Electrical calculators
Free conduit fill, box fill, derating and voltage drop calculators, plus conduit bend radius tables, built on NEC tables.
Conduit fill calculator
NEC Chapter 9, Tables 1, 4 and 5. Uses THHN/THWN-2 conductor areas. Fill limits are 53% for one wire, 31% for two and 40% for three or more.
Box fill calculator
NEC 314.16. Counts conductors, devices, grounds, clamps and fixture studs, then compares the total to your box volume.
Ampacity derating calculator
NEC Table 310.16 with 310.15(B) temperature and conductor-count adjustments, and the 110.14(C) termination limit.
Voltage drop calculator
Uses the circular-mil formula: VD = 2 x K x I x L / CM for single phase, 1.732 x K x I x L / CM for three phase. K is 12.9 for copper and 21.2 for aluminum at 75 C.
NEC 210.19 and 215.2 suggest keeping branch circuits to 3% and the total feeder plus branch to 5%. These are recommendations, not requirements, but long runs that miss them cause dim lights and motor trouble.
Conduit bend radius table
NEC Chapter 9, Table 2. Minimum radius to the centerline of the conduit, in inches, for conductors without lead sheath.
| Trade size | One-shot and full-shoe benders | Other bends |
|---|
Offset bend multipliers and shrink
Multiply the offset height by the multiplier to get the distance between bends. Shrink is how much shorter the conduit gets per inch of offset.
| Bend angle | Multiplier | Shrink per inch of offset |
|---|---|---|
| 10 degrees | 5.76 | 1/16 in. |
| 22.5 degrees | 2.6 | 3/16 in. |
| 30 degrees | 2.0 | 1/4 in. |
| 45 degrees | 1.414 | 3/8 in. |
| 60 degrees | 1.155 | 1/2 in. |
The total of all bends between pull points cannot exceed 360 degrees (four quarter bends), per the NEC conduit articles such as 358.26 for EMT. Bender take-up varies by brand, so check the marks on your bender.
These tools are for planning and education. Results follow the NEC tables (2020/2023 values) and do not replace the code book, the manufacturer's instructions or your local inspector. North Carolina's 2024 State Building Code, in effect since July 1, 2025, adopts the 2023 NEC with state amendments, so confirm the current edition and any local rules before you build.
Common mistakes these tools catch
- Forgetting that the ground wire counts toward conduit fill.
- Counting a switch or receptacle as one wire in box fill, when it counts as two.
- Running 3 circuits in one conduit and not derating for the extra current-carrying wires.
- Using the 90 C wire rating for the final answer when the breaker terminal is only rated 60 or 75 C.
- Ignoring 240.4(D), which caps 14, 12 and 10 AWG copper at 15, 20 and 30 A.
Frequently asked questions
Are these calculators accurate enough to use on a real job?
They use the NEC tables for conduit area, box volume and conductor ampacity, so they are a solid planning check. Always confirm against the NEC edition North Carolina has adopted and with your local inspector, because local amendments and special conditions can change the answer.
What is the conduit fill limit for three or more wires?
NEC Chapter 9, Table 1 allows 40% of the conduit's cross-section for three or more conductors, 31% for two and 53% for one. A nipple of 24 inches or less can be filled to 60%.
Do I count the ground wire for conduit fill?
Yes. Every conductor in the conduit counts for fill, including the equipment grounding conductor. For ampacity derating, grounds and a balanced neutral are not counted as current-carrying.
How do I count the ground wires for box fill?
All equipment grounding conductors together count as one allowance, based on the largest ground in the box. Each device yoke counts as two allowances, and internal cable clamps together count as one.
Why can my wire carry less than the table ampacity?
Hot ambient temperatures and more than three current-carrying conductors in one conduit reduce ampacity. Terminations also cap it, since most breakers and lugs are rated for 60 or 75 degrees C no matter how hot the wire insulation can run.
How much voltage drop is too much?
The NEC recommends no more than 3% on a branch circuit and 5% for the feeder and branch circuit combined (210.19 and 215.2 informational notes). Going bigger on wire size is the usual fix for long runs, such as to a detached garage, well pump or EV charger.
What is the minimum bend radius for conduit?
NEC Chapter 9, Table 2 sets it by trade size. For example, 3/4 inch conduit needs a 5 inch radius with a one-shot or full-shoe bender, and 1 inch needs 6 inches. The table above lists every size.
Can you do the calculations and pull the permit for me?
Yes. Reliance Electrical sizes the wire, conduit and boxes for every job and works to code in Rowan County and the surrounding area. Call or text for a free estimate.
Rather have a licensed electrician handle it?
We size the wire, conduit and boxes for every job, and pull the permit when one is needed.
Have a project in mind? That is fine. Text a photo or ask a question and we will tell you what we would do, with no obligation.