Short Circuit DC Power Calculator

DC Fault Input Panel

Source voltage Use open circuit or maximum DC voltage in volts.

Source resistance Enter internal resistance per source in milliohms.

Parallel sources Batteries or supplies feeding the same bus.

Current limit per source Enter 0 for no electronic current limit.

Cable length Use one way conductor length in meters.

Loop multiplier Use 2 for outgoing and return conductors.

Conductor area Cross sectional area in mm².

Parallel conductors per pole Conductors sharing the same polarity path.

Conductor material Custom uses the fields below.

Conductor temperature Temperature in °C for resistance correction.

Custom resistivity Ω·mm²/m at 20 °C.

Custom temp coefficient Per °C coefficient.

Contact resistance Total lugs, terminals, and joints in milliohms.

Bus resistance Bus bar, breaker, and shunt resistance.

Fault resistance Arc or bolted fault resistance in milliohms.

Extra resistance Add any filter, coil, or device resistance.

Clearing time Protection clearing time in milliseconds.

Fuse or breaker rating Nominal protective device rating in amperes.

Interrupt safety factor Multiplier for required interrupt capacity.

Formula Used

R_cable = ρ_T × L × loop multiplier ÷ (A × parallel conductors)

R_total = R_source + R_cable + R_contact + R_bus + R_fault + R_extra

I_sc = V ÷ R_total

P_source = V × I_sc

E = P × t, and I²t = I_sc² × t

When a current limit is entered, the final current is the lower value between ohmic current and total source limit.

How to Use This Calculator

Enter the maximum DC source voltage first. Add source internal resistance in milliohms. Enter the number of parallel sources if batteries or supplies share the same bus. Add cable length, loop multiplier, conductor area, material, and temperature. Then add contact, bus, fault, and extra resistance.

Set clearing time for energy and I²t estimates. Enter the fuse or breaker rating to compare available fault current with the device rating. Use the safety factor field to estimate a minimum interrupting capacity with margin. Press the calculate button to place the result above the form.

Example Data Table

Scenario	Voltage	Total resistance	Fault current	Source power	Use case
Small control battery	24 V	0.120 Ω	200 A	4.8 kW	Panel wiring check
Telecom battery string	48 V	0.012 Ω	4,000 A	192 kW	Breaker study
Solar battery bus	96 V	0.018 Ω	5,333 A	512 kW	Combiner design
EV low voltage bus	12 V	0.006 Ω	2,000 A	24 kW	Fuse selection

Understanding DC Fault Power

A direct current short circuit is a low resistance fault path. It can happen across battery terminals. It can also happen along a bus bar, cable, connector, or load. The current rises fast because no alternating zero crossing exists. The available power depends on source voltage and total circuit resistance. Small resistance changes can create large current changes. That is why milliohm inputs matter. A safe estimate should include every path element. Internal battery resistance, cable resistance, contacts, lugs, fuses, and fault resistance all add to the result.

Why Total Resistance Matters

The calculator treats the fault loop as one equivalent resistance. The loop may include outgoing and return conductors. It may include parallel conductors too. Copper and aluminum change resistance with temperature. Hot conductors have higher resistance. Corroded terminals can add extra milliohms. A poor joint can reduce current, but it can also overheat badly. The highest current usually appears near a strong source. The lowest current may appear at the far end of a long feeder. Both cases affect protection.

Power, Energy, and Protection

Fault current alone is not the whole risk. Power shows the rate of heat release. Energy shows the total heat during the selected clearing time. A breaker or fuse must interrupt the available current. Its interrupting rating should exceed the calculated value with margin. Cable insulation must also survive the thermal stress. For DC systems, arc behavior can be severe. Polarity, inductance, and enclosure spacing also matter. Use calculated values as engineering estimates. Confirm critical work with standards and equipment data.

Advanced Input Strategy

Start with measured source voltage. Then enter internal resistance from a battery datasheet or test report. Enter cable length as one way distance. Select the loop multiplier for the complete fault path. Use two when current travels out and back. Enter conductor area and material. Add contact, bus, and fault resistance values. Use a current limit for regulated supplies. Use the number of parallel sources carefully. Equal sources share current only when wiring and condition are similar.

Common Design Checks

Compare results for normal temperature and worst hot temperature. Check both near source and remote faults. Review switch, contactor, and connector ratings. Consider freshly charged cells, because voltage can be high. Recalculate after layout changes. Keep notes for inspectors, maintenance teams, and future upgrades. Record ambient assumptions and selected clearing times. List all resistance sources for review.

Interpreting the Output

The result block reports ohmic current, limited current, total resistance, power, and energy. It also estimates cable voltage drop and fault point heating. A fuse ratio above one means the fault exceeds the selected fuse rating. That does not prove instant clearing. Time current curves still control operation. A low ratio may indicate a hidden protection problem. Increase conductor size, reduce distance, or improve protection when needed. Document assumptions before approving any final protective device setting.

FAQs

What is DC short circuit power?

It is the power released when a DC source feeds a low resistance fault. It equals source voltage multiplied by fault current. The calculator also estimates heat power in the fault path.

Why do milliohms matter so much?

DC fault current can be very high. A few milliohms can change the result by hundreds or thousands of amperes. Battery, cable, and contact resistance should be entered carefully.

Should I use open circuit voltage?

Use the highest credible source voltage for conservative checks. A fully charged battery or raised charger voltage may produce more current than nominal voltage.

What does loop multiplier mean?

It converts one way cable length into fault loop length. Use 2 when current travels through an outgoing conductor and returns through another conductor.

How is cable resistance calculated?

The tool uses resistivity, cable length, loop multiplier, conductor area, and parallel conductors. It also adjusts copper or aluminum resistance for conductor temperature.

What is current limit per source?

Some power supplies limit output current electronically. Enter that value per source. The final current will not exceed the total current limit of all parallel sources.

Can this replace a protection study?

No. It gives useful engineering estimates. Final protection work should use equipment data, time current curves, applicable standards, and qualified review.

Why is fault point power separate?

Total source power covers the whole loop. Fault point power uses only the fault resistance. It helps estimate local heating near the shorted connection or arc.

What does I²t mean?

I²t is a thermal stress indicator. It multiplies current squared by clearing time. Cables, fuses, and devices often use this value for withstand checks.

How do parallel sources affect current?

Ideal equal sources reduce equivalent source resistance and increase available current. Real sharing depends on wiring, state of charge, age, and internal resistance differences.

What safety factor should I use?

Use a factor that matches design policy and risk. Higher factors give more interrupting margin. Document assumptions before approving any final protective device setting.