DC current vs phase current in electric boat propulsion: explained

When you ask a motor supplier for a quote, they'll send you two current figures: DC bus current and phase current (RMS). Many boat owners are uncertain which one drives what — so they either over-specify (expensive) or under-specify (dangerous).

This guide explains both figures clearly, what they size, and how to use them.

→ Calculate both values for your specific boat and speed target →

The basic circuit: battery → controller → motor

Electric boat propulsion uses a three-phase permanent-magnet motor driven by an inverter/motor controller. The controller converts the battery's DC power into three-phase AC for the motor windings. This creates two distinct circuits with different current levels:

1. The DC side: battery → fuse → main contactor → motor controller input
2. The AC side (inside the controller): controller → three motor phase wires → motor

DC current: what it is and what it sizes

DC current is the current drawn from the battery bank. It flows through:

• Battery terminals and internal resistance
• Main fuse and contactor
• Main DC cables

The formula:

DC current (A) = Motor shaft power (W) ÷ (Battery voltage × System efficiency)

For a 10 kW motor at 85% system efficiency on a 48 V system:

DC current = 10,000 ÷ (48 × 0.85) = 245 A

This is the continuous DC current — the figure used to size:

• Main cable cross-section
• Fuse and contactor rating
• BMS continuous discharge rating

At peak power (typically 1.5–2× continuous for 10–30 seconds), DC current rises proportionally. Peak DC current sizes the short-duration rating of the contactor and fuse.

Phase current (RMS): what it is and what it sizes

Phase RMS current is the current flowing through each motor phase wire and through the switching transistors (IGBTs or MOSFETs) inside the motor controller.

It is often assumed to be a fixed multiple of DC current. It is not. The two are linked by a power balance:

P_dc = V_dc × I_dc
P_ac = √3 × V_LL × I_phase × power factor

The term that moves is V_LL, the line-to-line voltage the controller is actually synthesising — which depends on how much of the DC bus it is using (the modulation index), and that in turn depends on motor speed.

At rated speed, the controller runs near full modulation, V_LL ≈ 0.61–0.71 × V_dc, and the two currents come out in the same ballpark — phase RMS typically lands somewhere around 0.85–1.1 × DC current. At full modulation it can be slightly below DC current.

At low speed — manoeuvring, coming astern, accelerating hard — the motor's back-EMF is low, so the controller uses only a fraction of the bus voltage. It still has to push full current through the windings to make full torque, but it draws far less DC current to do it. This is where phase RMS substantially exceeds DC current.

That low-speed case is the one that sizes the hardware, which is why motors and controllers are rated in phase RMS rather than DC amps: the worst moment for the switching devices and the windings is full torque at low revs, not full speed. The configurator applies a 1.25 factor to the DC figure to envelope it.

Phase RMS current sizes:

• Motor controller continuous current rating (almost all controller datasheets quote phase RMS)
• Motor winding rating (motors are specified in phase RMS, not DC)
• Phase cables (the three wires between controller and motor)

Peak vs continuous: the distinction that matters most

Almost every motor and controller has two current ratings:

• Continuous: what the component can sustain indefinitely without thermal damage
• Peak (10 s or 30 s): what it can handle for a short burst

For boat propulsion, continuous rating is what determines long-term reliability. Peak rating matters for:

• Hard maneuvering in a marina (high throttle, short duration)
• Pushing off a lee shore into waves
• Emergency full-power situations

Always size cables and fuses for continuous current. The motor controller's internal protection handles peak limiting.

Which current figure do you quote to suppliers?

Two figures the spec calculator outputs look superficially similar but serve different purposes:

• Peak current — useful for understanding worst-case loads and short-burst headroom (acceleration, maneuvering, emergency full power).
• Continuous RMS current — what the motor controller and motor windings can sustain indefinitely, and the number suppliers actually use to size their products.

When requesting quotes from motor or controller manufacturers, always quote continuous phase RMS current. That is the number they use to size their products.

Practical example: 10 kW system at 48 V

The phase figures here are the DC current with the configurator's 1.25 low-speed factor applied. That factor is a sizing margin covering the manoeuvring case, not a physical constant — at cruising revs the real ratio is nearer 1.

The most common sizing mistake

Under-sizing the controller by reading DC current from the datasheet. Many online "10 kW motor controllers" quote their DC bus current rating in the headline figure but bury the phase current rating in the datasheet — which may be significantly lower than required.

Always compare your calculated phase RMS continuous current against the controller's continuous phase current rating. If the controller's phase rating is below your calculated figure, it will thermal-limit or fail under sustained load.

→ Get your exact DC and phase current figures →