1. The Romex Wire Guide
The most common cable in American homes, explained from the jacket inward: construction, markings, and where it goes.
READ GUIDE →FRIENDLY GUIDES • ALL LEVELS • 12 MIN READ • UPDATED OCTOBER 2026
YOUR FRIENDLY GUIDE TO ALL THINGS ELECTRICAL WIRE
I am Sonny, and I buy copper wire for a living — which means I have handled more of this stuff than most people will ever see. This guide is everything I wish someone had handed me on day one: what each wire is, where it goes, and how the pros pick the right one.
Romex / NM-BMC CableTHHN & ConduitAWG Gauge Chart
THE SHORT VERSION
TL;DR
- Wire is a single conductor; cable is two or more wires in one jacket. Romex and MC are cables, THHN is a wire.
- Pick the wire by where it lives: dry interior means Romex (NM-B), commercial or exposed means MC cable or conduit, wet or underground means UF-B or THWN-2.
- Size the wire by gauge and ampacity: bigger number means thinner wire, and NEC Table 310.16 sets the safe current for each gauge.
- Service and panel work belongs to licensed electricians. This guide teaches you to understand the work, not to replace the pro.
- Every leftover foot of copper wire has real value by the pound. The cutoff section at the bottom explains what to do with it.
READ THIS FIRST
Licensed-pro framing.
THE RULE OF THIS GUIDE
Understanding is free. Panel work is licensed.
This guide explains how electrical wire is chosen, sized, and installed — and it does it well enough that a licensed electrician can nod along. But service panels, new circuits, and anything behind the meter belong to licensed electricians, and for good reason: the work is inspected, permitted, and held to the National Electrical Code. Read these pages to understand the trade, to ask smarter questions, and to know what you are looking at. When the work starts, bring in the pro. Every code reference here names the NEC article and edition so you can verify it yourself, and you should always check your local amendments.
WHY THIS GUIDE EXISTS
Why I wrote this.
I am Sonny Johns, and I run Ironcrest Industrial Surplus out of Chicago. I buy copper wire and cable from electricians, demolition crews, and contractors — new surplus, clean pulls, and the cutoff buckets from jobs just like yours. Because of that, I touch more kinds of wire in a month than most people touch in a career: Romex from house rewires, MC from commercial buildouts, THHN pulled out of conduit by the thousand-foot reel, tray cable, service drops, all of it.
What I kept noticing is that nobody had written the friendly version of all this. The manufacturer spec sheets are accurate and unreadable. The forum threads are full of answers but you have to wade through arguments to find them. And the beginner standing in the wire aisle at the supply house is guessing. So I wrote the guide I wish existed: every common wire type explained in plain language, in one place, organized so a beginner can start at the top and a master can still find something worth reading. If you learn something here, that is the whole point. And if one day you have wire to sell, you already know who to call.
PICK YOUR PATH
Start here.
Four reading paths, one for each level. Start at yours and work down the list in order.
BEGINNER
New to wire — start at the top
WORKING PRO
On the tools — the working knowledge
VETERAN
Foreman-level — the heavy stuff
MASTER
Read for fun — the deep cuts
THE WIRE FAMILY
The four wires that run America.
Almost every building in America is wired with some combination of four products. Learn these four and you can read any job. This hub covers each of them in depth, but here is the one-paragraph version so the rest of the guide makes sense.
NM-B (Romex): the house wire.
NM-B — nonmetallic-sheathed cable, sold under Southwire's Romex brand name — is the flat-ish cable with the colored plastic jacket you see in every residential wall. Inside are two or three insulated copper conductors plus a bare ground wire. It is cheap, fast to install, and limited by the NEC (Article 334) to dry interior locations with its ampacity capped at the 60°C column. Our Romex wire guide takes it apart layer by layer.
MC cable: the commercial wire.
MC — metal-clad cable — is what goes in the walls of offices, stores, and strip malls. THHN copper conductors live inside a corrugated aluminum armor that protects them from physical damage, plus a dedicated green ground wire. It is governed by NEC Article 330, it is tougher and more expensive than Romex, and it is constantly confused with old "BX" cable (which is actually AC cable — a different product). Our MC cable guide settles the confusion for good.
THHN in conduit: the pro's wire.
THHN — thermoplastic, high heat-resistant, nylon-coated — is the single insulated conductor electricians pull through conduit (the metal or plastic pipe that protects wire in commercial and industrial work). It is rated 90°C in dry locations, it comes in every gauge, and it is the most flexible option because each conductor is pulled individually. The how to choose electrical wire guide explains when conduit and THHN beat cable.
UF-B: the outdoor wire.
UF-B — underground feeder cable — is the gray, sunlight- and moisture-resistant cable rated for direct burial and wet locations. Where Romex stops, UF-B starts: outdoor receptacles, detached garages, landscape lighting, and well pumps. The comparison table in how to choose electrical wire puts it side by side with the rest of the family.
TOOLS
Tools worth bookmarking.
The references electricians actually reopen on the job.
WHY THIS GUIDE IS FREE
No paywall. No email required. Here is why.
I buy scrap metal for a living — copper wire, cable, and electrical surplus from the same crews this guide is written for. These guides are how I give back to the trades, and yeah, if you ever have copper or carbide to sell, I hope you call me first. That is the whole business model, stated openly.
— Sonny
SELLING SCRAP FROM WHAT YOU LEARNED?
Talk to Sonny, not a department
Every quote, pickup, and payment runs through one person who answers the phone: Sonny Johns at 312-880-8174. Wire, cable, and electrical surplus — by the bucket or by the truckload.
QUESTIONS, ANSWERED
Wire questions, answered.
Who is this wire guide written for?
Everyone with an interest in electrical wire: curious homeowners, career-changers, first-year apprentices, working journeymen, foremen, and master electricians who enjoy reading about the craft. The guide is organized into four learning paths — Beginner, Working Pro, Veteran, and Master — so you can start at your level and go deeper from there.
Does this guide replace an electrician or the NEC?
No. This guide explains how electrical wire works and how the pros choose it, but it does not replace training, apprenticeship, a license, or the National Electrical Code. Service and panel work belongs to licensed electricians, and every code reference here names the NEC article and edition so you can verify it — and always check your local amendments.
What is the difference between wire and cable?
A wire is a single conductor — one piece of copper (or aluminum) with insulation around it, like THHN. A cable is two or more wires bundled inside an outer jacket, like Romex (NM-B) or MC cable. People mix the terms up constantly, including pros, so do not feel bad about it.
Why does wire gauge run backwards?
Because the AWG (American Wire Gauge) numbers come from how many times the wire was drawn through the sizing dies: more draws means thinner wire and a bigger number. So 14 AWG is thinner than 12 AWG, and 12 AWG is thinner than 10 AWG. Once wire gets thicker than 1 AWG, the system switches to zeros: 1/0, 2/0, 3/0, and 4/0, with 4/0 the thickest of that set.
Can I run Romex outside or underground?
No. Romex is the trade name for NM-B nonmetallic-sheathed cable, and the NEC (Article 334) limits it to dry interior locations. For outdoor, wet, or underground runs, the right products are UF-B cable (rated for direct burial) or THHN/THWN-2 conductors in conduit — each covered in its own guide in this hub.
What is the wire gauge chart for?
It is the quick-reference tool of the hub: every common copper gauge from 14 AWG through 4/0 AWG with its diameter, typical uses, and ampacity from NEC 2023 Table 310.16 at 75°C. Electricians keep it open on the job when they need a number fast, and beginners use it to learn what each gauge is for.
Is this guide really free?
Yes. The entire guide and every page in it is free to read, with no sign-up and no paywall. Ironcrest Industrial Surplus publishes it as a resource for the trades, supported by our scrap-buying business: we buy copper wire, cable, and electrical surplus from the same crews this guide is written for.
CUTOFF BUCKET
What to Do With the Cutoffs
Every electrical job leaves copper behind. Romex cutoffs from the rough-in. THHN ends too short to pull. MC scraps from the commercial floor. Old wire pulled out during a rewire. On most crews, it all goes into a bucket — and on most crews, that bucket gets split.
Here is the honest part: insulated copper wire is bought by weight, and the copper inside it is the same copper running through every wall in America. Strip it or sell it as-is, keep the grades separated (Romex in one pile, THHN in another, bare bright by itself), and never mix aluminum in. When the bucket is full, call Sonny at 312-880-8174 for a number on it. One call, one person, and the bucket turns into money instead of a dump run.
Suggested next read: The Romex Wire Guide — start with the cable you will touch most.
ONE DIRECT CONTACT
Talk to Sonny, not a department
Every quote, pickup, and payment runs through one person who answers the phone: Sonny Johns at 312-880-8174. No portals, no ticket queues, no waiting on a committee.
- PHOTO QUOTES — Send photos of the material and get a real number back.
- TRADE FRIENDLY — Built for crews: wire, pipe, brass, and carbide, by the bucket or by the truckload.
- US ONLY — Your material never leaves the United States.