In introductory industrial control labs, use blue wire for the +24 VDC control circuit with white-with-blue-stripe wire for all return conductors. This is a student-centric approach to learning that is adjacent to the wiring conventions used for control panels. An example of this scaffolding is shown in Figure 1.
This companion article is a prerequisite. It describes the output terminal of a DC bench power supply with an emphasis on the floating output, including the option of using the ground jumper from the black negative terminal to the green earth safety terminal.
Key Takeaways
- Bench power supplies feature red and black DC output terminals along with the implied terms of ‘positive’ and ‘negative’.
- The terms positive and negative, while useful for general electronics, are better understood when replaced by the terms +24 VDC and return, abbreviated RET.
- Industrial controls primarily use blue wire for the +24 VDC control conductors.
- A white-with-blue-stripe wire is recommended for the RET conductors.
- The article features 22 AWG Alpha Wire blue and white-with-blue-stripe.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Relay Logic → Fundamentals
Learning Pathway: Industrial Automation Technician → Fundamentals
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 28 Jul 2026
Figure 1: Blue and white-with-blue-stripe 22 AWG wire is shown in this PLC-based project.
Red and Black vs Blue and White-With-Blue-Stripe
The traditional laboratory-grade power supply has red and black output terminals as described in the prerequisite article. Almost all voltmeters share this color convention, with red as the positive terminal and black as the negative. We routinely see the color scheme in battery-powered devices.
However, we do not see this color convention in +24 VDC industrial controls. The distinction is important as students need a conceptual bridge from the red and black terminals of the power supply to the blue wires in the industrial control panel.
Use the Term Return Instead of Negative
The term negative is mostly inappropriate as the +24 VDC power supply within most control panels is floating relative to earth. Like a bird on a power line, there is no direct connection to earth. Instead of using the term negative, industrial control systems refer to a 24 VDC return line or simply return or RET. A classic mistake is to attempt to read the +24 VDC with the meter’s black probe on the exposed metal of the control panel. It will read about zero volts as the meter circuit violates the closed loop stipulation of Kirchhoff’s voltage law because the chassis and the +24 VDC are not directly connected.
Tech Tip: The classroom ban on the use of the term negative is deliberately overstated. The intent is to break a bad habit before it is formed. Yes, the term is often used along with zero and DC common. The problem is that most 24 VDC control systems float relative to earth ground. The other terms are convenient but lead to unnecessary complications in later lessons.
Red and Black Are Not Used for +24 VDC Control Wiring
Black and red colors signify very different conductors in the industrial control panel.
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Black is associated with the heavy power circuits such as 208 and 460 VAC mains.
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Red is associated with AC control circuits.
Instead, industrial control designers associate the color blue with the +24 VDC control conductors. Likewise, some organizations designate the white-with-blue-stripe wire for the RET node.
Blue (+24 VDC) and white-with-blue-stripe (RET) will be used exclusively in these introductory lessons. I’ve found that the clear color distinction helps students with their wiring. The constant questioning of wire color forces them to think about the wire nodes at every junction.
Standards for Wire Color
The wire color used in industrial control panels is not universal, as there are many standards. The IEC 60204-1 and NFPA 79 (Electrical Standard for Industrial Machinery) are most applicable to our discussion:
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The IEC 60204-1 standard associates blue wire with DC control circuit conductors. Other colors such as white-with-blue-stripe wire are not called out, but are allowed if properly documented. The exceptions are green and yellow conductors which are reserved for earthed safety wire and bonding.
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Within NFPA 79, blue is specified for the ungrounded +24 VDC conductors. Technically, the white-with-blue-stripe wire is for grounded DC control circuits. If we are critical, we would need to change the lab’s wire color depending on presence or absence of the bench power supply’s negative-to-earth bonding jumper. Without the jumper, we use blue wire for all +24 VDC control conductors. With the bonding jumper in place, we use the white-with-blue-stripe wire for the RET line.
In all cases, it is good practice to include a legend on the inside of the control panel describing the panel’s wire color convention.
Tech Tip: The blue and white-with-blue-stripe recommendation prioritizes student scaffolding over adherence to any single standard.
Think of this recommendation as an in-house standard for wiring the control panel. Once students understand the concepts of nodes, logic, grounding, and bonding, they can adapt to the applicable standard. For example, there may be a difference between a machine designed for the European vs the American market.
Parting Thoughts
This entire article is concerned with student scaffolding. At one extreme, we declare that a “wire is just a wire,” and use gray wire for the entire learning sequence. Instead, we intentionally use blue and white-with-blue-stripe wire. This is a constant reminder to focus on nodes with a clear distinction between +24 VDC and RET. It’s the little things like wire color that carry an unspoken language. It sets the expectations for what they will encounter in a control panel with ramifications touching maintenance and ease of troubleshooting.
Continue Exploring Industrial Control Systems
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- Full Catalog: Industrial Control & Automation
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.