If you have ever looked at an AC or DC contactor, you have probably noticed a confusing maze of labels: NO, NC, L1, L2, T1, T2, A1, A2, and various pairs of two-digit numbers.
Let’s see what NO and NC mean, how a contactor NO/NC block works, and how to read connection diagrams.
What is NO and NC in a Contactor?
At their core, contactors are heavy-duty, electrically controlled switches used to turn high-power loads on and off. NO and NC describe the physical state of the electrical contacts inside the device when it has no power (de-energized state).
-
NO (Normally Open): The contact circuit is disconnected by default. No electricity flows through it. When the contactor coil receives power, the contacts pull shut, completing the circuit.
-
NC (Normally Closed): The contact circuit is connected by default. Electricity flows through it continuously. When the contactor coil receives power, the contacts pull open, breaking the circuit.
The Contactor NO/NC Symbol
On electrical schematic diagrams, these components are represented by unique standard symbols (IEC and NEMA formats):
-
The NO symbol looks like two parallel lines with a physical gap, or a switch hovering open.
-
The NC symbol looks like two parallel lines with a diagonal line cutting through them, representing a complete, closed bridge.
When should I use NO vs. NC?
-
Use NO (Normally Open) for: Safety-critical power connections, starting motors, or turning on lights. If control power cuts out, the equipment defaults to a safe, unpowered state.
-
Use NC (Normally Closed) for: Electrical interlocking (e.g., preventing forward and reverse contactors from engaging simultaneously), emergency stop loops, or activating standby backup alarms when primary power fails.
Main Contacts vs. Contactor NO/NC Auxiliary Blocks
A standard contactor usually consists of two distinct sets of contacts: Main Contacts and Auxiliary Contacts.
Main Contacts (Power Circuit)
These are robust contacts designed to handle high currents and voltages—like powering a three-phase industrial motor or a large heating element. They are almost always Normally Open (NO) for safety reasons; if control power is lost, the machine shuts off automatically.
Auxiliary Contacts (Control Circuit)
These contacts handle very low current and are used for signaling or logic controls (e.g., lighting up a status indicator lamp or sending an "On/Off" feedback signal to a PLC). If your contactor doesn't have enough built-in auxiliary contacts, you can snap on a contactor NO/NC block (also called an auxiliary contact block) right onto the top or side of the main unit. These blocks move mechanically in tandem with the main contactor core.
What are the Numbers on a Contactor (L1, L2, T1, T2, A1, A2)
People frequently ask: What do the numbers on a contactor mean? Let's decode the standard labeling system.
The Coil Terminals: A1 and A2
A1 and A2 are the connections for the internal electromagnet coil. When you apply the correct control voltage (e.g., 24VDC, 120VAC, or 230VAC) across A1 and A2, the contactor activates.
The Main Power Terminals: L1, L2, L3 & T1, T2, T3
-
L1, L2, L3 (or 1, 3, 5): "Line" inputs. This is where your incoming electrical power supply connects.
-
T1, T2, T3 (or 2, 4, 6): "Load" terminals. This is where you connect the wires leading to the actual equipment, like a motor or pump.
The Two-Digit Auxiliary Numbers (e.g., 13, 14, 21, 22)
Auxiliary terminals use a brilliant two-digit numbering system based on international standards:
-
The Second Digit (Function Digit):
-
.1 and .2 always mean the contact is Normally Closed (NC) (e.g., 11/12, 21/22).
-
.3 and .4 always mean the contact is Normally Open (NO) (e.g., 13/14, 33/34).
-
-
The First Digit (Sequence Digit):
-
This simply tells you which number contact it is on the device. For example, 13/14 is the first set of auxiliary contacts (which is NO), while 21/22 is the second set of auxiliary contacts (which is NC).
-
Contactor NO/NC Connection & Wiring Diagram
How do we put this all together into a functioning contactor NO/NC connection? Below is a basic wiring diagram for a start/stop motor control circuit using a seal-in circuit.
-
Line Power: 3-phase power enters at L1, L2, L3 and passes through to T1, T2, T3 when closed.
-
The Stop Button (NC): Power flows straight through a normally closed STOP push button to the START button.
-
The Start Button (NO): When you push the momentary START button, power travels to A1, completing the coil circuit to A2 (Neutral/-). The contactor clicks shut.
-
The Seal-In (Auxiliary NO 13-14): Because the START button is momentary, if you let go, the power would drop out. To fix this, we wire an auxiliary NO contact (13-14) in parallel with the START button. Once the contactor pulls in, 13-14 closes, keeping the coil powered even after you release your finger!
FAQs about Contactor NO NC
What is "NC and non-NC"?
This is usually a phrasing confusion for NC and NO. There is no standard term "non-NC"; it simply refers to a contact that is not normally closed—meaning it is Normally Open (NO).
My contactor has terminals marked 13NO and 14NO. Is this one circuit or two?
It is one single circuit. The two-digit system splits the pair: terminal 13 is the input side of the first auxiliary set, and 14 is the output side of that same set. The "NO" suffix is just a visual reminder that this specific path is Normally Open.
Why is my contactor "chattering" (buzzing loudly and rapidly flipping on/off)?
Contactor chattering is almost always caused by a voltage drop at the control coil (A1/A2). When the coil receives power, it pulls the contacts closed. This sudden load causes the control circuit voltage to drop below the holding threshold, causing the coil to drop out. Once it drops out, the voltage bounces back up, pulling it back in, creating a rapid, destructive looping buzz. Check for loose control wires or an undersized control transformer.
What happens if I swap A1 and A2 wires on an AC contactor?
If it is a standard AC coil, swapping the wires on A1 and A2 makes no difference; the electromagnet will function perfectly either way because alternating current changes direction constantly. However, if it is a DC coil with an internal surge suppression diode, polarity matters—swapping them could short out the diode or prevent the coil from firing. Always check the coil specifications printed on the device.
Post time: Jun-15-2026