Solar & DC
PV / Battery DC Transfer Switch Calculator
Size the DC design current and organise the voltage, pole, and switching requirements for a DC transfer switch between two DC sources — for example solar and battery, or two battery banks. DC transfer is arc-critical: the switch must make and break at full DC voltage with the correct polarity handling.
How the rating is built
The transfer switch carries the full DC load continuously, so the current rating takes the design current with a duty margin. The voltage rating must cover the highest DC bus voltage, and both poles are normally switched so the load is fully isolated during transfer between the two sources.
Why DC transfer is different
Direct current has no natural zero crossing, so a DC transfer switch must quench the arc entirely on its own during the make-break of each transfer. That demands a device with a genuine DC voltage rating, DC-rated contacts, and — depending on the topology — correct polarity handling, not an AC changeover switch pressed into DC service. In solar-plus-storage systems the transfer also has to respect source characteristics: a battery can deliver very high fault current, while a PV source is current-limited near its short-circuit value.
Frequently asked questions
Can I use an AC ATS on a DC system?
No, unless it carries an explicit DC voltage and current rating with DC-rated contacts. AC transfer switches rely on the AC current zero to clear the arc, which a DC circuit does not provide.
Two poles or four?
Two poles isolate a single DC bus (positive and negative). A bipolar or 4-pole arrangement suits split or bipolar DC systems, or where each source’s poles are switched independently. Follow the system topology.
How do I handle the different fault levels?
Size the current and interrupting duty for the worst case — usually the battery, which can source very high fault current — while respecting the PV source’s current-limited behaviour. Coordinate with the DC protection on each source.
Select the DC transfer switch
Match the current, DC voltage, and pole arrangement to a transfer switch rated for DC source changeover.
Planning estimate for guidance only. Final transfer-switch ratings, DC interrupting duty, polarity handling, and control sequence must be confirmed by a qualified person against the system topology and the manufacturer’s DC data.