The short answer
NFPA 70 and the Control Panel
NFPA 70, the National Electrical Code, governs the installation of the control panel and, through Article 409 and its reference to the UL 508A listing, most of what is inside it. The code cares that the panel carries a short-circuit current rating adequate for the available fault current where it is installed, that it is listed or field evaluated, that its supply conductors and overcurrent protection match its nameplate, that the control circuits inside follow the rules for their class, that it is grounded and bonded, and that there is working space in front of it. The panel builder meets the internal requirements through the listing; the installer meets the external ones; and the engineer who specifies the panel is responsible for the ratings, the nameplate, and the installation drawings that let both of them do that.
Key points
- Article 409 covers industrial control panels and requires a nameplate with the short-circuit current rating and supply data.
- The short-circuit current rating must be at least the available fault current at the line terminals; the installer proves the second number.
- Control circuits follow Article 725 for class and wiring method, or Article 430 Part VI for motor control circuits.
- Grounding and bonding inside the panel follow Article 250; the enclosure, the doors with devices, and the subpanel are bonded.
- Working space and the disconnecting means are installation items that the panel design must not defeat.
Where the code reaches
| Article | What it covers in a panel | Who meets it |
|---|---|---|
| 409 | Industrial control panels: listing, nameplate, short-circuit current rating, conductor sizing, overcurrent protection | Builder and engineer |
| 110 | Working space, marking, available fault current documentation, terminal and conductor ratings | Installer and engineer |
| 240 | Overcurrent protection of the supply and, with 409 and 430, of the internal circuits | Builder and engineer |
| 250 | Grounding and bonding of the enclosure and internal parts | Builder and installer |
| 430 | Motor circuits, controllers, overload protection, disconnects, motor control circuits | Builder and engineer |
| 725 | Class 1, 2, and 3 remote control and signaling circuits: power limits, wiring methods, separation | Builder and installer |
| 700, 701, 702 | Emergency, legally required, and optional standby systems where the panel is part of one | Engineer |
| 500 to 516 | Hazardous locations, with UL 698A for the panel | Engineer and builder |
Article 409 and the listing
An industrial control panel is an assembly of two or more power circuit components, control circuit components, or both, with wiring and terminals, in an enclosure. Article 409 requires it to be marked with the supply voltage, phases, frequency, full-load current, the short-circuit current rating, and the enclosure type, and it permits the inspector to require a listing or a field evaluation. UL 508A is the listing standard, and building to it is how a panel shop demonstrates that the internal conductor sizing, overcurrent protection, spacing, component ratings, and the short-circuit current rating were done correctly. A panel that is not listed is not automatically non-compliant, but the inspector who asks for a field evaluation is within the code.
Short-circuit current rating
The panel nameplate states the short-circuit current rating, the fault current the assembly can withstand. The code requires that the available fault current at the line terminals, which depends on the utility transformer and the conductors between it and the panel, not exceed that rating, and it requires the available fault current to be documented and marked at the service and at equipment. A panel rated 5 kA, the default for an unstudied assembly, installed on a 480 volt service with 30 kA available is a violation and a hazard. The engineer determines the available fault current and specifies a panel rating above it; the builder achieves the rating through the components and the current-limiting devices in the supply path.
Control circuits
- Class 1 control circuits run at up to 600 volts with conductors sized and protected by the rules in Article 725 and the panel standard; they use standard wiring methods.
- Class 2 circuits are power limited by a listed source and can use lighter wiring methods and cables; 24 volt instrument and network circuits are often Class 2.
- Class 2 conductors are kept separate from power and Class 1 conductors unless the separation rules are met, which matters inside the panel and in the field conduits.
- Motor control circuits tapped from the motor branch circuit follow Article 430 Part VI for their overcurrent protection and are the reason a control transformer secondary is fused.
Grounding and bonding
The enclosure is bonded to the equipment grounding conductor of the supply; the subpanel is bonded to the enclosure; a door carrying devices is bonded with a strap; the control transformer secondary is grounded on one side if it is a grounded control circuit; and the equipment grounding conductors for every outgoing circuit land on a bar bonded to the enclosure. A separate instrument ground bar is bonded to the same system; an isolated ground is still bonded, just by a controlled path. None of it is optional and none of it is done through paint.
At the door
- 1
Nameplate
Voltage, phases, frequency, full-load current, short-circuit current rating, enclosure type, and the listing mark or evaluation label.
- 2
Available fault current
The documented value at the panel, below the nameplate rating.
- 3
Disconnect
A disconnecting means in sight or lockable, rated for the load, with the door interlock if the panel has one.
- 4
Supply conductors
Sized for the full-load current and protected by the upstream device per the nameplate.
- 5
Working space
Clear depth, width, and height in front of the panel per Article 110 for the voltage.
- 6
Bonding
Enclosure, door, and subpanel bonded; grounding conductors landed.
- 7
Wiring methods
Conduit fill, conductor types, separation of classes, and the seals for a hazardous location.
Frequently asked questions
- Does every control panel need a UL listing?
- The code requires the short-circuit current rating and the nameplate, and permits the authority to require a listing or field evaluation. In practice most jurisdictions expect a listed panel, and a listing avoids a field evaluation at the site. Build to the standard.
- Who is responsible for the available fault current?
- The installation side: the engineer of record or the installer documents the available fault current at the service and at the panel. The panel builder supplies the rating; the engineer must make sure the rating exceeds the available current.
- Can 24 volt instrument wiring share a wireway with 120 volt control wiring?
- Class 2 conductors are kept separate from Class 1 and power conductors unless the code separation conditions are met, and noise considerations argue for separation anyway. Separate wireways in the panel and separate conduits in the field.
- Is a control panel a service disconnect?
- Not unless it is built and listed as service equipment. A panel fed from a service needs the service disconnect upstream, or must itself carry a service-rated listing, which is a different design.
Related topics
- UL 508A Industrial Control PanelsWhat the listing actually covers, what has to appear on the label, and the requirements that most often cause a panel to fail inspection.
- Short-Circuit Current Rating (SCCR)What a control panel’s short-circuit current rating means, how it is determined under UL 508A, why the NEC requires it on the label, and why the number is usually smaller than people expect.
- UL 698AIndustrial control panels for hazardous locations: what UL 698A adds to UL 508A, a panel in a classified area or a safe-area panel with intrinsically safe circuits, barriers and isolators, the control drawing, and where utilities meet classified areas.
- NFPA Standards for ControlsThe National Fire Protection Association documents that reach into a control system: NFPA 70, the National Electrical Code, and the articles that govern panels, motors, and instrumentation; NFPA 70E for electrical safety at work; NFPA 79 for industrial machinery; NFPA 820 for wastewater plant hazardous area classification; and NFPA 110 where standby power meets the controls.
- How to Calculate a Panel SCCRDetermine the short-circuit current rating of a control panel by the UL 508A Supplement SB method: list every power circuit component, find each rating, apply published series combinations and current limiting, and mark the panel with the lowest result.
- Ground FaultsGround faults in and around control panels: what happens on grounded 120 VAC circuits, on grounded and ungrounded 24 VDC systems, and on drive outputs; where they hide, from chafed wire to flooded boxes; and the divide-and-conquer method that finds them.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.