AC-15 vs DC-13: Auxiliary Contact Ratings Explained

Every auxiliary contact block carries two ratings that have nothing to do with its 10 A thermal current: an AC-15 rating at one voltage and a DC-13 rating at a much lower current on the same hardware. Read the wrong one — or assume “10 A means 10 A” — and a contact that has switched a coil happily for years will weld the first time it drops a DC load, or read “closed” into a PLC input that never sees a signal. This page explains both categories, shows the rating collapse voltage by voltage, and gives the selection rules for relays, PLC inputs and dry circuits.

Quick Answer: What the Codes Mean

Auxiliary (control-circuit) contact ratings are defined in IEC 60947-5-1, not the 60947-4-1 main-contact standard. The utilization categories that matter for control panels:

The 72 VA boundary splits AC-14 from AC-15 — it is a surrogate for how hard the coil’s inductance punishes the contact on break. In practice, “what contactor coil can this aux contact switch” is the AC-15 row, and “what DC coil or PLC load” is the DC-13 row.

One Contact, Two Very Different Ratings

Catalogue data for a typical 10 A thermally rated aux contact block (IEC 60947-5-1, values in line with published ABB CA5-type data) shows how fast the DC column collapses:

At 250 V the same contacts carry 4 A on AC but only 0.3 A on DC — a 13× collapse. The reason is physics, not conservatism: AC gives the contact a free arc extinction at every zero crossing, twice per cycle. DC has none, so the arc burns until the gap is physically wide enough to force the current down, and the inductive load keeps feeding it from its stored ½LI² energy — worst of all when the “load” is the relay’s own coil, which is exactly what a DC-13 duty usually is.

Make vs Break: Why AC-15 Is Tested Harder on Close

The test duties behind the two columns differ in shape:

AC coil loads demand an inrush magnetizing current far above sealed current — hence the 10× making test, which is why an aux contact that survives AC coil duty can still fail on a DC switch: DC break duty is continuous arcing rather than a sharp inrush. Note that DC-13 is defined by a time constant proportional to power (T = 6P): bigger DC loads come with proportionally longer arc durations. This is the same zero-crossing physics behind main-contact DC ratings in our DC arc extinction guide.

The 17 V / 5 mA Floor: Dry Circuits and PLC Inputs

There is a bottom to the scale too. Contact faces grow oxide films, and below a threshold voltage/current the contact never wipes the film away — the circuit reads “open” while the mechanism is closed. Standard silver contacts quote a minimum switching capacity around 17 V / 5 mA (failure rate ≤ 10⁻⁶ per IEC 60947-5-4 Annex K).

That matters because modern PLC and DCS inputs draw 5–10 mA at 24 V — right on the floor. Consequences:

• For dry-circuit and low-level analog duty, specify gold-clad contacts or solid-state coupling; gold does not form an insulating oxide.
• Don’t assume a contact that “tests 0 Ω” with a multimeter will pass 5 mA reliably — the meter’s test current is above the floor.
• Where a relay is unavoidable, one with bifurcated or gold-plated contacts is the cheapest reliability upgrade in the panel.

VA, W, and the UL Pilot-Duty Crossover

AC ratings are quoted in apparent power (VA), DC ratings in real power (W) — they are not interchangeable in either direction. A coil “rated 24 V / 10 VA” draws its inrush in VA, not W; a DC brake magnet’s rating is its real heating. When the buyer sits in a UL market, the same contact appears a third way: pilot duty classes per UL 508, typically A600 (7200 VA make / 720 VA break) or A300 / B300 for smaller contacts. Quote all three languages on an export RFQ: AC-15 value at the control voltage, DC-13 value if DC coils are in the scheme, and the pilot-duty class.

Selection Rules That Prevent the Callbacks

1. Rate the contact for the coil it switches, not the wire size: a 24 VAC coil on a 9 A contactor is an AC-14 duty; a 220 V coil on a 115 A frame can exceed 72 VA and needs AC-15 headroom.
2. Never translate AC-15 amps into DC amps. Use the DC-13 column, and for inductive DC loads above a few tens of watts consider a freewheeling diode — with the trade-off that the diode lengthens the release time of the coil (see the release-delay numbers in our surge suppressor comparison).
3. DC at higher voltage: wire poles in series. Two or three aux contacts in series split the break duty — the same trick used on DC main contacts in our DC contactor selection guide.
4. PLC inputs: stay on the safe side of 17 V / 5 mA with gold contacts, or choose solid-state feedback for critical interlocks.
5. Keep the contact count honest — the available NO/NC combinations on a contactor body are laid out in our auxiliary contact configuration guide, and the physical NO/NC side-by-side logic in the aux vs main contact explainer.

For the switching hardware itself, our CJX2-F225 3-pole contactor accepts F-series add-on contact blocks with published AC-15/DC-13 ratings — the frames and accessories are listed in the AC contactor category.

Real Loads Against Real Ratings

Translate the categories into the loads a panel actually contains:

Two boundary cases deserve a second look. A contactor coil rated 220–240 V on a large frame (say a 115 A or 225 A body) can draw 20–30 VA sealed and several hundred VA inrush — comfortably AC-15 territory, and the reason thin “contactor relay” contacts fail when someone reuses them for a big coil. And a DC control circuit at 220 V for a DC brake is exactly where the 0.3 A DC-13 ceiling bites: a brake magnet pulling 1 A will weld the contact on the first drop. The correct fix is poles in series plus a suppressor across the coil, as quantified in our surge suppressor guide.

FAQ

Is AC-15 the same as AC-14?

Same contact, different load size. AC-14 covers control of AC electromagnets up to 72 VA; AC-15 covers above 72 VA. The 72 VA boundary reflects the higher inductive stress of larger coils on break.

Why is the DC-13 rating so much lower than AC-15?

DC has no zero crossing, so the contact must stretch and cool the arc on its own while an inductive load keeps feeding it. At 250 V the same physical contact carries 4 A on AC-15 but only about 0.3 A on DC-13.

Can I use one aux contact to switch both AC and DC?

Yes, within each column’s limits — they are independent ratings of the same contact. The classic mistake is protecting the DC leg with nothing: add a freewheel diode or RC across inductive DC loads and keep the current in the DC-13 band.