Short answer: neither leg of a 24 VDC control supply has to be grounded. The metal boxes do. Exposed metal of enclosures bonds to an equipment grounding conductor regardless of the voltage inside. A 480-to-24 VDC source feeding five sub panels therefore owes an EGC to every metal sub panel and to the metal housings beyond them. The DC+ and DC- legs stay ungrounded unless you choose to bond DC common, once, at the source. Run the Class 2 arithmetic before you pull wire: a 1000 VA source at 24 VDC delivers 41.7 A, ten times the 100 VA Class 2 ceiling. Wire and protect the feed as Class 1 control wiring, and give any device whose datasheet demands a listed Class 2 supply its own converter.
What grounded can mean on a 24 V system
The argument usually tangles three different jobs under one word. Separating them settles most of the question before any table gets opened.
System grounding bonds one current-carrying conductor to a grounded system: the neutral at a service, or the DC common of a supply designed that way. The NEC names the systems it wants system-grounded, and a 24 V control output is not one of them.
Equipment grounding bonds exposed, normally non-current-carrying metal — enclosures, raceway, housings — to an equipment grounding conductor. That duty applies at every voltage, the 24 V control circuit included.
Earthing means ground rods in the soil. NEC 250.4(A)(5) treats the equipment grounding conductor as the effective ground-fault current path and the earth as no part of it, so a rod driven at a sub panel never does an EGC's job. The grounding and bonding guide walks each job for enclosures in general; the rest of this answer applies those jobs to the low-voltage DC feed.
Neither DC leg has to be grounded
Nothing in the NEC requires you to ground either output terminal of a control power supply, and you can leave both legs floating by design. Floating has a real advantage in process plants: the first accidental ground on DC+ or DC- puts a reference on the system but trips nothing, so the line keeps running while maintenance chases the fault.
Most panels still bond DC common, because a grounded 0 V gives analog and communication circuits a stable reference and a place for noise to go. Make it one bond, at the supply. When DC- lands on the ground bus at the source and again at each sub panel, return current takes both paths at once. The split shows up as jitter on 4-20 mA loops and drift in encoder counts. Plan the 0 V bond alongside the backplate layout, and keep the return conductors away from the high-current feed on the DIN rail and backplate layout guide.
If the drawing already shows DC- landed on the ground bus at the source cabinet, treat that as the single bond for the whole system and land nothing downstream of it.
The metal gets an EGC at any voltage
The grounding duty follows exposed metal, not the voltage inside. NEC 250.96(A) requires metal parts intended to serve as equipment grounding conductors — raceways, cables, equipment, enclosures — to be bonded together. The bond has to conduct any fault current likely to be imposed on it.
Why bond a box that only ever sees 24 V? Because panels mix voltages. The 480 V primary shares backplates, doors and gland plates with the DC control circuits, and one slipped lug or one failed insulation can put 480 V on any of that metal. The EGC gives the fault a low-impedance path back to the source, so the overcurrent device clears. NEC 250.4(A)(5) adds the corollary: the current returns through metal, and the earth is no part of it.
The metal-housed probes and actuators get the same treatment as the sub panels, so bond them the way you bond the boxes. Fed through metallic conduit, a housing bonds through its gland and hub; fed by cable, it takes an insulated EGC run with the DC legs, landed on the housing or its mounting stud. Where you drill the gland entries matters too: the cutouts, glands and HMI windows guide shows how to keep the entry clean and the bond path intact.
Before you land any of this inside a cabinet with a 480 V supply, lock out and verify absence of voltage.
Working a 1000 VA, 24 VDC source against the 100 VA Class 2 line
Inputs. The source is rated 1000 VA at 24 VDC (the rating on the 480-to-24 VDC supply in this case). Class 2 circuits are capped at 100 VA of output, the limit the NEC and NFPA 70 set for the class, and device datasheets turn that into a 4 A line at 24 V. Current follows I = VA ÷ V.
Arithmetic. Full-load current: 1000 VA ÷ 24 V = 41.7 A. Class 2 ceiling current at 24 V: 100 VA ÷ 24 V = 4.2 A, which datasheets round to 4 A (4 A × 24 V = 96 W). The margin: 1000 VA ÷ 100 VA = 10 times the Class 2 power limit, and 41.7 A is roughly 10 times the 4.2 A figure.
Result and decision. Anything above 100 VA of output is outside Class 2. Wire and protect the feed as Class 1 control wiring: overcurrent protection at the source, conductors sized from the ampacity tables. The 1000 VA rating sits exactly at the NEC's ceiling for a power-limited Class 1 supply at 30 V or less, so the design lands cleanly inside Class 1 territory. At 41.7 A, one 8 AWG copper conductor (50 A at 75 C) carries the load; the two #6 per leg in the drawing give 130 A of combined ampacity (65 A each at 75 C), which is voltage-drop margin rather than a leftover.
Run the check the other way too: a supply below 100 VA is not automatically Class 2, because UL has specific tests behind the Class 2 mark. When a datasheet says a device needs a listed Class 2 supply, it wants the listed source its listing was evaluated with. The same logic drives when a custom machine enclosure needs a UL listing or an IP sticker.
What size EGC the sub panels get
NEC Table 250.122 sizes the equipment grounding conductor from the rating of the overcurrent device ahead of the circuit, not from the load conductors and not from the voltage. Read at the device row: 10 AWG copper EGC at a 60 A device, 8 AWG at 100 A, and 6 AWG above 100 A up to the 200 A row. The pair of #6 legs in this drawing carries 130 A of combined ampacity, so its protecting device sits somewhere in the 60-125 A range. One EGC of 10, 8 or 6 AWG copper answers the table; you pull one conductor, not two #6, and not the #14 either.
Two adjustments move the number. When the DC legs are enlarged beyond what the overcurrent device needs, as voltage-drop work does, the EGC grows in the same proportion. And where the conduit between source and sub panel is metallic and continuous, the raceway itself can serve as the EGC. That keeps new wire out of the crowded 3/4 in run a retrofit like this leaves behind. The metal raceway article explains when the raceway counts as the conductor and when it needs a wire beside it.
Insulated EGCs of 6 AWG and smaller run green or green with one or more yellow stripes (NEC 250.119), so a 10 AWG pull lands inside that rule. The bonding conductor sizing article walks the same table for accessible enclosures.
What changes in the panel
Nothing about the enclosure rating changes; the wiring and the protection do. Work the list in order.
| Where | What to do | The number behind it |
|---|---|---|
| Source output | Fit overcurrent protection rated to the conductors | 41.7 A load on 8 AWG copper (50 A at 75 C) |
| Feeder | Pull one EGC sized at the device rating | 60 A device → 10 AWG copper |
| Sub panels | Land the EGC on bare metal at a lug | NEC 250.12 clears coatings from contact surfaces |
| Device power | Add a listed Class 2 converter for every datasheet that demands one | 100 VA / 4.2 A ceiling |
| DC common | One bond at the supply, or a deliberate float | One bond per system |
| Signal wiring | Give Class 2 wiring its own duct or a barrier | Separation from the 41.7 A feed |
The 480 V side of the supply chassis keeps its own bond to the 480 V EGC, because that is where the fault energy lives; the low-voltage label changes nothing about it. Brushed stainless helps at the bond points: there is no paint to clear, so a lug with star washers bites clean metal and meets the NEC 250.12 duty without extra work.
What goes on the drawing
The order that comes out of the analysis:
- Source: 480-to-24 VDC, 1000 VA, DC common bonded once at the supply, overcurrent protection at the output rated to the conductors.
- Feeders: two #6 per leg as drawn, plus one EGC sized at the protecting device — 10 AWG copper at a 60 A device.
- Sub panels: EGC on bare metal; probe and actuator housings bonded through gland entries or through the raceway.
- Devices: a datasheet check for the Class 2 condition, converters added where it appears.
- Enclosures: 304 or 316L stainless wall-mount boxes rated NEMA 4X / IP66 by the manufacturer; not UL listed, in outside sizes from 6 x 4 x 4 in to 60 x 36 x 16 in (H x W x D), cutouts placed where the glands land through the custom enclosure builder; the stainless enclosure size finder filters the stock sizes you can quote from. When conductor fill and bending space push the box up a size, the enclosure sizing guide carries the NEC numbers.
And the decisions, case by case:
- Any low-voltage DC system with metal boxes: bond every metal box; the voltage inside never changes that duty.
- Cable-fed sub panels: one EGC, sized from NEC Table 250.122 at the device rating.
- Continuous metallic conduit: let the raceway carry the EGC and leave the crowded pipe alone.
- Devices marked for a Class 2 supply: their own listed Class 2 converter; a 1000 VA bus cannot shrink into a Class 2 mark.
- DC common: bond it once at the source or float it deliberately, and write the choice on the drawing so nobody adds a second bond later.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Do the metal sub panels need an equipment grounding conductor on a 24 VDC feed?
Yes. Exposed metal of enclosures is bonded to an equipment grounding conductor regardless of the voltage inside, so a low-voltage sub panel gets an EGC exactly as a 480 V panel does. Size it from NEC Table 250.122 at the rating of the overcurrent device ahead of the feeder: a 60 A device takes a 10 AWG copper EGC, a 100 A device takes 8 AWG, and both are far above a #14. Land the lug on bare metal.
Is a 1000 VA, 24VDC power supply a Class 2 source?
No. Class 2 circuits top out at 100 VA, which is 4.2 A at 24 V; a 1000 VA supply delivers 41.7 A, ten times that. Even a supply below 100 VA is not automatically Class 2: the mark comes from listing tests, not arithmetic. Feed any device whose datasheet demands a listed Class 2 source from its own small converter.
Should the DC common be bonded to the ground bus?
It is a design choice, and most panels take it. Bonding 0 V to the ground bus at the supply gives analog and communication circuits a stable reference and a place for noise to go. Keep it to one bond per system: extra bonds between the source and the sub panels split return current between DC- and the bonding paths, and the split shows up as noise on 4-20 mA loops.
Do metal-housed 24-volt devices like level probes need an EGC?
Give the metal housing a bond path. Fed through metallic conduit, the raceway does the job; fed by cable, add an insulated EGC run with the DC legs, green or green with yellow stripes at 6 AWG and smaller per NEC 250.119. A probe housing bonded through its gland entry cannot float up to 480 V if a primary-side fault ever touches the metal.
Why bond low-voltage enclosures at all?
Because a control panel mixes voltages. The 480 V supply shares backplates and gland plates with the 24 VDC circuits, and one failed lug can put 480 V on any metal part. NEC 250.4(A)(5) calls the EGC the effective ground-fault current path, and the earth is not part of it: the fault returns through metal, so the breaker clears instead of the housing staying live.
