ASIC Circuit Breaker Selection: Sizing for Safety

The circuit breaker is the fire-prevention device on a mining circuit. It exists to trip before the wiring overheats, so choosing the wrong size, too large for the wire, or too small for the load, either removes the protection or trips constantly. Correct asic circuit breaker selection comes down to a short chain of math: the miner’s continuous current, the 80 percent rule for continuous loads, and matching the breaker to the conductor it protects. This guide works through that math with examples for common miners, explains breaker types, and shows how to size for one unit or several on a circuit. The breaker and its wiring must be installed by a licensed electrician to local code; this is operator-level education to plan and verify, not a substitute for a professional.

The core rule: 80% for continuous loads

A mining load is a continuous load, meaning it runs for three hours or more, in fact, around the clock. Electrical code treats continuous loads specially: a breaker and circuit should be loaded to no more than 80 percent of their rating on a continuous load. Put another way, the breaker must be rated at least 125 percent of the continuous current.

The reason is heat. A breaker carrying its full rated current continuously runs warm and can nuisance-trip or degrade; the 80 percent derating gives thermal headroom for a load that never lets up. A miner is the textbook continuous load, so this rule is not optional padding, it is the governing constraint.

The selection chain is therefore:

• Find the miner’s current draw from its power and voltage (current = watts ÷ volts).
• Apply the 80% rule: the breaker rating must be at least the current ÷ 0.8 (the same as current × 1.25).
• Round up to a standard breaker size and confirm the wire is rated for that breaker.

That last step is critical and the one amateurs skip: the breaker protects the wire, so the wire must be sized for the breaker, never the other way around.

A worked example: one S21-class miner

Take a single air-cooled flagship miner drawing about 3,500 watts on a 240V circuit. (Pull the exact wattage from the manufacturer spec sheet; figures here are illustrative of an S21-class unit.)

Step 1 — current. 3,500 W ÷ 240 V = 14.6 amps continuous.

Step 2 — apply the 80% rule. 14.6 A ÷ 0.8 = 18.2 A. The breaker must be rated at least 18.2 A.

Step 3 — round up. The next standard breaker is 20 A. A 20 A breaker, loaded to its 80 percent continuous limit of 16 A, comfortably carries the 14.6 A draw.

Step 4 — match the wire. A 20 A circuit requires conductor rated for 20 A. Sizing that conductor, accounting for length and voltage drop, is covered in the 240V circuit sizing guide; the breaker and wire are a matched pair.

So one S21-class miner lands cleanly on a dedicated 20 A, 240 V circuit. Note the margin: 14.6 A on a 20 A breaker leaves only 1.4 A of continuous headroom (16 A limit minus 14.6 A), which is why this circuit holds exactly one such miner and not two.

A worked example: two miners on one circuit

Two of those 14.6 A miners draw 29.2 A continuous combined.

Apply the 80% rule: 29.2 A ÷ 0.8 = 36.5 A. The breaker must be at least 36.5 A.

Round up: the next standard size is 40 A. A 40 A breaker, with its 32 A continuous limit, carries the 29.2 A draw with modest headroom.

Match the wire: a 40 A circuit needs conductor rated for 40 A.

This shows why the circuit size grows in steps. A 30 A breaker (24 A continuous limit) could not legally carry two of these miners, so the jump is straight to 40 A. Planning the miner-per-circuit count around standard breaker sizes avoids the trap of an awkward load that no standard breaker fits cleanly. When a circuit fills up, the next step is more circuits or a sub-panel; the sub-panel installation guide covers adding capacity once the existing panel is full.

Breaker types that matter for mining

Beyond the amp rating, the breaker type affects whether a mining circuit behaves.

Standard thermal-magnetic breakers

The common breaker trips on sustained overcurrent (thermal) and on a sharp fault (magnetic). For a dedicated miner circuit, a standard breaker of the correct rating is the baseline.

GFCI and AFCI breakers

Ground-fault (GFCI) and arc-fault (AFCI) breakers add protection that code may require in certain locations. Both can nuisance-trip on the leakage and switching noise of ASIC power supplies. The leakage profile of a switching PSU can exceed a GFCI’s threshold, especially with several units, causing trips unrelated to a real fault. The answer is not to remove required protection but to have the electrician select the appropriate breaker type and distribute the load; the leakage behavior is the same one discussed in the electrical grounding guide. Where AFCI/GFCI is code-required, work with the electrician on the layout that satisfies code without constant trips.

Double-pole for 240V

A 240V circuit uses a double-pole breaker that opens both hot legs together. A miner on 240V needs a double-pole breaker of the calculated rating, not two single-pole breakers.

Matching breaker, wire, and load together

The three elements, load, breaker, wire, form a chain where the breaker is the linchpin. The breaker is sized from the load using the 80 percent rule, and the wire is sized to match the breaker. Getting any link wrong creates a hazard:

• Breaker too large for the wire is the dangerous error: the wire can overheat and start a fire before the oversized breaker trips. This is why you never simply upsize a breaker to stop nuisance trips.
• Breaker too small for the load trips constantly, which is annoying but safe, and usually means the miner needs its own larger circuit.
• Wire too small for the breaker is the same fire hazard as an oversized breaker, viewed from the other end.

The whole point of the load calculation is to get these matched. Summing the loads across multiple circuits and balancing them against the panel’s capacity is covered in the electrical load calculation guide, which is the panel-level companion to this circuit-level breaker selection. Use the manufacturer’s wattage figures, never estimates, because the entire chain scales off that input number, and a low estimate produces an undersized breaker and wire.

Breaker rating, voltage, and interrupting capacity

Two specifications beyond the amp rating decide whether a breaker is correct for a mining circuit, and getting them wrong is unsafe even with the right amperage.

Voltage rating. A breaker must be rated for the circuit voltage. A breaker intended for a 120V branch circuit is not appropriate for a 240V miner; the 240V circuit needs a double-pole breaker rated for that voltage. Using a breaker below the circuit voltage is a serious error.

Interrupting capacity. Every breaker has an interrupting rating, the maximum fault current it can safely interrupt without failing. During a dead short, the available fault current depends on the service and the wiring, and a breaker whose interrupting rating is below the available fault current can fail catastrophically instead of cleanly tripping. In most home settings the standard breakers an electrician installs are adequate, but in larger installations with high available fault current, interrupting capacity becomes a real design consideration that the electrician evaluates. The operator’s takeaway is that the breaker is not just an amperage number; voltage and interrupting rating are part of why this is professional work.

The miner’s draw also is not perfectly steady. A unit pulls a brief inrush surge at startup as its power supply energizes, and the steady draw shifts with temperature and any tuning. A breaker sized only to the steady draw with no margin can nuisance-trip on inrush, which is another reason the 80 percent rule’s headroom matters: it absorbs the normal variation rather than tripping on it.

How tuning changes the breaker math

Undervolting and overclocking change a miner’s power draw, and any breaker calculation has to be based on the draw the unit will actually run at, not the factory default.

Undervolting lowers power consumption to improve efficiency, which reduces the current and could in principle allow a smaller circuit, though the safer practice is to size for the higher of the possible draws so a later change of tuning does not exceed the circuit. Overclocking does the opposite: it raises power draw, sometimes substantially, and a circuit sized for the stock draw can be overloaded by an aggressive overclock. The overclocking guide and the undervolt safe-ranges guide cover how far these adjustments move the power figure.

The prudent approach is to size the breaker and wire for the maximum draw the unit might run at, then verify the actual draw with a meter once the final tuning is set. Sizing to a low undervolted figure and later overclocking onto the same circuit is how a setup quietly ends up overloaded. Because the entire breaker calculation scales off the power figure, the figure must reflect how the unit will really be run, and it should come from the manufacturer spec sheet for the stock case and a meter reading for the tuned case, never a guess.

Verifying the circuit before running

Once the electrician has installed the circuit, a few operator checks confirm it is right before the miner runs continuously:

• Confirm the breaker rating matches the calculated size for the load, and that it is double-pole for a 240V circuit.
• Measure the actual draw with a clamp meter or power meter once running, and confirm it sits below the breaker’s 80 percent continuous limit. The power meter and monitoring guide covers measuring real draw.
• Check for warmth at the breaker after hours of running; a breaker that runs hot is overloaded or loose.
• Confirm one miner per dedicated circuit unless the breaker and wire were sized for more.

A breaker selected by this math and verified after install protects both the wiring and the hardware. The recurring reminder applies throughout: the breaker and its conductor are installed by a licensed electrician to local code, and the operator’s job is to plan the load, supply accurate wattage, and verify the result.

References

• Antminer S21 power and current specifications — Bitmain
• Avalon miner power specifications — Canaan
• Use of electricity and continuous loads — U.S. Energy Information Administration

Size the circuit to the real hardware draw. The Coin Web Mining catalog lists each model’s rated power so you can run this breaker math before buying, and we can quote multi-unit orders for operators planning a full panel.