Size. American Wire Gauge runs backwards: 14 AWG is small, 4/0 (0000) is large. Above 4/0, conductors are sized in thousands of circular mils (kcmil, formerly MCM): 250, 350, 500, 750, 1000. DLO cable uses its own odd sizes (262.2, 313.3, 444.4, 535.3, 646.4, 777.7 kcmil) that fall between the standard ones.

Metal. Copper carries about 60 percent more current than aluminum of the same size and terminates in any listed connector. Aluminum (AA-8000 series for building wire) is lighter and cheaper on large feeders but needs dual-rated AL9CU connectors, oxide inhibitor and correct torque, and it cannot land on a connector marked CU only.

Stranding. Class B (concentric, 7 to 61 strands) is standard building wire. Class C is finer. Class I and K are flexible fine-strand (DLO, welding, battery cable). The class decides the lug: a standard compression lug is listed for Class B/C; fine-strand needs a flex-rated lug (UL 486A) because the conductor diameter is larger and the strands flow differently under the die.

Insulation. THHN/THWN-2 (90 °C dry and wet, nylon jacket, raceway only), XHHW-2 (cross-linked polyethylene, better in cold and wet), USE-2/RHW-2 (direct burial and service entrance), DLO (2 kV EPR, locomotive and industrial), PV wire (UL 4703). The letters are the rating: T thermoplastic, H heat (75 °C), HH 90 °C, W wet, N nylon, -2 means 90 °C wet.

NEC Table 310.16 gives the allowable ampacity of up to three current-carrying conductors in a raceway or cable at 30 °C ambient, in three temperature columns. Two corrections stack on it: ambient temperature (Table 310.15(B)(1): 0.91 at 36 to 40 °C, 0.82 at 41 to 45 °C for the 90 °C column) and conductor count (Table 310.15(C)(1): 80 percent for 4 to 6 current-carrying conductors, 70 percent for 7 to 9).

The trap is 110.14(C): the ampacity you can actually use is limited by the lowest temperature rating of any termination in the circuit. Breakers and lugs are typically 75 °C, so a 90 °C conductor is derated from the 90 °C column but the result cannot exceed the 75 °C column value. Equipment rated 100 A or less, or conductors 14 to 1 AWG, default to the 60 °C column unless marked otherwise.

Continuous loads (3 hours or more) are sized at 125 percent (210.19, 215.2). Motor circuits, welders and capacitors have their own rules.

A compression lug is a seamless copper tube with a tongue, listed under UL 486A (copper) or 486B (dual-rated). Crimped with the matched die, it is a cold-welded, gas-tight termination that outlasts any mechanical connection. Getting it right is a system: lug, conductor class, die, tool and inspection.

• Barrel length. Standard barrel takes one crimp (two on larger sizes); long barrel takes two full crimps and is specified for vibration, utility, switchgear and anywhere a pull test matters. Long barrel is the default for feeders 1/0 and up.
• Tongue and stud. One-hole lugs by stud size (1/4, 5/16, 3/8, 1/2, 5/8 in). Two-hole lugs on NEMA spacing (1/2 in holes on 1-3/4 in centers for 250 kcmil and up, 3/8 in on 1 in for smaller) for switchgear and bus; telecom two-hole is 1/4 in holes on 5/8 in centers. 90° and 45° flag lugs where the conductor enters from the side.
• Die color codes. Industry dies follow the T&B Color-Keyed index: 8 AWG red (21), 6 blue (24), 4 gray (29), 2 brown (33), 1 green (37), 1/0 pink (42), 2/0 black (45), 3/0 orange (50), 4/0 purple (54), 250 yellow (62), 350 red (66), 500 brown (71), 750 black (99). Lug and die carry the same color; the crimp leaves the die number embossed on the barrel for inspection.
• Tool. 12-ton hydraulic (manual or battery) with U-dies through 750 kcmil; dieless crimpers on their listed range only; hand ratchet tools for 8 AWG and smaller. Hammer crimps and vise crimps are not listed terminations.
• Inspection. Correct die number embossed, barrel fully formed with no cracks, no insulation under the barrel, strand ends flush with the barrel end (not protruding more than one strand diameter), then heat shrink or tape to the insulation rating.

Torque the bolt per the manufacturer or 110.14(D) (which requires a calibrated tool or torque-indicating device): a perfect crimp on a loose stud is still a hot joint.

Where the conductor has to be reconnected, resized or tapped, mechanical and insulated connectors do the job without a crimp tool.

Mechanical lugs (set-screw, 1 to 4 conductor, dual-rated AL9CU, 14 AWG to 1000 kcmil) are the panelboard and switchgear standard. The rule is the wire range marking and the torque table on the connector: set screws on aluminum are the single most common loose connection in a service.

Insulated multi-tap and in-line splice connectors replace tape-wrapped split bolts in panels and junction boxes: UL 486A/B rated, 600 V insulated housing, single or dual entry, 14 AWG to 750 kcmil. Dual-entry taps let the run conductor pass through uncut. Zig-zag dual-entry taps stagger ports for wider ranges.

Split bolts are the field tap for grounding and bare or insulated conductors where tape is acceptable; they must be rated for the metals joined (CU, AL or CU/AL) and torqued. Panelboard connectors and splicer-reducers (1 to 4 barrel, 1 and 2-hole, AL9CU) adapt large feeders to lugs and bus.

Butt connectors, ring terminals and ferrules cover the 22 to 10 AWG control and OEM world: insulated nylon or heat-shrink barrels, crimped with the matching ratchet tool and never with pliers.

Conduit is the equipment grounding path, the physical protection and the fill limit all at once. EMT (Article 358) is thin-wall steel joined by set-screw (dry, indoor) or compression fittings (wet locations with raintight versions, concrete-tight). RMC and IMC (344, 342) are threaded heavy-wall; threadless compression fittings let you couple cut lengths without threading in the field. Conduit bodies (LB, LL, LR, C, T) turn corners and give pull access; the Form 7 and Form 8 patterns are standard across manufacturers.

• Fill (Chapter 9, Table 1): 40 percent for three or more conductors, 31 percent for two, 53 percent for one. Annex C tables give conductor counts by size.
• Support (358.30 / 344.30): EMT within 3 ft of every box or termination and every 10 ft; rigid within 3 ft and then 10 to 20 ft by trade size.
• Bends: no more than 360° between pull points. Conduit bodies reset the count.
• Bonding: a grounding locknut or bonding bushing with a jumper where a raceway lands on a concentric or eccentric knockout on a service or over 250 V (250.92, 250.97). Insulated throat bushings on 4 AWG and larger (300.4(G)).
• Expansion: steel conduit moves about 0.07 in per 100 ft per 10 °F; expansion fittings on long outdoor runs and across building joints.

Straps and hangers, locknuts, bushings, reducing washers and couplings are the fittings that show up on every job and where a supplier's depth of stock and cross-reference matter most.

Inside the enclosure the work is landing conductors on the right bar. Neutral bars are isolated from the can except at the service; ground bars bond to the can; the main bonding jumper joins them only at the service disconnect (250.24, 250.28). Sub-panels keep neutral and ground separate, always.

Power distribution blocks (UL 1953) split one feeder into several branch conductors inside a panel or enclosure with a 600 V insulated base. Busbars (tinned copper, 150 to 600 A and beyond) are the low-voltage DC and marine answer, with MRBF and blade fuse blocks for individual circuits. Enclosures are chosen by NEMA type: 1 indoor, 3R outdoor rain, 4 and 4X washdown and corrosion, 12 dust and drip for industrial.

Keep the bar sized to the largest lug position you will need, use the manufacturer's listed lugs on bars that require them, and label every conductor. A clean panel is an inspection that takes five minutes.

The tool list follows the course: cable cutters sized for the conductor (ratcheting above 2/0), strippers that do not nick strands, a 12-ton hydraulic crimper with a full die set, ratchet crimpers for terminals and MC4, a calibrated torque wrench and screwdriver for 110.14(D), a fish tape and a bender for EMT.

Verification is the last step on every circuit: continuity and insulation resistance (megger at 500 or 1000 V on feeders), phase rotation on three-phase, and a torque audit. Thermal imaging at first load catches the loose terminations the torque wrench missed.

NFPA 70E governs working on or near energized equipment: establish an electrically safe work condition, lockout tagout, arc-rated PPE by the incident energy on the label, and insulating gloves tested and in date. Hi-vis, hard hat and eye protection are the floor, not the ceiling.