Chuck Yung 
EASA Technical Support Specialist
 
When repairing motors, we often take the lead wire ampacity charts for granted, without giving much thought to how they were developed. Who .gured out how much current is acceptable for specific lead wire, and why are there different ratings for different types of insulation? 

It might be helpful to consider some of the variables that influence what looks – on the surface at least – like a simple subject. And as we will see shortly, “the surface” is one of the variables to consider. 

Since most electric motors and generators use lead cable rather than bus bar, the occasional motor with bus bar leads to questions about “circular mils per amp” for bus bar. Is the current density of bus bar com­parable to that for lead cable? 

The short answer is no, but this article will cover some often-over­looked facts about ampacity. This subject is complex, and space is lim­ited, so I have included some refer­ence resources. 

First, a definition: ampacity is the current-carrying capacity of a conductor. 

Here is a shortlist of items that influence the current-carrying ability of a conductor such as lead wire or bus bars: 

• Stranded or solid wire? 
• Cross-sectional area and dimen­sions 
• Is the bolted connection silver-plated or bare? 
• Color and surface finish 
• AC or DC current? 
• Orientation and proximity to other conductors 
• Availability of cooling/venti­lation 
• Allowable temperature rise 

Cable size 
The cross-sectional area of a con­ductor is only one factor when con­sidering the current-carrying capacity of a conductor. You have probably noticed how lead cable resembles rope – small strands twisted together to make larger bundles, which in turn are twisted together to make still larger bundles (Figure 1) until the desired ca­ble size is achieved. The main reason for this construction method is that AC current is carried on the surface of a conductor (skin effect). 

The higher the frequency of the AC voltage, the greater the skin effect becomes. High-frequency conductors will have smaller strands. So it follows that DC current is fairly uniformly dis­tributed through the conductor. Numerous smaller strands have more surface area, and therefore greater current-carrying capacity, than a single large conduc­tor with the same cross-sectional area. With more surface area per unit volume, there is also more surface area to dissipate heat. Another reason for stranding the cable is to make it more flexible. 

Table 1 compares the current rating for several conductors with the same cross-sectional area, for a 30° C rise. Current passing through a conductor generates heat, and any­thing that traps the heat can cause the temperature to increase. If the conduc­tors are bundled together, enclosed in a conduit or terminal-box, or insulated, the temperature of the conductors will increase. If a higher temperature rise is acceptable, a higher current rating can be assigned to a conductor. 

Ampacity and temperature 
For example, copper bus ampacity increases for higher temperatures, so different standards establish different maximum allowable temperatures: ANSI C37 allows a 65°C rise above a 40°C ambient if the connection sur­faces are silver-plated. US National Electrical Code (NEC) offers different ampacity depending on temperature rises of 30°, 50° or 65° C. British Standards Institution standard BS 159 permits up to 50°C rise above a 35°C ambient. As Table 2 illustrates, the higher the allowable temperature rise, the higher the ampacity. 

The insulation class of the lead insulation determines the maximum ampacity. The same size lead wire can have several different current ratings, each limited by the temperature class of the respective insulation. The permis­sible current density is not linear – smaller lead wire can have much higher current density (lower circular mils per amp/ higher amps per mm squared) than a larger wire. To compare the cross-sectional area of a stranded lead wire to the group ends to which it will be connect­ed is pointless. Using such a comparison to determine what size lead wire to use would cause us to use unnecessarily larger lead wire, increasing cost without benefit while crowding the inte­rior of the motor. 

Shown in Figure 2 are two identical copper bars; the one oriented vertically (left) dissipates heat better than when laid flat (right).

Bus positioned on edge (Figure 2a) has a higher the identical bus placed .at (Figure 2b), even when both are in free air. The larger the temperature difference (delta T) between a hot object and a surround­ing medium, the more efficient the heat transfer will be. Heat rises, so the copper bus on edge has a larger thermal gradient from top to bottom than when laid .at. Due to the greater delta T for the bar on edge, it can dissipate heat better, so a higher current rating is assigned. Paral­leling conductors is one more area where we need to be careful. Generally, the spacing of paralleled rectangular bus should be at least as great as the thickness of the bus bar. If paralleled buses are spaced closer than this minimum distance, the ampacity must be further reduced to limit the temperature rise. 

Increasing the surface area 
If a motor uses multiple thin sheets of copper for lead straps, the manufac­turer may have done so to increase the surface area for the improved ampac­ity and heat dissipation. Be especially cautious of motors rated for higher­than-normal (i.e., above 50 or 60 Hz) frequencies. 

It should be obvious that if a bus is taped with insulation, heat transfer is inhibited, and the temperature will increase. But sometimes the obvious must be stated. This is especially true of many of the industries we serve, where legitimate safety concerns might result in a bus being insulated to reduce a perceived “touch hazard,” even though lock-out/tag-out rules prohibit opening the enclosure to expose that bus while it is energized. 

But while insulation wrapped around the bus bar traps heat, would you believe that the correct coating of paint actually helps to dissipate heat? 
Emissivity (the rate at which heat is transferred from a part through radiation) is affected by surface .nish and color. So a copper bus bar painted .at black – with higher emissivity – can carry more current than the same bus bar with a shiny finish. 

For comparison, shiny metal has an emissivity of 0.1 compared to a .at black (or .at white) painted surface emissivity in excess of 0.9; a nine-fold increase in emissivity is possible by painting a bus bar with a dull non-me­tallic paint. The improvement in emis­sivity permits a 23% increase in AC current-carrying capacity with the same bus temperature. You read that correctly the same bus bar painted .at black can carry 23% more current at the same temperature rise, because the .at dark surface improves the heat dissipation. 

Surroundings 
The lead wire tables published by manufacturers of lead cable include several tables. One table gives the am­pacity for a single lead in free air, with a second table for the ampacity of “no more than 3 conductors in a raceway.” It is that table (usually Table 3 on the most common lead wire manufac­turer's chart) that should be used for sizing motor lead wire. The ampacity for “not more than 3 conductors in a raceway or conduit” is reduced by ap­proximately 40% from the rating for a single conductor in free air. The built in de-rating factor accounts for the heat given off by the clustered con­ductors, and the reduced ventilation in a relatively closed space. When more than 3 leads are grouped together, further de-rating should be applied. 

Our customers usually route much larger conductors to the motor, com­pared to our motor leads. The reason for doing so has nothing to do with ampacity. Rather, there is a voltage 
drop across a long conductor. The longer the cable run to the motor, the greater the voltage drop. To reduce the voltage drop, a larger conductor is used. The more oversized the cable is for the current, the more the volt­age drop can be reduced, but cable is expensive so a practical compromise must be reached. The NEC recom­mends limiting the voltage drop to less than 3%. 

Another consideration 
There is still another consideration when selecting lead wire for 3-phase electric motors. When a winding has a Y-delta external connection (that includes most motors with either 6 leads or 12 leads), the paralleled leads do not carry half the current. Leads attached to the open corners of a delta connection each carry the phase cur­rent, or 58% of the line current. 

Figure 3 shows two identical windings, one (Figure 3a) with the delta closed internally and 2 paral­lel leads. Figure 3b shows the same exact winding, but with the leads brought off the corners of the open delta. The individual leads in Figure 3b will carry 16% higher current than in the closed delta configuration. 

When considering what lead wire size to use, there are some more cautions to keep in mind. First, if the winding was factory original, you should be safe in using the same size and temperature class of lead wire (un­less you found evidence of overheated lead wire, such as brittle insulation.) 

When the leads are paralleled ex­ternally, they won't necessarily share the current equally. For example, a motor that uses Y-delta starting operates in the delta mode, with 1-6, 2-4, 3-5 paired to the line. It is not uncommon to measure 20% or more difference in the current of two paral­leled leads. 

That is one factor that has led some industries to require the repairer to use “one size larger” lead wire than necessary. That is a good practice, especially when the current rating of a particular lead wire is only marginally higher than the motor full load amps. If the lead routing / terminal box opening is going to be tight, a good solution is to use lead wire with a higher tem­perature rating.

That usually permits the use of one size smaller lead wire, which can go a long way toward solv­ing space problems. 

For more detail about this subject, refer to the Copper Development As­sociation Web site (www.copper.org), and the Belden (www.belden.com) lead wire chart. Posting a wall chart with the ampacity tables is another good way to make sure your winders have the information they need to make the right decisions about selecting lead wire. 

Categories: Design/theory/application, Wire

Tags: Motor leads

Rate this article:

4.0

Print