Practical tools for measured work
Wire sizing guide and DC worksheet
Compare copper DC voltage drop with a chosen limit and independently check supplied corrected ampacity.
Calculate the layout, check the diagram, then print the result for the shop or jobsite.
Checks against your entered limits
| Check | Result |
|---|---|
| Voltage drop | Exceeds entered limit |
| Supplied ampacity | Within entered limit |
How it works
First identify the governing installation standard, cable material and insulation, routing method, ambient conditions, grouped conductors and termination ratings. Obtain the applicable base ampacity and correction factors from that standard or the cable manufacturer. The prefilled numbers are a calculation example, not a cable rating or installation recommendation.
This two-wire DC worksheet uses loop resistance = 2 × one-way length × 0.017241/area at 20°C, with copper resistance temperature coefficient 0.00393/°C. Voltage drop is current × loop resistance. Adjusted supplied ampacity is the smaller of the corrected base ampacity and termination limit; compare it with load current × your required sizing multiplier.
Reference: manufacturer ampacity and correction factors · Reference: copper wire resistance
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- DC supply voltage
- 12 V
- Actual load current
- 10 A
- One-way copper cable length
- 5 m
- Selected conductor area
- 2.5 mm²
- Estimated conductor temperature
- 20 °C
- Entered voltage-drop limit
- 3 %
- Published base ampacity for installation
- 30 A
- Applicable temperature correction factor
- 0.8
- Applicable bundling adjustment factor
- 0.7
- Termination current limit
- 20 A
- Required load-sizing multiplier
- 1.25
Primary result: Calculated voltage drop: 0.68964 V.
Before using the result
- Check the selected mode, units and input values before using the result.
- Read the assumptions and rounding rules; compare the worked example with your intended calculation.
- A numerical result does not supply missing context or change the stated limitations.
Limitations
A voltage-drop result alone is not a safe wire size. This worksheet does not look up local-code ampacity, select overcurrent protection, evaluate fault current, or check AC reactance, aluminium connections, burial or conduit fill. Its temperature is an input, not a predicted cable temperature. Verify the complete installation and protection requirements before selecting cable.
Frequently asked questions
What should I verify before using this result?
Check the input values, units, dates and assumptions. Compare the method and its stated limits with the source relevant to your task.
Why might the finished result differ?
Results depend on the entered data and the model’s assumptions. Rounding, changing conditions or omitted factors can affect the comparison. Read the limitations before using the result.
Copper conductor reference selector
How it works
Reference scope: US 2023 NEC table basis; individual copper THHN conductors in dry raceway; no more than three current-carrying conductors; ambient temperature at or below 30°C; every termination and connected component explicitly suitable for 75°C ampacity; supply no more than 600 V. Confirm every condition before a result is shown.
Enter the required conductor design current after all applicable load factors, not just the measured running current. The lookup starts at 14 AWG and ends at 4/0 AWG. It applies conservative 15 A, 20 A and 30 A caps to 14, 12 and 10 AWG respectively. For example, a 20 A design current selects 12 AWG in this reference.
Southwire SPEC 10003: 14–10 AWG · Southwire SPEC 10000: 8 AWG–4/0 AWG
Before using the result
- Check the selected mode, units and input values before using the result.
- Read the assumptions and rounding rules; compare the worked example with your intended calculation.
- A numerical result does not supply missing context or change the stated limitations.
Limitations
This is an ampacity reference selection, not a complete cable design or a breaker recommendation. It excludes other jurisdictions or editions, 60°C terminals, NM-B cable, aluminium, hotter ambient conditions, additional conductors, motor-specific rules and special equipment. Check voltage drop separately in the worksheet, plus protection, fault current, grounding, conduit fill and local adoption. A larger size may be required.
Frequently asked questions
What should I verify before using this result?
Check the input values, units, dates and assumptions. Compare the method and its stated limits with the source relevant to your task.
Why might the finished result differ?
Results depend on the entered data and the model’s assumptions. Rounding, changing conditions or omitted factors can affect the comparison. Read the limitations before using the result.
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