Calculator Inputs
Example data table
These sample scenarios show how different derating and reserve assumptions change the chosen standard breaker size.
| Scenario | Base Current (A) | Voltage (VDC) | Continuous | Ambient % | Group % | Growth % | Margin % | Required Breaker (A) | Recommended (A) |
|---|---|---|---|---|---|---|---|---|---|
| 12 V battery pump | 48.00 | 12 | Yes | 100 | 100 | 15 | 10 | 75.90 | 80 |
| 48 V telecom load | 29.17 | 48 | Yes | 90 | 95 | 20 | 10 | 56.29 | 63 |
| 125 V panel branch | 72.00 | 125 | No | 85 | 100 | 10 | 15 | 107.15 | 125 |
| 250 V battery string | 180.00 | 250 | Yes | 90 | 90 | 10 | 8 | 330.00 | 400 |
Formula used
This calculator estimates the breaker ampere rating first, then checks supporting conditions. Final selection must also satisfy conductor protection, device voltage rating, interrupt rating, enclosure rules, and the manufacturer’s DC application limits.
How to use this calculator
- Choose whether you know the DC load in amps or watts.
- Enter system voltage and the base load value.
- Set continuous duty, ambient derating, and grouping derating.
- Add any future expansion or nuisance-trip margin you want to reserve.
- Select the breaker rating series you want to round into.
- Optionally enter conductor ampacity, breaker voltage rating, and interrupt rating for validation checks.
- Press Calculate Breaker Size to show the result above the form.
- Use the CSV or PDF buttons to export the calculation summary.
FAQs
1) What does this calculator size?
It estimates a DC breaker ampere rating by starting with load current, then applying continuous-load rules, derating, reserve growth, and nuisance-trip margin before rounding to a standard size.
2) Why is continuous load multiplied by 125%?
Many design practices size overcurrent protection above continuous current so the device carries sustained load without overheating or nuisance operation. Always verify the exact requirement for your code and equipment.
3) Why do derating factors increase breaker size?
When temperature rises or conductors are bundled, usable capacity falls. Dividing by these factors increases the required breaker size so the chosen device still handles the effective design current.
4) Is the recommended breaker enough by itself?
No. The final breaker must also have adequate DC voltage rating, interrupt rating, correct trip curve, proper conductor coordination, and manufacturer approval for the exact circuit application.
5) Can I size from watts instead of amps?
Yes. In power mode, the calculator converts watts to current using I = P ÷ V. That is useful when equipment power is known but operating current is not listed.
6) Why add nuisance-trip margin?
Some DC loads have startup surges, cycling behavior, or short spikes. A modest margin helps avoid repeated trips, but it should never replace proper device curve selection.
7) What if the required current exceeds the listed series?
That means the selected breaker family may be too small. Consider another product range, different coordination method, parallel architecture, or a fresh design review before installation.
8) Does this replace engineering review?
No. Use it as a planning and documentation tool. Final protection design should be checked against local code, manufacturer data, fault calculations, conductor limits, and project-specific safety requirements.