WireGaugeCalc Free calculator · NEC cited · works offline
1oo Amp Urd Voltage Drop
1oo amp urd voltage drop is quick to size with WireGaugeCalc, since the tool reads the load, run length, and conductor and returns the loss.
You enter 100 amps, the one-way distance, and aluminum or copper, so the calculator reports both the volts and the percent drop.
That tells you if the underground feeder needs an upsize before the trench is closed.
Open the calculatorUses Vd = (phase factor × K × I × L) / circular mils, K = 12.9 copper, 21.2 aluminum. Field aid only, verify against the current NEC and your local authority having jurisdiction.
Why Long URD Runs Push The Drop Toward The 3% Guideline
Underground residential distribution feeders often run long distances from the transformer to the panel, so resistance adds up over the length.
At 100 amps that added resistance can push voltage drop past the 3 percent feeder guideline, since current and distance both work against you.
The calculator finds the drop on aluminum URD so you can step up the conductor before pulling cable.
What Drives Voltage Drop On A 100 Amp URD Feeder
The drop on the underground run comes from these factors:
- The 100 amp load current
- One-way run length in the trench
- Aluminum URD versus copper, since aluminum drops more
- System voltage, usually 240V single phase
- Conductor size you plan to bury
One app for every NEC calculation
WireGaugeCalc keeps the calculations you reach for most in one place:
- Voltage drop and wire sizing for any run
- Conduit fill for EMT, PVC, and RMC
- Box fill and junction box sizing
- Ampacity and temperature derating
- Motor circuit and load calculations
- Conduit bend offsets and saddles
Built for the field, works offline
The whole app runs on your phone, so it keeps working in a basement, an attic, or a job site with no signal.
There is no account to create and nothing to load. Open it, run the number, and get back to work.
- No signup and no signal needed
- Answers in a tap, not a spreadsheet
- Same tool on phone, tablet, and desktop
Every result cites the NEC article
Each answer shows the table or formula it came from, so you can check the method and learn the code as you go.
That makes the app useful on the job and during exam prep, since the reasoning is right next to the number.
Switch the code year your job runs on
Jurisdictions adopt the NEC at different times, so you can match the calculation to the code in force:
- NEC 2017, 2020, and 2023 tables
- Copper and aluminum conductors
- 60, 75, and 90 degree C terminations
- Single-phase and three-phase systems
Run the number, then get back to work
Stop flipping through a paper book or hopping between calculator sites. Enter your values, read the code-cited answer, and move on. Free to use, no signup.
Open the calculatorFrequently asked questions
What size aluminum URD is used for a 100 amp service?
A 100 amp 240V service commonly uses 2 AWG or 1/0 aluminum URD depending on the run length and the drop you can accept. Run your distance through the calculator and verify the size against the current NEC and your local authority having jurisdiction.
How much voltage drop is acceptable on a 100 amp URD feeder?
Many installers hold a feeder near 2 to 3 percent so the branch circuits still have room. The NEC treats this as a recommendation, not a hard rule, so the design drop is yours to set.
Is WireGaugeCalc free to use?
Yes. Every calculator is free to run with no signup. A paid tier adds saved projects, PDF reports, and extra code years, but the core math stays free.
Does it work without internet?
Yes. The app runs on your device, so it keeps working with no signal on a job site, in a basement, or in an attic.
Are the results code accurate?
Results follow published NEC tables and standard formulas, and each answer shows the article it came from. It is a field aid, not a stamp of approval, so verify against the current code and your local authority before you wire anything.
Which NEC code year does it use?
You can switch between NEC 2017, 2020, and 2023, since jurisdictions adopt the code at different times. Pick the year your job runs on.