire brush the conductor and apply a listed joint compound.
* Tighten the connection per the connector manufacturer's recommendation.
* Wipe off any excess joint compound.
Using mechanical compression-type connectors.
* Use connectors that are dual rated (AL7CU or AL9CU) and listed by UL for use with aluminum and copper conductors and sized to accept aluminum conductors of the ampacity specified.
* Use lugs that are marked with wire size, die index, number and location of crimps, and suitably color coded. Lug barrel must be factory prefilled with a joint compound listed by UL.
* Use a stripping tool to avoid damage to the conductor and to remove insulation from the required length of the conductor.
* Wire brush the conductor.
* Crimp the connection per the connector manufacturer's recommendation.
* Wipe off any excess joint compound.
Termination of aluminum conductor to aluminum bus.
* Prepare a compression connection adhering to procedure outlined above for mechanical compression-type connectors.
* Use anodized alloy 2024-T4 bolts that conform to ANSI B18.2.1 and to ASTM B211 or B221 chemical and mechanical property limits.
* Use aluminum alloy 6061-T6 or 6262-T9 nuts that conform to ANSI B18.2.2.
* Use flat aluminum alloy 2024-T4, Type A plain, standard wide series washers that conform to ANSI B27.2.
* Lubricate and tighten hardware as per the manufacturer's recommendations.
Termination of aluminum conductor to copper bus.
* Prepare a compression connection conforming to procedure outlined above for mechanical compression-type connectors.
* Use plated or galvanized medium carbon steel; heat treated, quenched, and tempered bolts equal to ASTM A-325 or SAE grade 5.
* Use heavy semi-finished hexagon nuts that conform to ANSIB18.2.2; threads to be unified coarse series (UNC), class 2B.
* Use steel, Type A plain standard wide series washers that conform to ANSI B27.2.
* Use Belleville conical spring washers of hardened steel, cadmium plated, or silicone bronze.
* Lubricate and tighten hardware as per the manufacturer's recommendations.
Termination of aluminum conductor to equipment not equipped for termination of aluminum conductor.
* Prepare compression connection using an adapter listed by UL for the purpose, or by pigtailing a short length of suitable size of copper conductor to the aluminum conductor with a compression connector listed by UL.
* Provide an insulating cover over adapter body or the compression connector.
* Terminate the adapter or the pigtail on to the equipment per manufacturer's recommendation.
RELATED ARTICLE: CONNECTOR TEST
Listed connectors for use with aluminum conductors have to meet the requirements of UL Standard 486B. In particular, the accelerated performance evaluation test, such as the current cyclingin this standard, is used to assess the stability of the connector-and-conductor combination. Further, most connectors listed in accordance with this standard are also evaluated or listed for use with copper conductors and marked, for example "AL7CU" or "AL9CU".
The same or similar connectors are used by many utilities. To approve them for use on their systems, utilities rely on a similar assessment that is in accordance with ANSI C119.4 Standard. The current cycling test per ANSI C119.4 is more severe than the UL 486B test, and it uses bare conductors.
Conductors and cables of 8000 Series aluminum alloy materials are recognized in the NEC, specifically in Sec.310-14.
Performance of 8000 Series aluminum alloy conductor connections have been compared with similar connections of copper conductors to demonstrate the reliability of performance. These comparisons have been based on utilizing the heat cycling test per UL 486B Standard. Results show that connections of 8000 Series conductors perform as well as those made with electrically equivalent copper conductors.
To provide additional substantiation, the performance of 8000 Series aluminum alloy conductor connections with mechanical screw-type connectors was compared with similar connections of electrically equivalent copper conductors in accordance with the heat cycling test per ANSI C119.4 using insulated conductors. The current required to raise the temperature of the connection to the required level for the current cycling test increased significantly as a result of the use of insulated conductors. Thus, the current cycling test protocol became more severe than the protocols described in both standards. Additionally, two different methods of preparation to make the connections and three different levels of tightening torque were investigated.
To evaluate conductor performance at the connections, UL listed (AL9CU) mechanical screw type connectors were tightened to five sizes of insulated Type XHHW-2 conductors with an 8000 Series aluminum alloy and their electrically equivalent sizes of Type THHN conductors with copper, as shown in the Table, on page 72.
Temperature performance. When 8000 Series aluminum alloy conductors are used in place of their equivalent ampacity copper conductors, the connections with these conductors are likely to operate at a lower temperature than equivalent copper conductors. A detailed examination of the temperature data suggests that if 8000 Series aluminum alloy conductors were subjected to the same level of current as their copper ampacity equivalent sizes, then all the 8000 Series connections would operate at a lower temperature than the connections with copper conductors.
Resistance performance. When 8000 Series aluminum alloy conductors are used in place of their equivalent ampacity copper conductors, the connections are likely to operate at a lower resistance than equivalent copper conductors. Test results show that the resistance stability of connectors installed on 8000 Series aluminum alloy conductors are equally stable as those installed on copper conductors of equivalent ampacity.
Torque retention test performance. Torque measurements show that the connectors installed on 8000 Series aluminum alloy conductors retain similar, but slightly lower, torque than the connectors installed on electrically equivalent copper conductors. More importantly, the data suggests that the difference between the overall average values of percent retained torque from connections installed on these conductors and cop per conductors is significantly less than the standard deviations observed from connections installed on either of these conductors. Therefore, given the variability in measurement of torque retention, there is little difference between the performances of the connections of 8000 Series aluminum alloy and equivalent copper conductors. The absolute temperature, temperature stability, and resistance stability criteria of the heat cycling test are far better predictors of the long-term performance of a connection, whether it is with copper or 8000 Series aluminum alloy conductors, than the retained torque.