Mike Howell, PE
EASA Technical Support Specialist
EASA recommends using the lead wire specified by the original equipment manufacturer (OEM) whenever possible. If not available, some guidance is provided in section 6 of the EASA Technical Manual and an online calculator is available at easa.com/calculators to determine a minimum recommended size based on temperature rating, expected current, number of leads and type of connection. This article will describe the calculator’s function. It’s important to note that there is no one right answer in this process when the original information is unknown. When selecting a lead wire, the following topics should be considered.
Material-Related Issues
If there are any known issues with the original lead wire insulation and the application’s environment, this should be considered when selecting the replacement lead wire. Gather all pertinent information and consult your preferred lead wire manufacturer for recommended products.
Thermal Class of Insulation System
The lead wire thermal rating should be close to the insulation system’s thermal rating. In many cases, the lead wire can be one thermal class lower if needed. For example, using lead wire rated 150°C or higher is desirable when choosing lead wire for a class F (155°C) insulation system, but it may be acceptable to use 125°C rated lead wire. Conversely, using a lead wire with a higher thermal rating may allow for a smaller size.
Rated Voltage
The lead wire voltage rating should be greater than or equal to the machine’s rated voltage. For example, a service center rewinding low voltage machines might use 600 V rated lead wire for machines rated 230 V, 380 V, 415 V, 460 V and 575 V. For medium voltage machines, a 7500 V lead wire might be used for machines rated 2300 V, 3300 V, 4000 V and 6600 V. Likewise, a lead wire rated for 15000 V might be used for 11000 V, 13800 V and 14400 V rated windings.
Balanced Three-Phase Currents
The relationship between line and phase currents for wye (Y) and delta (D) balanced 3-phase loads is shown in Figure 1. This figure shows that for wye connections, phase current is line current, and for delta connections, phase current is 58% of line current.
Sizing Leads
The machine’s rated current and connection are required to determine the expected current in each lead. The machine’s full-load rated current should be available on the nameplate. If this information is missing or illegible, a reasonable estimate can be made by referring to the EASA Technical Manual (Section 2) or the EASA Electrical Engineering Pocket Handbook. However, a better estimate might be obtained by comparing similarly rated machines using EASA’s Motor Rewind Data software, and you can always contact EASA Technical Support for assistance. A conservative approach is to size the lead wire based on service factor current, especially when this is a known operating condition.
The procedure to determine the lead current used for sizing in the online calculator is described in Table 1, and it can also be used for manual sizing. Manufacturers do use connections, winding arrangements and numbering schemes not accounted for in Table 1 or the online calculator.
Column A contains information about the number of leads and an abbreviated description of the connection. For example, the third row Column A entry = 6 (YD). This indicates that the three winding phases are accessible and can be externally connected in wye or delta.
Column B contains the expected lead current as a percentage of line current. For example, the third row Column B entry = 100% (Y) or 58% (D). This indicates that if the winding is designed to operate connected externally in wye, the expected lead current for that connection will be 100% of the line current. If the winding is designed to operate connected externally in delta, the expected lead current for that connection will be 58% of the line current.
Column C contains a correction factor based on the number of leads. This correction factor is consistent with technical information published by at least one major lead wire manufacturer, and it is sourced from NFPA 70 (National Electrical Code), though the application of motor leads is not explicitly stated there. It is worth noting here that many lead wire manufacturers direct users to the equipment manufacturer for sizing. That is, they do not publish ampacity tables. As has already been stated, the purpose of this procedure is to make a reasonable selection when manufacturer information is not available. For example, the third row Column C entry = 1.25. This indicates that the current determined in Column B should be multiplied by 1.25 to obtain a current to use for the lookup table. The purpose of this safety factor is to account for multiple conductors in close proximity to one another and existing in a space that is not open or ventilated for cooling.
Column D contains the multiplier or factor to be used when calculating the lookup current in the
EASA Technical Manual Lead Wire Chart. For example, the third row Column D entry = 1.25 (Y) or 0.73 (D). If sizing leads rated 150°C for a machine whose rated current is 100 A and assuming a 1.00 service factor, the lookup value would be 1.25*100 = 125 A if designed for an external wye connected or 0.73*100 = 73 A if designed for an external delta connected. Figure 2 provides a screenshot of the calculator output for both options. For the external wye connection and 125 A, 3 AWG (121-143 A) or 25 mm2 (117-130 A) is selected. For the external delta connection and 73 A, 8 AWG (56-76 A) or 10 mm2 (61-78 A) is selected.
How Does This Compare?
As stated, there is no one right answer in this process when the original information is unknown. However, 12 cases were evaluated to compare the calculator output with one OEM’s selected lead size. Each machine had class F or class H insulation systems, so a 150°C rated lead wire was chosen. The leads were sized based on full-load amps; that is, the service factor in the calculator was set to 1.00. The comparisons were done using AWG sizing to be consistent with the OEM’s published data. A summary of this information is presented in Table 2. Of the twelve cases, the calculator recommended a larger size than the manufacturer used five times, a smaller size than the manufacturer used four times, and the same size as the manufacturer used three times. Additional information is available in the handout for the May 2023 webinar recording “Lead Wire Sizing 101” which is available at easa.com/training/webinar-recordings.
AVAILABLE IN SPANISH
EASA Technical Manual
More information on this topic can be found in EASA's Technical Manual- Section 6: Magnet Wire & Lead Wire
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