Why data centres are revisiting direct current for AI workloads

The debate between alternating current (AC) and direct current (DC) dates back to the 1880s. Nikola Tesla and Thomas Edison famously argued over the best method for distributing electricity. AC ultimately won the ‘war of the current’ and has since underpinned global power systems for more than a century.  

Despite this, DC power didn’t fully disappear. Batteries, renewable energy, and telecommunication equipment have all relied on DC distribution.  

Today, the conversation is resurfacing with the data centre sector being the staging ground. As AI and high-performance computing workloads increase rack densities to 1MW and beyond, engineers are exploring whether high-voltage DC distribution - specifically 800V initially followed by 1500V in the future- could be more efficient in delivering power.  

Why AC became the dominant power distribution model

While both AC and DC deliver electrical energy, they behave differently in power systems.  

A useful way to think about AC is to imagine waves on the ocean. The energy moves forward whilst the water itself moves up and down remaining in place. In electrical systems, AC current similarly alternates direction, creating a sinusoidal waveform that reverses polarity fifty times per second, or 50 hertz, in European grids.  

DC current behaves differently and more like a river. Rather than alternating direction the way AC does, the electrical current flows continuously from the source to the load (and back to the source).  

Historically, AC became the preferred approach because its alternating waveform allows voltage levels to be transformed efficiently using conventional transformers. This made it possible to step the voltage up for transmission and then reduce it again for end use.  

Raising the transmission voltage reduces current, thereby minimising resistance losses. Losses are proportional to the current squared (I²) times conductor resistance (R). For example, doubling the current increases losses by a factor of 4 (2² = 4). Increasing voltage and reducing current lets networks deliver power efficiently over distance.  

This flexibility is what enabled the development of large, interconnected energy grids and established AC as the standard.  

Why AI workloads are renewing interest in DC distribution

The data centre industry is facing a new challenge. The power requirements of AI infrastructure are rapidly increasing. Some roadmaps suggest that individual racks may soon exceed 1MW of power demand - a huge leap from the norm we’ve seen in the last decade.  

At this scale, even small improvements in electrical efficiency can deliver significant operational savings across large campuses.  

Higher-voltage DC systems can reduce current for a given power level, minimising resistance losses and copper usage. Fewer AC-to-DC conversions further simplify electrical distribution in data centres.  

As a result, DC, and in particular, 800V DC distribution, is being tested as a way to support AI data centres.

The engineering challenges of adopting 800V DC systems

While DC distribution offers potential efficiency benefits, adopting it at scale presents several technical challenges. These are mainly around protection and power conversion strategies.  

In conventional AC systems, when a fault occurs, a circuit breaker ‘trips’ and isolates the fault, however, the air becomes ionised and conductive. This creates an arc between the two contacts and sustains the fault current. These breakers use the natural zero crossing of the alternating current to clear the fault.  

As DC systems lack this natural zero crossing, interrupting faults become more complex and require specialised protection strategies. Standard mechanical DC circuit breakers require special arc chutes to cool and extinguish this arc and interrupt the fault.  

Another solution emerging is solid-state circuit breakers (SSCBs). These use semiconductor devices like insulated-gate bipolar transistors (IGBTs). These have no moving parts, so they can interrupt fault currents faster than traditional breakers. Though again, this creates new challenges for designs as they have continuous losses which generate heat and require liquid or forced fan cooling.  

Solid-state transformers (SSTs) replace traditional transformers for DC systems. They convert AC medium voltage to DC at distribution voltage levels, such as 800V DC, without relying on electromagnetic induction.  

The process is typically two stages.  

• Stage one: An AC/DC rectifier converts AC to medium-voltage DC.
• Stage two: A DC/DC converter steps the voltage down to the required distribution level.  

This allows for the full voltage AC distribution component to be removed from the electrical architecture, removing multiple transformations between AC and DC power.  

Moving directly forward

As data centre power densities continue to increase, engineers must rethink traditional power distribution strategies. Whilst the debate between AC and DC has been going on for over a century, in truth, the correct answer is simply to use the right tool for the job. DC will not replace AC everywhere, but as the data centre industry is at an inflexion point, operators need to rethink their approach to better address the demand for high-density infrastructure.