Selecting the right wire and cable starts with understanding your project's requirements — environment, voltage, current, code compliance, and installation method. This guide walks you through each decision point so you can specify cable confidently, avoid costly mistakes, and pass inspection the first time.
What Environment Will the Cable Be Installed In?
Environment is the single biggest factor in cable selection. The wrong jacket or insulation in a given space can create a fire hazard, fail electrical inspection or violate fire code, or degrade prematurely.
Indoor — Plenum, Riser, and General Purpose
Building interiors are classified by airflow. Plenum spaces (above drop ceilings, under raised floors where HVAC air circulates) require CMP or CL2P/CL3P-rated cable with low-smoke, low-flame-spread jackets. Riser spaces (vertical shafts between floors) require CMR or CL2R/CL3R-rated cable that resists flame propagation floor to floor. General purpose areas with no plenum or riser classification can use CM or CMG-rated cable. You can always use a higher-rated cable in a lower-rated space (CMP cable works in riser and general purpose areas), but never the reverse. See our Plenum vs. Riser Cable guide for a detailed comparison.
Outdoor — Direct Burial, Aerial, and Conduit
Outdoor cable installations must withstand UV exposure, moisture ingress, temperature swings, and physical damage. Direct burial cable is designed to be installed underground without conduit — it has moisture-resistant jackets and often includes a gel-filled core or armor. Aerial cable (messenger wire or self-supporting) must resist UV and wind loading. Cable run through outdoor conduit still needs a wet-rated insulation (look for the "W" in ratings like THWN or XHHW-2). For underground runs, see our Direct Burial Cable guide.
Hazardous and Specialty Environments
Some environments demand specialty cable. High-temperature areas (boiler rooms, furnaces, engine bays) need insulation rated above standard 90°C — silicone, fiberglass, or PTFE-insulated cable can handle 150°C–250°C. Chemical exposure environments require chemically resistant jackets like XLPE or fluoropolymers. Oil and gas installations often require cable with chemical resistance and high-temperature jackets. See our High Temperature Cable guide for specialty insulation options. Also see gas pump cables for fueling infrastructure applications.
What Voltage Does Your System Require?
Cable is manufactured and tested for specific voltage classes. Using cable below the system voltage is a safety hazard; using cable rated far above it wastes money.
| Voltage Class | Typical Rating | Common Cable Types | Applications |
|---|---|---|---|
| Low Voltage | 0–600V | THHN/THWN, NM-B, MC, AC, UF-B | Building wire, branch circuits, feeders |
| Low Voltage (signal) | Up to 300V rated (actual signals typically <60V) | Cat5e/Cat6/Cat6A, coax, fire alarm | Data, voice, video, security, fire alarm |
| Medium Voltage | 5 kV – 35 kV | MV-105, MV-90, EPR/XLPE insulated | Utility feeds, industrial power distribution |
| High Voltage | 69 kV+ | Utility transmission cable | Transmission lines, substations |
For most commercial and residential projects, 150V–600V-rated cable covers your needs. Industrial power distribution and utility feeds typically require medium voltage cable (MV-105) rated 5 kV to 35 kV. Always confirm the system voltage with your electrical engineer before specifying cable.
How Much Current Will the Cable Carry?
Current-carrying capacity (ampacity) determines the conductor size you need. Undersized conductors overheat; oversized conductors cost more and are harder to install. The NEC® ampacity tables (310.16 for copper, 310.17 for aluminum) provide the starting point, but several factors require derating.
Key Factors That Affect Ampacity
- Conductor material — Copper carries more current per gauge than aluminum. For the same ampacity, aluminum typically requires a wire two AWG sizes larger.
- Insulation temperature rating — 90°C insulation (THWN-2, XHHW-2) allows higher ampacity than 75°C or 60°C insulation, assuming terminals are also rated for 90°C.
- Ambient temperature — Installations above 30°C (86°F) require derating per NEC® Table 310.15(B)(1).
- Conduit fill — More than three current-carrying conductors in a conduit requires derating per NEC® Table 310.15(C)(1).
- Continuous vs. non-continuous loads — Continuous loads (3+ hours) must be calculated at 125% of the actual load.
Use our AWG Wire Gauge Guide for a complete ampacity reference table with derating factors.
Need help selecting the correct cable size? Our team can help you choose the right cable based on voltage, environment, and installation type. Request a quote →
What Codes and Standards Apply?
Code compliance is non-negotiable. The applicable codes depend on your project type and location.
| Code / Standard | Governs | Key Cable Requirements |
|---|---|---|
| NEC® (NFPA 70) | Electrical installations in the U.S. | Conductor sizing, conduit fill, derating, cable types by location |
| NEC® Article 725/760 | Low-voltage and fire alarm circuits | Power-limited vs. non-power-limited, plenum/riser ratings |
| NEC® Article 800 | Communications circuits | Category cable ratings (CMP, CMR, CM) |
| UL® Listed | Product safety certification | Cable tested and listed for intended use |
| Local AHJ amendments | Your specific jurisdiction | May be more restrictive than NEC® — always check |
Always confirm which NEC® edition your jurisdiction has adopted (some lag behind the current edition by several cycles) and check for local amendments. Your authority having jurisdiction (AHJ) has final say on cable specifications for any permitted project.
What Type of Circuit Are You Wiring?
Different circuit types call for different cable constructions.
Power Distribution
Branch circuits, feeders, and service entrances typically use THHN/THWN building wire in conduit, or Type MC (metal-clad) and AC (armored) cable where allowed. For outdoor or underground service entrances, USE-2 or direct burial cable is common.
Data and Communications
Structured cabling for Ethernet networks uses Cat5e, Cat6, or Cat6A cable depending on speed requirements and run length. Video distribution may use coaxial cable (RG6 for most runs, RG11 for long distances). Always match the cable category to your network equipment's capabilities.
Fire Alarm and Security
Fire alarm cable must be specifically listed for fire alarm use (FPLP for plenum, FPLR for riser, FPL for general purpose). NEC® Article 760 governs these circuits. Security and access control systems typically use similar cable but follow Article 725 for power-limited circuits.
Industrial and Specialty
Industrial environments often require tray cable for cable tray installations, instrumentation cable for process control signals, or VFD cable for variable frequency drives. Portable cords (SOOW, SJOOW) are used for temporary power and equipment connections.
Not sure which cable type fits your application? Tell us about your project and we'll recommend the right product. Talk to our sales team →
How Will the Cable Be Installed?
The installation method determines which cable constructions are permitted and affects pulling tension, fill calculations, and protection requirements.
| Installation Method | Common Cable Types | Key Considerations |
|---|---|---|
| Conduit (EMT, RMC, PVC) | THHN/THWN, XHHW-2, individual conductors | NEC® conduit fill limits (40% for 3+ conductors); derating for bundled conductors; wet rating required in underground or outdoor conduit |
| Cable tray | Tray cable (TC), MC cable, MI cable | Must be listed for cable tray use; open-air ampacity tables apply; single-layer vs. stacked affects derating |
| Direct burial | UF-B, USE-2, direct burial-rated cables | Moisture-resistant jacket required; minimum burial depth per NEC® Table 300.5; consider conduit for added protection at transitions |
| Aerial / messenger | Self-supporting cable, lashed to messenger wire | UV-resistant jacket; wind and ice loading calculations; sag and tension specs |
| Portable / flexible | SOOW, SJOOW, SJT, Type W | Rated for repeated flexing; choose jacket and temperature rating based on duty cycle and environment |
Match the cable type to the wiring method before finalizing your specification. A cable that's perfectly rated for the voltage and environment may still fail inspection if it's not listed for the installation method — for example, NM-B (Romex) is not permitted in conduit in many jurisdictions, and standard THHN cannot be direct-buried without conduit.
Copper or Aluminum?
Both conductor materials are used in modern installations, each with trade-offs.
| Factor | Copper | Aluminum |
|---|---|---|
| Conductivity | Higher — smaller conductor for same ampacity | Lower — typically 2 AWG sizes larger for equivalent ampacity |
| Weight | Heavier (~3x aluminum by volume) | Lighter — easier to pull in long runs |
| Cost | Higher per foot | Lower per foot — significant savings on large feeders |
| Terminations | Compatible with most lugs and connectors | Requires AL-rated connectors and anti-oxidant compound |
| Best for | Branch circuits, signal cable, smaller feeders | Large feeders (4/0 and up), service entrances, utility work |
For feeders 4/0 AWG and larger, aluminum is often the more economical choice — you get equivalent ampacity at significantly lower cost and weight. For branch circuits and signal-level wiring, copper is the standard.
Wood vs. Plastic Reels: Which Do You Need?
Cable packaging affects handling, storage, and shipping cost — especially on large orders. Understanding the trade-offs helps you specify the right reel type when placing orders.
Wood Reels
Wood reels are the industry standard for large cable runs. They're durable enough for heavy cable (500 kcmil+), stackable in warehouses, and reusable. Most distributors and manufacturers ship large orders on wood reels by default. Wood reels can handle rough jobsite conditions and are easy to set up on reel jacks for pulling. The downsides: they're heavy, take up space, and some jurisdictions charge for disposal.
Plastic Reels
Plastic reels are lighter and increasingly common for smaller gauge cable and shorter put-ups. They're easier to handle on smaller jobs, won't splinter, and are recyclable. However, they can't support the weight of heavier cables and are more prone to cracking in extreme cold.
Coils and Cartons
Smaller quantities (typically under 1,000 ft of lighter gauge cable) may ship in coils, shrink-wrapped bundles, or cartons. These are fine for short runs and smaller projects but are harder to pay out cleanly on longer pulls.
When ordering, specify your preferred packaging if it matters for your jobsite logistics. Not all cable types are available in every packaging option — for example, heavy-gauge power cable typically ships on wood reels only. Ramcorp can accommodate reel size and put-up length preferences on most orders — consult our sales team to confirm availability and discuss options.
Cable Selection Quick-Reference Checklist
- Define the environment — Indoor (plenum/riser/general), outdoor (direct burial/aerial/conduit), or specialty (high-temp, chemical, hazardous)?
- Confirm the voltage class — 600V (most building wire), 300V (signal/data), 5 kV – 35 kV (medium voltage)?
- Calculate ampacity — Load current × 125% for continuous loads → select AWG from NEC® tables → apply derating.
- Identify the circuit type — Power, data, fire alarm, control, or portable?
- Determine installation method — Conduit, cable tray, direct burial, aerial, or portable/flex?
- Choose conductor material — Copper for branch circuits and signal; aluminum for large feeders and services.
- Verify code compliance — NEC® edition, local AHJ amendments, UL® listing requirements.
- Specify packaging — Reel type, put-up length, and delivery requirements.
Frequently Asked Questions
Can I use outdoor-rated cable indoors?
In most cases, yes — but the cable must also carry the appropriate indoor rating. For example, THWN-2 is rated for wet and dry locations, so it works indoors and outdoors in conduit. However, a cable rated only for direct burial may not have the flame-spread rating required for indoor use. Always check that the cable carries the specific listing (CMP, CMR, CM, etc.) required for the indoor space.
What's the difference between stranded and solid conductors?
Solid conductors are a single piece of metal — they're easier to terminate on screw terminals and more rigid, which is preferred for in-wall residential wiring (like NM-B). Stranded conductors are made of multiple smaller wires twisted together — they're more flexible, easier to pull through conduit, and better for applications with vibration or repeated bending. Most commercial and industrial cable uses stranded conductors.
How do I know if my cable needs to be plenum-rated?
If the cable runs through a plenum space — any area used for air circulation in an HVAC system, including the space above drop ceilings and below raised floors — it must be plenum-rated (CMP, CL2P, CL3P, or FPLP depending on cable type). Your building plans should identify plenum spaces, or your AHJ can confirm. When in doubt, use plenum-rated cable — it's code-compliant everywhere a lesser rating would be.
What does the "2" mean in THWN-2 or XHHW-2?
The "2" indicates a 90°C wet-location insulation rating. Without the "2," THWN is rated 75°C in wet locations and 90°C in dry locations. THWN-2 is rated 90°C in both wet and dry. The higher wet rating means better ampacity in certain conditions and makes the cable more versatile. Most modern THHN/THWN cable is dual-rated THHN/THWN-2.
Can I mix copper and aluminum conductors in the same circuit?
Only with connectors specifically listed for copper-to-aluminum connections (marked "AL/CU" or "CU/AL"). Direct contact between copper and aluminum without a rated connector causes galvanic corrosion, which leads to high-resistance connections, overheating, and potential fire. Never splice copper to aluminum with standard wire nuts or terminals.
How much cable should I order beyond my measured run lengths?
A common rule of thumb is 10–15% overage to account for routing changes, termination waste, pull-through, and mistakes. For large projects, review your cable schedule with your electrician and add a contingency factor based on the complexity of the installation. It's far cheaper to order a bit extra upfront than to place a rush order for a short piece later.
Related Resources
- Plenum vs. Riser Cable: CMP, CMR, CL2P & CL3P Ratings Explained
- AWG Wire Gauge Guide: Sizes, Ampacity & Selection
- THHN vs THWN Building Wire: Ratings & Applications
- Coaxial Cable Guide: RG59, RG6, RG8, RG11 & LMR
- Fire Alarm Cable: Types, Ratings & NEC Requirements
- Direct Burial Cable: Types, Depth Requirements & Selection
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Disclaimer: This guide is provided for informational purposes only and is not installation advice. It does not constitute professional electrical, engineering, or code-compliance advice. Installing wire & cable can be dangerous and pose a risk of possible electric shock or other hazards. Building codes, NEC editions, and local amendments change periodically. Always consult a licensed electrician and your local authority having jurisdiction (AHJ) before specifying or installing cable. Images are for illustration purposes and may not reflect actual installed products.
The information on this page is provided for general reference only and may contain errors or omissions. NEC® is a registered trademark of the National Fire Protection Association (NFPA®). UL® is a registered trademark of Underwriters Laboratories. All other trademarks, product names, and brand names referenced on this page are the property of their respective owners. Ramcorp Wire & Cable is not affiliated with or endorsed by these organizations unless explicitly stated.