v1.0 · NEC 2023 · low and medium voltage

A Neher-McGrath cable ampacity calculator.

Web-based. NEC 2023 cross-references. Validated against the 17 published cases in CIGRE Technical Brochure 880. From Deliverable Labs, makers of Power Sketch Pro.

Try the calculator → Read the methodology View validation results →
Free in-browser demo · no account · perpetual license, $1,500/user during founder's window
CableSketch Pro canvas showing a duct bank with circuits and a result panel reporting 1599 A available, 1200 A needed, 75% loaded, with limiting Section A at 4 inches and 84°C.
Calculation Parameters (left), Interactable Canvas View Designer (center), result panel (right)
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Method Neher-McGrath, per Anders (2005).
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Code references NEC 2023, Article 310.
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Validation 10 of 17 CIGRE TB 880 cases pass.
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Project files Local JSON, documented schema.
02 · Scope

What v1 calculates.

Steady-state ampacity for the configurations below. Detailed assumptions are on the methodology page; cases not yet covered are on the validation page.

Installation
conduit · duct bank · direct burial

Underground installations.

Single conduits, concrete-encased duct banks up to 12×12, and direct-buried cables. Backfill, native soil, and burial depth are configurable per section.

Cable
Low and medium voltage · Cu / Al · NEC 2023

Single and multi-conductor.

THWN-2, XHHW, USE, RHH/RHW for low voltage; shielded XLPE and EPR constructions for medium voltage, in copper or aluminum. Custom cables can be defined when the library entry doesn't match.

Heating
mutual · Kennelly images · sun

Multi-circuit interaction.

Mutual heating between adjacent circuits via Kennelly's method of images. Solar gain for cables exposed to sun. Site-specific soil resistivity and ambient temperature.

03 · Features

What's in the app.

Six things the app does.

/01

Interactive visual canvas

The duct bank is a drawn object. Drag conduits to add, remove, or rearrange them.

CableSketch Pro canvas showing trefoil cable conduits with per-conduit ampacity readouts of 365A and 350A
/02

Intermediate values exposed.

The result panel and the report list the full thermal-circuit breakdown — R₁ through R₄, Kennelly mutual terms, skin and proximity loss factors, and conductor temperature at the limiting section — not only the final ampacity number.

// section_a.result.json (excerpt)
{
  "ampacity_A": 387.2,
  "limiting": "section_a · conduit_4 · 84.0°C",
  "thermal_resistances_K_m_W": {
    "R1_insulation": 0.412,
    "R3_jacket":     0.000,
    "R4_earth":      1.847,
    "Rmutual_kennelly": 0.612
  },
  "loss_factors": { "Yc": 0.018, "Yp": 0.011 }
}
/03

Cable manager.

Library of NEC 2023 cable types, indexed by AWG / kcmil and insulation class. Each entry carries conductor and insulation dimensions, jacket, and shield construction. Custom cables can be defined per project or saved to the account library.

Cable manager showing 750 kcmil Cu THWN-2 600 V with construction details
/04

Site conditions lookup.

Pin a project location on the map. The app reads SSURGO soil thermal resistivity and 30-year ERA5 ambient design temperatures and writes them into the project file as defaults. Both values are editable; the data sources and timestamps appear in the report.

CableSketch Pro site lookup map showing a pin in Bend, OR with soil rho 90 K·cm/W from SSURGO and a 32°C P95 design ambient from ERA5
/05

Thermal resistance stacking

Stack trench layers. And layers of concrete-encased or special thermal backfill layers.

Canvas in dark mode showing a duct bank under edit
/06

JSON project files.

A project is a single .cablesketch.json document, saved locally. The schema is documented and stable across versions; files open in any text editor. You own your project data.

// bend-substation.cablesketch.json
{
  "version": "1.0",
  "project": "Bend Substation 13.8 kV Feeder",
  "site": { "lat": 44.0786, "lon": -121.2702, "rho_soil": 90, "ambient_C": 32 },
  "sections": [ /* … */ ],
  "circuits": [ /* … */ ]
}
04 · Adjacent tools

Other Neher-McGrath calculators.

For reference. CableSketch Pro is one option in this category; here's how the others differ.

CableSketch Pro AmpCalc Cableizer CYMCAP ETAP / SKM
Calculation method Neher-McGrath (NEC) Neher-McGrath IEC-focused IEC + FEM IEC + Neher-McGrath
Runs in browser ✓ (Demo Only) — ✓ — —
Platform Windows desktop, Mac (coming soon) Windows desktop Browser Windows desktop Windows desktop
U.S. NEC focus ✓ ✓ — Partial Partial
Pricing tier $ $$ $$ $$$$ $$$
Project file portability JSON · user-owned Proprietary Cloud-stored Proprietary Proprietary

Pricing tiers are order-of-magnitude only. Specific numbers and feature sets change; check each vendor directly for a current quote.

05 · Pricing

A perpetual license, per user.

$2,000 per user. Buy once, run forever. One year of updates and email support included; renew at $400/user/year if you want to stay current after that.

Free demo
$0no account

Run a real calculation in your browser.

  • Full canvas, trimmed cable library
  • Real Neher-McGrath solve
  • Validation page access
Open the demo
Founder's edition
Per user
$1,500/user, perpetual

$2,000 after the founder's window closes. One year of updates and support included.

  • Perpetual license, owned not rented
  • Unlimited calculations and reports
  • 1 year of updates + email support
  • Renew at $400/user/yr to stay current
See pricing
Multi-seat
5+users

Volume discount, single invoice, optional SSO. Email us for a quote.

  • Volume pricing from 5 users
  • Annual invoicing
  • SSO on request
Email us

See full pricing details →

06 · FAQ

FAQ.

/01 Is this validated? Against what? +
CIGRE Technical Brochure 880 (2022). It publishes 17 case studies for cable ampacity calculations with results to 10 significant figures. Our engine runs every case and publishes the deviations on the validation page. Re-run on every release.
/02 What revision of the Neher-McGrath method does the engine implement? +
The 1957 Neher-McGrath paper as extended by Anders (2005), with NEC 310.15(C) and 310.60 cross-references. Full derivation and citations on the methodology page.
/03 Can I use CableSketch Pro for stamped drawings? +
Yes, with the standard professional caveats — you remain responsible for the result. The engine is a calculation aid, not a substitute for engineering judgment. See the disclaimer.
/04 What happens to my projects if CableSketch Pro shuts down? +
Your project files are local .cablesketch.json documents. They continue to work in any text editor regardless of what happens to us. The schema is documented and stable.
/05 Do you support IEC 60287? +
Not yet. The architecture supports it; it's on the roadmap. If you're doing IEC work today, Cableizer is excellent — come back to us when you need NEC.
/06 Is there fully licensed cloud version without the download? +
Not today. The web platform is for demonstration and evaluation only.
/07 Do you store my project data on your servers? +
Only what's needed for billing, licensing, support, and marketing purposes. Calculation inputs to the web demo platform are not retained, and calculations in the downloaded app remain local.
/08 Can I get a team or multi-seat license? +
Yes — email us. From 5 users we can do volume pricing, annual invoicing, and SSO on request.
/09 What does "perpetual license" actually mean here? +
You buy the license once. It doesn't expire and it doesn't stop working when your support window lapses — the version you own keeps running. The first year of updates and email support is included; after that you can renew at $400/user/year to keep getting updates, or stay on your current version indefinitely.
/10 Who built this? +
Deliverable Labs — a small independent software studio. Two people, maybe three. More on the about page.

Try the calculator.

Free in-browser demo, no account. Or read the methodology and validation results first.

Open the demo →