Electrical and Controls Specifications

Control Panel from the late 1980s ready for a retrofit. How many things can you count that would violate YOUR specification?
Recently my wife took a job with a multinational OEM as a panelbuilder. This company recently built a new plant in our area and moved their panelbuilding operation here from Texas. They build custom refrigeration equipment and the associated compressors and as part of their product a control panel is included with every system.

She has been building panels for about 15 years and has worked for several different types of companies, machine builders and integrators including our own business. Usually specifications come from the customer rather than using our own, however sometimes when a customer didn’t have a written specification we would use our own.

Imagine her surprise when a panel showed up from Texas with all of the DC wiring in red. Of course she ended up rewiring that portion of the panel but then inquired as to what specification the Texas plant followed. Apparently no one really knew so she asked me if I would work up a sample specification for her to bring in as a starting point. With her input I started writing one yesterday. This is what I’ve got so far:

1 Scope

The purpose of this document is to define standards used in the fabrication, wiring and design of electrical control systems for [Company]. Exceptions to these standards must be approved in writing by the engineering department.

1.1 Identification
Table 1. Terminology and Definitions
1P Single Phase
3P 3 Phase
AC Alternating Current
COTS Commercial Off The Shelf
DC Direct Current
Hz Hertz

1.1.1 Standards and Handbooks Table 2 details preferred components that should be considered prior to any other component selection. Any substitutions must be approved by [Company].

Table 2. Preferred Components
Commodity Brand Series Part Number /Example
Circuit Breakers for AC (UL489 Only) Eaton/Cutler Hammer WMZT, C trip
Circuit Breakers for DC Phoenix Contact TCP
Contactors Allen Bradley 100/700
Disconnect, NFPA79 type, or equivalent Allen Bradley 194R 194R-FJ030P3 for example
Emergency Stops, LED illuminated Allen Bradley, Illuminated 800T Series 800T-FXQH24RA1
Ethernet Switches, managed Allen-Bradley, Phoenix Contact Ethernet/IP certified
Ethernet Switches, unmanaged Phoenix Contact, Moxa (2nd) 2891929 for example
Ethernet Utility connections Wago 51205068
Fasteners SAE or Metric
Hookup Wire MTW or Stranded THHN
Light Curtain Banner (14mm preferred)
Photo electric Sensors Banner QS18 (Expert preferred)
PLC Allen Bradley CompactLogix or ControlLogix
Pneumatics SMC
Power Supplies (24V, 12V, 5V) Phoenix Contact SFB 2866763 for Example
Proximity Sensors, < 8mm Keyence EM series Proximity Sensors, >=8mm diameter Turck (Keyence) uProx preferred
Pushbuttons, Illuminated, LED Allen Bradley 800H Series 800H-QRAH24GD1 for Example
Pushbuttons, non-Illuminated Allen Bradley 800T Series 800T-A2D1 for example
Safety Contactors Allen Bradley 100S/700S 100S-C23DJ404C for example
Safety Scanners Sick, Keyence, Banner, STI
Safety Relays Allen Bradley 440R 440R-N23-120 for example
Status Light tower, LED Allen Bradley, Banner (TL50)
Switches, Pilot Lights, LED Allen Bradley, Banner 30mm
Terminal Blocks Phoenix Contact ST Series
Touchscreens Need to be approved
USB Utility Connections Wago 51205157
Wireway in enclosure (fingered) Panduit or Iboco
Wireway in enclosure (solid) Panduit or Iboco or Hoffman type

Delete or modify items in this table that do not apply Table 3 below lists required methods associated with assembly of control systems.

Table 3. Required Documentation Methods
Subject Standard
Schematic Format AutoCAD, ANSI Ladder

1.1.2 Regulatory Documents
Document Title Document Number, Issue Date
Electrical Standard for Industrial Machinery NFPA79, 2007

1.1.3 [Company] Documents
Document Title Document Number, Issue Date
System Safety XXXXXX-XX, 2011
2 Specifications
Following are specifications for the fabrication of electrical controls and machine/system wiring.
2.1 Control Panel Wiring
2.1.1 Component Mounting Component locating shall be done per drawings using appropriate measuring, marking and squaring techniques with straight edges, squares and tape measures. Control components shall be attached to the enclosure backplane using tapped holes and appropriately sized machine screws. Exceptions may be made for components with specific manufacturer suggested mounting methods. Din rail shall be attached to the enclosure backplane using tapped holes and 10-32 machine screws. “Self-tapping” screws shall not be used for component mounting. Plastic wireway may be attached to the enclosure backplane using tapped holes and appropriately sized machine screws, metal rivets or plastic rivets. Standard pushbutton holes shall be made using an appropriately sized punch and die set. Rectangular or square operator interface cutouts shall be closely sized to the component using manufacturer’s recommendations and deburred with no sharp edges. Metal shavings shall be removed after drilling, cutting or deburring operations prior to further mounting of components. Enclosures shall contain 20% spare capacity.
2.1.2 Component Wiring DC wiring shall be blue TFFN, THHN or MTW. AC 120V wiring shall be red TFFN, THHN or MTW for hot or fused wires and white THHN, TFFN or MTW for neutral wiring. Ground wires shall be green TFFN, THHN or MTW. Ground wires shall be terminated by ground lug to the enclosure backplane and have a direct path to supplied power drop earth ground. Wire shall be sized for appropriate current carrying capability per NEC or NFPA79 sizing charts. Multiconductor wiring shall have their individual conductors terminated near each other. Conductors shall be stripped to a length such that the conductor is inserted entirely into the terminal block or termination point with no more than 1/16” or 1.5mm of exposed copper visible. Wiring shall be routed such that excess wire is not left in the wireway. Wiring shall also not be stretched tightly when rounding internal wireway corners. All conductors must have ferrules for termination wherever practical. Not required for wires larger than 14AWG. Cables entering through a metal space must be protected by a fitting or gland. Wire nuts shall not be used inside of enclosures. Wire extensions should be avoided whenever possible. If a wire must be extended, the extension wire must be the same color and gauge as the extended wire. It must be soldered and covered in shrinkable tubing. Electrical tape shall not be used to cover wire or wiring splices. Wireway shall be no more than 75% filled.
2.1.3 Wire Labeling Wiring and cabling will be labeled using machine printed adhesive wrap around or heat shrinkable COTS labeling system. All wires and cables shall be labeled on both ends and match the designation on the schematic. Wire labels shall be clearly legible and oriented toward the front of the panel opening. Wire labels shall all be oriented in the same direction; read bottom-top or left-right. Wire labels shall be placed approximately 1/8” or 3mm from the terminal point and be consistent in appearance.
2.2 Machine Wiring Wiring external to the control enclosure shall be contained in rigid conduit or EMT Minimum conductor size for motors shall be #14AWG.
3 Drafting and Design
4 Programming and Software

I apologize for the somewhat disorganized look of the text; WordPress (the application used to post this blog) doesn’t lend itself to formating text like a word document. Much of the content of this specification is fairly standard stuff. The NEC handbook and NFPA79 is a good reference for electrical assemblies built in the US. If anyone would like a copy of the original Word document and associated tables and formatting let me know.

Anyway the theme of this post is that it really is necessary to generate some kind of written specification for whatever your product is. Amazingly, most smaller companys have little or no specifications written for their electrical equipment.


Electrical Engineer and business owner from the Nashville, Tennessee area. I also play music, Chess and Go.

6 Comments on “Electrical and Controls Specifications

  1. Have you ever come across any specification which covers component location in a panel? I come from a background where the components are grouped by voltage, with higher voltage components towards the top and working down from there. Is this more of a preference thing or is there a spec?

    • I have seen that in a specification before but it was geared more towards putting the controller at an easily accessed point for diagnostics and cable connections. I’ve never seen one specifying higher voltage components at the top, but if you couple that with a fan/vent combination or air conditioner it could make sense.

      Where I currently work we typically put servos and 480VAC motor starters in the bottom part of the enclosure or in another compartment of a multi-door panel. This is pretty much for voltage separation. We depend on our customers for specifications usually though and they are rarely that specific other than separating high voltage AC from low voltage DC.

      Related to component location I have seen a lot of plants that specify no electrical components less than 18″ from the floor. This is probably water related though. Thanks for visiting the site!

  2. I was never good at languages, but you need to be if you are going to work with electrical systems. Specifically, you need to understand the unique symbols found in electrical systems.

  3. Hey Phranc, some great posts on panel building. I like the sample spec a lot.
    I still have a potential project on the horizon but higher priorities keep coming up for me. Your info on program design has been very helpful. I was wondering if you could comment some more and maybe suggest some good resources for the electrical and control cabinet design process? Once you have spec’d the major components for the machine, how to complete the electrical design including the cabinet and all the components in the cabinet that are needed.
    Your high level info on program design really helped me to identify areas I needed to research and educate myself more. I was wondering if you could help me similarly with this topic?

    Thanks as always,


  4. Doug,
    Great start! I am task with writing a panel specification for my project and would love to have what you have done, in WORD format for a starter.