This is the front door to everything we know about electrical wire: sizing, identification, code basics, and the physical differences between wire types. No jargon, no datasheets — the same plain talk you'd get standing in the shop holding a roll.
Copper ampacity from the NEC table (Table 310.16), 14 AWG through 4/0, at the three insulation temperature ratings. These assume no more than three current-carrying conductors, 30°C ambient — a normal room. Which column you're allowed to read is decided by your terminals, not by the wire.
| Wire size (AWG) | COPPER — allowable amps | ||
|---|---|---|---|
| 60°C | 75°C | 90°C | |
| 14 | 20 | 25 | 30 |
| 12 | 25 | 30 | 35 |
| 10 | 30 | 35 | 40 |
| 8 | 40 | 50 | 55 |
| 6 | 55 | 65 | 75 |
| 4 | 70 | 85 | 95 |
| 3 | 85 | 100 | 115 |
| 2 | 95 | 115 | 130 |
| 1 | 110 | 130 | 145 |
| 1/0 | 125 | 150 | 170 |
| 2/0 | 145 | 175 | 195 |
| 3/0 | 165 | 200 | 225 |
| 4/0 | 195 | 230 | 260 |
Based on NEC Table 310.16. General guidance only — local Chicago-area code amendments can be stricter, and a licensed electrician has the final word on any real installation.
Want the full version — aluminum columns, the 60°C-vs-75°C-vs-90°C rules, why a 12 AWG circuit lands on a 20-amp breaker, and the 125% rule for continuous loads? Read the complete wire gauge chart →
Before you dig into anything specific, here's the lay of the land. Nearly every wire you'll touch falls into one of these four families — and each one has its own guide below.
These are the questions that come up on nearly every job — how to size wire, how to read what's printed on it, and the three cables every branch circuit runs on. If you read nothing else, read these.
Every roll of wire carries its resume printed right on the jacket — gauge, conductor count, insulation type, voltage rating, UL listing. This article walks you through decoding all of it, letter by letter, with three real jacket printings worked through line by line. Once you can read a roll, you know exactly what you're holding.
The full NEC ampacity table in plain English, from 14 AWG to 4/0, copper and aluminum. Plus the parts the table doesn't teach you: which temperature column to read, why a 12 AWG circuit is on a 20-amp breaker, and the 125% rule for continuous loads.
Three ways to run a branch circuit, three very different answers about where each is allowed. NM-B, metal-clad cable, and the old armored BX you'll find in half the Chicago-area housing stock — compared side by side on cost, protection, grounding, and code uses.
Sometimes — with conditions. The code allows it when the insulation is sound, the gauge is right, and the product is listed. But cloth-covered wire, brittle jackets, and unknown history all fail. This one walks the decision tree and tells you what to do with the wire you can't reuse.
EV charging is a continuous load, so the circuit gets sized at 125% of the charger rating — and that one rule decides everything else. Charger amps to breaker to copper wire, why Romex needs bigger wire than THHN in conduit for the same job, and when distance forces an upsize.
The working knowledge behind the essentials: what the insulation letter-codes actually mean, the specific answers for common projects, and the small details — colors, stripping, burial depth — that decide whether a job passes inspection.
Three letter-codes that cover most of the single-conductor wire electricians pull through conduit. What the insulation layers are, where wet locations change the answer, and how to tell them apart by the jacket marking.
One question, one answer: 10 AWG copper — plus the 80% rule that explains it, the aluminum equivalent, and what goes wrong when someone skips a size. Dryers, water heaters, and small AC units, covered.
Long runs eat voltage, and the NEC's 3% guideline decides when you have to upsize. The simple math, a worked 100-foot example, and the rule of thumb electricians actually use for garages and sheds.
Black, white, red, green, bare — and the commercial 277/480-volt palette of brown, orange, yellow, and gray that most homeowner articles skip. What each color means, and the one rule that outranks all of them: test before you trust color.
Why 1960s–70s aluminum branch wiring fails — creep, oxidation, dissimilar metals — how to tell copper-clad aluminum from solid copper with a file-a-strand test, and the approved fixes. Insurance implications included.
Solid terminates better on devices; stranded pulls better through conduit — that's the whole debate in one sentence. Where each wins, the backstab warning, and how to land stranded wire under a screw terminal properly.
The burial depth table in plain English: 24 inches for direct-burial UF, 18 for PVC conduit, 12 with GFCI protection — plus warning tape, what inspectors check, and the shed-project walkthrough.
A nicked conductor is a future failure — hot spots, breakage, callbacks. The right stripper hole, the right technique, step by step, and the common mistakes that nick wire without you noticing.
Feeder sizing for 60-amp and 100-amp subpanels, the four-wire rule that separates ground from neutral — the #1 subpanel mistake — and why aluminum feeders are legitimately cheaper at these sizes.
Same idea, different jacket — and the jacket is the whole story. Why UF-B's solid gray sunlight-resistant construction is rated for wet locations and burial while Romex isn't, and the #1 mistake: thinking conduit makes Romex outdoor-legal.
The rest of the territory: what's already in old walls, when to worry about it, and the code-legal answers for the projects people actually tackle — splices, conduit, spas, and the armored cable half of Chicago is wired in.
Porcelain knobs, ceramic tubes, open conductors — what knob-and-tube looks like in a Chicago-area attic or basement, why insurers won't touch it, and when replacement stops being optional.
An honest checklist: flickering lights, cloth-insulated wire, two-prong outlets, aluminum branch wiring, warm switch plates, breakers that trip for no reason. What each sign means and which ones can't wait.
Junction boxes, wire nuts vs. lever connectors, and the splices that fail inspection — including why buried splices are never code-legal. The methods that pass, shown where they actually happen.
A 3/4-inch conduit holds fewer wires than you'd guess — fill limits are about heat, not just space. The 40% rule in plain English, with the worked example for 12 AWG THHN in 3/4-inch EMT.
Wire sizes for 50-amp and 60-amp spa circuits, plus the detail most articles skip: the GFCI disconnect's location rules — line-of-sight, the 5-foot minimum, and why it matters.
"BX" is the old trade name for Type AC cable — interlocked armor, bonding strip, and conductors inside. Where you'll still find it (half of Chicago's older buildings), and how it differs from modern MC cable.
Silver-looking strands that are still copper — the tin coating fights corrosion, especially in marine and damp environments. What tinned copper is, where it's used, and how to tell it from aluminum at a glance.
Ironcrest Industrial Surplus is a wire buyer based in the Chicago area — this guide is how we think about wire every day. Reading a jacket, knowing which wire type does what, sizing a circuit right — that's the same skill we use when we evaluate a roll. We wrote it down so you can use it too. We buy usable wire anywhere in the country — for the right load, we’ll travel. The code notes about conduit requirements, BX in the older housing stock, and local amendments were written with Chicago-area houses in mind.
Finished the project and still have half a spool — Romex, THHN, MC cable, extension cords? Usable wire on the roll is often worth more than scrap. See how we buy leftover wire — fast, free, no-obligation quote.
Get Your Fast, Free QuoteThe Wire Guide is Ironcrest Industrial Surplus's plain-English reference for electrical wire: sizing, identification, code basics, and the physical differences between wire types. Every article teaches first — no sales pitches inside the educational content.
Start with the quick-answer chart at the top of this page, then read the full wire gauge chart for the temperature-column rules and the small-conductor limits. For project-specific answers — EV chargers, subpanels, hot tubs, 30-amp circuits — each has its own guide above. Your local code and a licensed electrician have the final word on any real installation.
Because wire insulation comes in 60°C, 75°C, and 90°C ratings — but the column you read is decided by the terminals the wire lands on, not by the wire itself. Most residential work reads the 60°C column; the 90°C column is mostly for derating math. The full chart explains the whole thing.
In the jacket-printing guide: it decodes every letter and number — gauge, conductor count, insulation type, voltage rating, UL listing — with three real jacket printings worked through line by line. It's the single most useful wire skill in the whole guide.
Often, yes. Usable wire still on the roll — Romex, THHN, MC cable, extension cords — may be worth more than scrap because it goes back into someone's project instead of into a melter. We buy usable leftover wire anywhere in the country — based in the Chicago area, traveling for the right load. Fast, free, no-obligation quote.
Yes — written from Chicago, where the code notes cover the city’s conduit requirements, the BX and armored cable in the older local housing stock, and local amendments where they matter. The buying side is broader: Ironcrest buys usable wire anywhere in the country, and for the right load, we’ll travel.
Every article in this guide is a skill we use when a roll comes across the counter: read the jacket, identify the type, check the gauge, then value it. Teaching it here is how we show our work — and it's why we can pay more than the scrapyard for usable wire.