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Eaton: Wire control panel - UL 508A, Verdrahtung nach UL 508A

How should you wire a control panel in accordance with UL 508A?

The regulations in the North American control panel standard UL 508A cover every single area of a control panel—up to and including the wiring of main and control circuits. 

cUL certification is similar to CSA (Canadian Standards Association) standards and is therefore observed and recognized by the Canadian authorities as CSA certification. UL and CSA are applicable in both markets.

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Which rules apply to the wiring of the main circuit inside the control panel?

For feeder circuits and branch circuits, UL 508A distinguishes between internal wiring, which does not leave the control panel, and field wiring.

First and foremost, internal wiring must be made of copper. In addition, main current wiring should not be less than 14 AWG and should be marked at the connection point with letters or numbers that match the circuit diagram provided in the control panel.

For individual loads, UL 508A stipulates that the main current wiring for motors or heating systems should be designed for a current carrying capacity not less than 125 % of the full load current. For multiple loads, the main current wiring should be designed for a current carrying capacity not less than 125 % of all heating loads, plus 125 % of the largest motor load, plus the amperage for all remaining motors when running at full load and other loads that can be operated simultaneously.

The conductor cross-section for the conductor is selected from NEC Table 310.15(B), based on the calculated current carrying capacity of the wire. Wires used in group motor applications should comply with Table 66.2 (reference range 66.7.5). Table 28.1 of UL 508A can also be consulted when designing the internal wiring in the control panel.

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What do you need to consider when wiring the main circuit in the field?

As with the internal wiring, UL 508A states that the field wiring should be a minimum of 14 AWG. For individual loads, the wiring in the field should be designed for a current carrying capacity of 125 % of the full load current. For multiple loads, such as multiple motors or a motor with different loads, the field wiring is based on the sum of 125 % of the largest motor full-load current, plus the sum of the full-load current of the other remaining loads. The conductor cross-section is also selected using NEC Table 310.15(B) or Table 28.1 of UL 508A, based on the calculated current carrying capacity of the wires.

Which wire colors should be used for the main circuit?

In the world of IEC, DIN EN 60204-1 does not give clear specifications for cable colors—the only colors that are clearly defined are green-yellow for the protective conductor and light blue for the neutral conductor. Other than that, the standard contains only recommendations for color designation. In contrast, UL 508A provides clear specifications:

For the internal main current wiring, all non-earthed electrical circuit wires must be black regardless of voltage. Grounded AC-carrying conductors should be either white or gray regardless of voltage—or feature three white stripes on a surface color that is not green, blue, orange or yellow.

Exception: Insulated conductors with a minimum size of 4 AWG (21.2 mm) and insulation in a color other than those indicated above should be marked at each connection point using a white marker such as insulating tape wrapped around the conductor.

According to UL 508A, protective conductors, insulated earthing conductors and equipotential bonding conductors should be green—with or without one or more yellow stripes. 

How do you need to design control circuit wires to comply with UL 508A?

The wire size for control cables within the control panel must be a minimum of 18 AWG, with the exception of control cables for PLC inputs/outputs. The conductor cross-section is determined using Table 38.1 from UL 508A. This is based on the amperage of the overcurrent protective device used for the control current, or the amperage of the output cable from the control transformer or of the power supply.

What colors do the wires need to be?

UL 508A also specifies firm rules for the coloring of core cables:

  • All ungrounded AC or DC main power lines = black
  • Non-earthed AC control circuit cables = red
  • Non-earthed DC control circuit cables = blue
  • Non-earthed control circuit cables or other wiring, such as for control panel lighting, which remains energized even when the main switch is OFF = yellow or orange
  • Earthed AC-carrying control circuit cables, regardless of voltage = white or gray, or feature three white stripes on a surface color that is not green, blue, orange or yellow
  • Earthed DC-carrying control circuit cables = white with blue stripes
  • Earthed AC-carrying control circuit cables that remain energized even when the main switch is OFF = white with yellow or orange stripes
  • Protective conductor = green or green and yellow
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Conclusion

If switchgear systems are to be exported to North America (the US and Canada), attention must also be paid to the wiring. The IEC and UL 508A specifications for this differ considerably in some parts. Detailed information on this can be found in the "Design guides for control systems in accordance with UL 508A," which can be downloaded from the Eaton website.

Control panel design guide according UL 508A

Our control panel design guide helps machine and system builder to design control cabinets for the North American market and to select the right products for their application. It informs as well about the selection for the correct wire for control and power circuit wiring.

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