PV DC transfer-switch sizing checks
The transferred DC load establishes current while the highest of either source establishes the voltage screen.
Iload = P/VDCIdesign = Iload × FcontinuousIe,DC >= IdesignUe,DC >= Umaxwhere:
- Iload
- Maximum transferred load current[A]
- Ie,DC
- DC operational-current rating for the documented switching duty[A]
- Umax
- Highest voltage available from either source[V]
DC source transfer requires purpose-rated switching, interlocking, conductor isolation, and a controlled transition sequence.
How to size a PV or battery DC transfer switch
Enter transferred power or current and the highest voltage available from either source. Apply the continuous-current factor, choose the DC conductor arrangement and transition method, then check the candidate operational current, voltage, and poles.
- Use the highest source voltage under all operating conditions.
- Calculate maximum transferred load current.
- Apply the required continuous-current factor.
- Use open transition unless a purpose-designed source-paralleling system exists.
- Verify DC utilization category and pole wiring from the exact product data.
Why a DC ATS is not an AC ATS
DC interruption lacks the regular current zero of AC. Contact spacing, arc control, polarity, number of series poles, voltage, current direction, and source characteristics all affect switching capability. An AC ATS label does not establish DC suitability.
- Both sources require isolation and backfeed prevention.
- Battery sources can deliver high fault current.
- PV voltage varies with temperature and string configuration.
- Transfer logic must define source loss, recovery, delay, and failed-transfer behavior.
Open transition and source interlocking
Break-before-make operation prevents the two DC sources from being connected together during ordinary transfer. Closed transition is not a simple timing option: it requires equipment designed for parallel operation, current sharing, protection, and the exact source technologies.
PV DC ATS sizing example
A 20 kW load at 500 V draws 40 A. Applying a 125% continuous-current factor gives 50 A, so the current class must be at least 50 A. The candidate still needs a DC voltage rating above the maximum source voltage and documented switching duty for its exact pole arrangement.
Assumptions
- Steady DC transfer load
- Entered voltage is the highest voltage that can appear from either source
- Current rounding is a product-family starting point
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Use only equipment explicitly rated for DC transfer duty at the actual voltage, current, polarity, utilization category, and pole wiring.
- Closed-transition or source paralleling can create uncontrolled circulating current and requires a purpose-designed interconnection system.