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: /electric-boat-spec

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 always higher than DC current for the same power.

Why? Because the controller is converting DC to three-phase AC. The power flows through only two of the three phases at any instant; the third phase is in transition. The mathematics of this conversion means phase current is approximately:

Phase RMS current ≈ DC current × √(3/2) ÷ Motor power factor

For a typical permanent-magnet motor with 0.95 power factor:

Phase RMS ≈ 245 × 1.22 ÷ 0.95 ≈ 314 A

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.

Installer mode vs DIY mode

The spec calculator includes an "Installer mode" setting that changes how it presents current figures:

• DIY mode: uses peak current figures (useful for understanding worst-case loads)
• Installer mode: emphasises continuous RMS ratings (what you quote to suppliers)

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

| Parameter | Value | Used to size | |---|---|---| | Continuous shaft power | 10 kW | Motor model selection | | System efficiency | 85% | — | | DC continuous current | ~245 A | Main cable, fuse, BMS, contactor | | DC peak (×1.5, 10 s) | ~368 A | Contactor short-duration rating | | Phase RMS continuous | ~314 A | Motor controller, motor winding | | Phase RMS peak (10 s) | ~472 A | Controller peak rating check | | Main cable (6 m run) | 70–95 mm² | Voltage drop <3% |

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: /electric-boat-spec