Voltage Drop
Estimate voltage drop and percentage drop for copper or aluminum conductors from 14 AWG to 1000 kcmil — single-phase or three-phase. Results update as you type.
For the trade
Free Electrical Calculators
We built these for our own estimators and field crews, then left them open. No account, no email wall, no upsell. Everything runs in your browser — we never see your numbers unless you deliberately send one to an estimator.
Built against the code, corrected when wrong
Most free calculators on the internet implement the tables and skip the rules that read them. Ours were swept against the NEC for exactly that: the 110.14(C) 75°C termination limit that stops a 90°C ampacity being used at a 75°C lug, the 240.4(D) small-conductor caps, the 314.16(B) box-fill allowances, and the 450.3(B) overcurrent rules on both sides of a transformer.
Phoenix makes two of these unavoidable. Ambient temperature correction and conduit-fill adjustment are optional arithmetic in a mild climate and mandatory here. A conductor sized off the bare Table 310.16 value, run in conduit on a Phoenix rooftop in July, is undersized. The ampacity calculator applies both corrections.
They are engineering aids, not a stamped design. Check every result against the code book and the approved drawings before you buy material. Where a calculation has to be sealed, our engineering team does that work.
Tuned for the Valley, not a mild climate
Every one of these runs the national tables. What makes them useful on a metro Phoenix job is that each page also carries the local rules the tables do not know about — the ones that turn a set drawn to the code book into a correction from a Valley plan reviewer.
- One more conductor in every raceway. Phoenix, Glendale and unincorporated Maricopa County amend NEC 250.118(4): EMT alone is not an acceptable equipment grounding conductor, so a wire-type EGC rides in every run. That changes the count in the conduit fill calculator and the box fill calculator, and it is the most common fill correction in the Valley.
- Ambient correction is not optional. A conductor sized off the bare Table 310.16 value and run in rooftop conduit in July is undersized. The wire ampacity calculator applies the ambient and conduit-fill derates at the 75°C termination column, which is where commercial gear actually lands.
- Long, hot site runs. Large flat commercial lots put lighting poles, gates and remote pads a long way from the service, and a hot conductor drops more than the table suggests. Use the voltage drop calculator for the geometry and confirm the ambient case.
- The utility question arrives with the kVA. APS and SRP each want the estimated load and the voltage and phase to open a commercial service file, and the 400 A Class 320 socket is residential-only at SRP, so commercial above 200 A is CT-rated. The transformer sizing calculator gives you the number the application is built from.
- The panel schedule goes in the permit set. In Phoenix it is reviewed with the load study and the one-line through SHAPE PHX, and a missing or inconsistent schedule is a correction. The panel schedule generator prints a sheet a reviewer can read.
Which code edition applies depends on the city, because Arizona has no statewide adoption: Phoenix, Mesa, Chandler, Glendale and Buckeye are on the 2023 NEC, Scottsdale on the 2020, unincorporated Maricopa County on the 2017, and for Gilbert, Peoria and Tempe we confirm with the jurisdiction before design starts. The position for each city we work in is on the service area page.
Need the calculation done as part of a permit set, sealed where the jurisdiction requires it? That is what engineering and design is for — and the form at the bottom of that page gets it priced.
The tools
Conduit Fill
Check conduit fill against NEC Chapter 9, Table 1 — any mix of conductor sizes and insulation types, in EMT, IMC, RMC, PVC or flex.
Wire Ampacity
Size conductors from NEC Table 310.16 with ambient-temperature correction and conductor-bundling adjustment — the two derates a Phoenix summer makes unavoidable.
Box Fill
Count box fill the way NEC 314.16 does — conductors, devices, clamps, fittings and grounds — and get the minimum volume in cubic inches.
Transformer Sizing
Size a dry-type transformer from the load: kVA, primary and secondary full-load current, and the NEC 450.3(B) overcurrent protection each side needs.
Panel Schedule
Lay out a panel schedule circuit by circuit, watch the phase balance as you go, and print or save a clean sheet. Everything stays in your browser.
Frequently asked questions
Do I need an account?
No. There is no signup, no email capture and no paywall. Open the page and use it.
Do you see my calculations?
No. Every calculator runs entirely in your browser. Nothing is transmitted to us unless you press the 'send this to an estimator' button, which opens a form with the numbers filled in — and even then nothing is sent until you submit it.
Which NEC edition do these follow?
The tables and rules implemented are from the 2023 NEC. Arizona has no statewide code adoption — each city adopts and amends the NEC independently — so confirm the edition and the local amendments with the authority having jurisdiction over your project before relying on a result.
Can I use these on a permit submittal?
Use them to check your work, not as the submittal. Plan review wants a calculation prepared and, where required, sealed by a licensed professional engineer. These will tell you whether your number is in the right neighbourhood before you pay for that.
The panel schedule — where does it save?
In your browser's local storage on that device. Nothing goes to a server. Print it or save it as a PDF from the print dialog to keep a copy.
Are these calculators specific to Phoenix?
The arithmetic is the national NEC, so they work anywhere. What is specific to Phoenix and the Valley is the local guidance on each page: the wire-type grounding conductor that Phoenix, Glendale and unincorporated Maricopa County require in every raceway, the ambient temperature derating that a Valley summer makes unavoidable, and the APS and SRP service rules that decide how a commercial service above 200 amps is metered.
Which Valley cities are on which NEC edition?
Phoenix, Mesa, Chandler, Glendale and Buckeye are on the 2023 NEC. Scottsdale is on the 2020. Unincorporated Maricopa County is still on the 2017. For Gilbert, Peoria and Tempe we confirm the adopted edition with the jurisdiction before design starts rather than assume. The tables these calculators implement are stable across those editions; the local amendments are what change the count.
Ready to price your Phoenix project?
Send us the plans, the scope, or just the problem. We will tell you what it takes. Call (602) 396-5216 or request a bid online.