For a standard US 15A, 120V branch circuit, the continuous load limit is 12A (1440W). Assuming a typical office PC draws 200W to 250W under load, you can safely put 5 to 6 computers on one 15A breaker. On a 20A circuit (1920W continuous limit), you can run 7 to 8 standard PCs, or roughly 2 to 3 high-end 800W workstations.

But there is a catch that trips up many DIY server rack and mining farm builders: inrush current. When you switch on multiple switch-mode power supplies (SMPS) simultaneously, the charging of their internal capacitors creates a massive, millisecond-level current spike that can easily exceed 100A. This spike will trip the magnetic mechanism of a standard breaker instantly. To safely control and sequence these loads without nuisance tripping, we use electromechanical contactors. Here is how to size the breaker and select the right contactor for the job.

The Breaker Math: Continuous Loads and Inrush Current

Under NEC Article 210.20, branch circuits supplying continuous loads (defined as operating for 3 hours or more) must be sized at 125% of the load. This is why a 15A breaker is limited to 12A of continuous draw, and a 20A breaker is limited to 16A.

Breakers vs. Fuses: Never treat fuses and breakers as interchangeable without looking at the trip curve. A standard 15A fast-acting fuse will blow immediately under the inrush spike of six PCs turning on at once. A 15A thermal-magnetic breaker, however, has a 'magnetic' trip curve (often Type C or D in Europe, or a specific magnetic threshold in US HACR breakers) designed to ignore brief sub-cycle inrush spikes while still protecting against hard short circuits. If you are building a high-density PC rack, use a breaker with a high magnetic trip threshold, not a standard fast-blow fuse.

To prevent the inrush current from tripping even a robust breaker, heavy-duty setups use electromechanical contactors to stage the power-up sequence, turning on computers in banks of two or three with a 500-millisecond delay between each bank.

Contactor Rating Table: What Governs the PC Load?

When selecting a contactor to switch your computer loads, you must look beyond the basic amperage. IEC and NEMA standards categorize contactor ratings by load type. Here is a breakdown of the critical rating columns.

Rating Category Load Type Typical Application Make/Break Capacity
AC-1 Non-inductive / Slightly Inductive Heaters, steady-state resistive loads 1.5x rated current
AC-3 Squirrel-cage Motors HVAC compressors, conveyor belts 10x rated current (make)
AC-6b Capacitive Loads SMPS, LED drivers, UPS banks High inrush tolerance

Which rating column governs this load? Computer power supplies are highly capacitive when first switched on. While the steady-state draw falls under AC-1, the inrush spike requires you to either select a contactor explicitly rated for AC-6b (capacitive switching), or heavily derate an AC-1 contactor. As a rule of thumb for SMPS loads, divide the contactor's AC-1 rating by 2 to find its safe continuous computer-load capacity.

Wiring the Control Circuit: Coil vs. Contact Side

Electromechanical contactors isolate your high-current load from your low-current control logic. Understanding the physical and electrical separation is critical for safe wiring.

  • The Contact Side (Load): These are the heavy-duty brass or silver-alloy contacts (typically labeled L1/T1, L2/T2, L3/T3). This side carries the 120V/240V mains power to the computers. Wire this side using the appropriate AWG (e.g., 12 AWG THHN for a 20A circuit) and torque the terminal lugs to the manufacturer's spec (usually around 1.2 to 1.5 Nm).
  • The Coil Side (Control): Labeled A1 and A2, this is the electromagnet. When you apply the coil voltage (e.g., 24V DC or 120V AC), it pulls the contacts closed. This side draws very little current (typically 5VA to 10VA) and can be driven by a smart relay, PLC, or microcontroller.
DC Coil Flyback Protection: If you are driving a DC coil (e.g., 24V DC) using a transistor, MOSFET, or microcontroller GPIO, you must wire a flyback diode (like a 1N4007) in reverse bias across the A1 and A2 terminals. When the coil de-energizes, the collapsing magnetic field generates a massive reverse voltage spike that will instantly destroy your driver transistor without this diode.

Decision Path: Selecting the Right Contactor by Load Type

Use this decision tree to select the correct contactor based on what you are actually switching on the bench or in the rack.

If your load is... Then prioritize this rating... And select this concrete part...
Purely Resistive (Space heaters, dummy loads) AC-1 (No derating needed) Eaton XTCE009B10 (9A AC-3 / 20A AC-1)
Inductive (Transformers, solenoids) AC-6a (Derate AC-1 by 30%) Schneider LC1D09U7 (TeSys Deca, 110V coil)
Motor (Compressors, pumps) AC-3 (Use rated HP/kW directly) Schneider LC1D09M7 (220V coil, 9A AC-3)
Capacitive / SMPS (Computers, Servers) AC-1 derated by 50%, or AC-6b Schneider TeSys LC1D09U7 (110V AC coil, 20A AC-1)

The Default Pick for PC Clusters: If you are wiring a 120V, 20A branch circuit to run a cluster of 6 computers (approx. 1500W steady state), do not use a cheap 9A relay. Terminate your decision path at the Schneider Electric TeSys LC1D09U7. It features a 110V AC coil (compatible with US 120V control circuits) and a 20A AC-1 rating. Derated by 50% for capacitive SMPS inrush, it safely handles 10A of continuous computer load while surviving the 80A+ inrush spikes without welding its silver-alloy contacts shut. You can verify its specifications and curve data via the Schneider Electric IEC contactor portal.

Testing Dead and Live: When to Repair vs. Replace

Contactors are mechanical devices; they wear out, pit, and eventually fail. Proper diagnostics require both de-energized and live testing. Always follow lockout/tagout procedures before opening a panel.

How to Test It Dead (De-energized)

  1. Disconnect all power and verify dead with a multimeter.
  2. Set your meter to Continuity or Ohms (Ω).
  3. Place probes across L1 and T1. It should read 'OL' (Open Loop).
  4. Manually press the contactor's mechanical plunger (the movable bridge) with a flathead screwdriver. The meter should now read less than 0.5 Ω. If it reads higher, the contacts are pitted or carbon-fouled.
  5. Check the coil resistance across A1 and A2. A 110V AC coil typically reads between 100 Ω and 400 Ω. If it reads 'OL', the coil is burned open.

How to Test It Live (Energized)

  1. Restore power and engage the coil.
  2. Measure the voltage directly across A1 and A2. It must be within ±10% of the nominal coil voltage. A 110V coil dropping to 90V due to undersized control wiring will cause the contactor to 'chatter' and destroy the contacts.
  3. Measure the voltage drop across the closed main contacts (L1 to T1). A healthy contactor under load will drop less than 0.5V. If you measure a 3V to 5V drop, the contacts are heavily degraded and generating dangerous heat.

When to Repair vs. Replace

For contactors under 40A (like the TeSys D-line or Eaton XT series), always replace, never repair. The contacts are factory-riveted and sealed; attempting to file down pitted silver-alloy contacts removes the protective coating and alters the contact pressure, leading to catastrophic arcing and potential fire. If your live voltage-drop test exceeds 0.5V, or if the coil is open, swap the entire unit. For a detailed field-testing methodology, refer to the Fluke troubleshooting guide for contactors.

By calculating your continuous load correctly, respecting the magnetic trip curve of your breaker, and staging the inrush current with a properly derated AC-1/AC-6b contactor like the LC1D09U7, your multi-computer setup will run reliably without nuisance trips or welded contacts.