Answers to common questions about electrical cable types, wire sizing, ampacity ratings, NEC code requirements, and how to order building wire, tray cable, and portable cord from Ramcorp Wire & Cable.
Building Wire Types
What is THHN wire?
THHN is a single-conductor thermoplastic wire with a nylon jacket, rated for 90°C in dry locations. It is the most widely used building wire in the United States for commercial and residential branch circuits. THHN is typically pulled through conduit or raceway systems and is available in copper from 14 AWG through 1000 kcmil. Most modern THHN is dual-rated as THHN/THWN-2, meaning it is also suitable for wet locations at 90°C. See our THHN vs. THWN Building Wire Guide for a detailed comparison.
What is the difference between THHN and THWN-2?
THHN is rated for dry locations at 90°C. THWN-2 adds a wet-location rating, also at 90°C. In practice, most building wire manufactured today carries a dual THHN/THWN-2 rating, so a single conductor meets both dry and wet requirements. The “2” in THWN-2 indicates the 90°C wet rating — older THWN (without the 2) was rated for only 75°C in wet conditions. When ordering, confirm the wire carries the dual THHN/THWN-2 marking if you need wet-location performance.
What is XHHW-2 wire?
XHHW-2 is a cross-linked polyethylene (XLPE) insulated wire rated for 90°C in both dry and wet locations. It offers superior moisture resistance and mechanical durability compared to THHN’s thermoplastic insulation. XHHW-2 is commonly used for larger feeder and service entrance conductors, and it is a preferred choice in industrial and commercial installations where environmental exposure is a concern.
What is welding cable?
Welding cable is a highly flexible, single-conductor cable with a fine-stranded copper center and a durable rubber jacket (typically EPDM or neoprene). It is designed to carry high currents at low voltages from welding machines to electrode holders and work clamps. Welding cable is also often used as a flexible alternative to battery cable, for power distribution in mobile equipment, and temporary high-amperage hookups. Common sizes range from 6 AWG to 4/0 AWG. See our Welding Cable Guide for sizing tables and application details.
What is tray cable?
Tray cable is a multi-conductor cable approved for installation in cable trays per NEC Article 392. It has a heavier jacket than standard building wire and is rated for exposed installation without conduit. Types include TC (thermoplastic), TC-ER (exposed run, which allows limited installation outside of cable tray per NEC), and VNTC (vinyl-jacketed nylon tray cable). Tray cable is widely used in industrial plants, power generation facilities, and commercial buildings where cable tray is the primary wiring method. See our Tray Cable Guide for details.
What is portable cord?
Portable cord (also called flexible cord) is a multi-conductor cable with a flexible rubber or thermoplastic jacket designed for temporary connections to portable equipment. Common types include SOOW (600V, oil-resistant), SJOOW (300V, oil-resistant), and SJT (300V, thermoplastic). Portable cord is used for temporary power distribution, tools, generators, and stage lighting. See our Portable Cord Guide for a full comparison of cord types and ratings.
What is the difference between single-conductor wire and multi-conductor cable?
Single-conductor wire (like THHN) is one insulated conductor that must be pulled through conduit or raceway. Multi-conductor cable (like tray cable or NM-B) bundles multiple insulated conductors together under a common jacket or armor. The choice depends on installation method, code requirements, and the environment. Conduit with individual conductors offers flexibility for future upgrades; multi-conductor cable simplifies installation and reduces labor.
Wire Sizing & Ampacity
How do I choose the right wire size for my project?
Wire size selection depends on three factors: the amperage of the circuit, the distance of the wire run (voltage drop), and the installation conditions (ambient temperature, conduit fill). Start with the calculated load for the circuit, then consult NEC Table 310.16 for the minimum conductor size at the appropriate temperature column. For long runs, use our Voltage Drop Calculator to determine if you need to upsize. Our AWG Wire Gauge Guide and Wire Ampacity Calculator provide additional sizing guidance for common applications.
What is ampacity?
Ampacity is the maximum continuous current a conductor can safely carry without exceeding its insulation temperature rating. Ampacity values are published in NEC Table 310.16 and vary by wire size (AWG or kcmil), insulation temperature rating, and conductor material. Actual ampacity in an installation may be lower due to derating for ambient temperature, conduit fill, or termination limitations. Additionally, NEC 210.20(A) requires that conductors supplying continuous loads be sized at 125% of the load, which further affects conductor selection.
What size wire do I need for a 200 amp service?
A 200-amp dwelling service entrance typically requires 2/0 AWG copper conductors, based on NEC Table 310.16 at the 75°C column and the dwelling unit service conductor allowance in NEC 310.12. Non-dwelling applications may require different sizing. Always verify with your local electrical inspector, as jurisdictions may have additional requirements. The neutral conductor may be sized smaller per NEC 220.61.
What size wire do I need for a 100 amp sub-panel?
A 100-amp sub-panel feeder typically requires 3 AWG copper conductors at the 75°C column. For longer runs, you may need to upsize to compensate for voltage drop — a general guideline is to limit voltage drop to 3% on the feeder (use our Voltage Drop Calculator to check). A 4-wire feeder (two hots, neutral, and separate equipment ground) is required for sub-panels in separate structures.
What is the ampacity of common building wire sizes?
Common copper THHN/THWN-2 ampacities per NEC Table 310.16 (90°C column, not more than 3 current-carrying conductors): 14 AWG = 25A, 12 AWG = 30A, 10 AWG = 40A, 8 AWG = 55A, 6 AWG = 75A, 4 AWG = 95A, 2 AWG = 130A, 1/0 AWG = 170A, 2/0 AWG = 195A. Note: actual circuit ampacity is typically limited to the 75°C column per NEC 110.14(C) for standard terminations. See our AWG Wire Gauge Guide for the complete table, or try our Wire Ampacity Calculator for quick lookups.
Does ambient temperature affect wire ampacity?
Yes. NEC ampacity tables are based on an ambient temperature of 30°C (86°F). If the installation environment is hotter — such as an attic, rooftop, or hot industrial facility — you must apply correction factors from NEC Table 310.16 that reduce the allowable ampacity. For example, at 40°C ambient, a THHN conductor’s ampacity is reduced to approximately 91% of its table value.
What is conduit fill and how does it affect wire sizing?
Conduit fill refers to the number and size of conductors installed inside a conduit relative to the conduit’s internal area. NEC Chapter 9 limits fill to 40% for three or more conductors. More conductors in a conduit generate more heat, which requires derating the ampacity per NEC Table 310.15(C)(1). For example, 4–6 current-carrying conductors in a conduit requires reducing ampacity to 80% of the table value.
Applications & Installation
Can THHN wire be used outdoors?
THHN by itself is rated for dry locations only. However, most modern THHN is dual-rated THHN/THWN-2, which is approved for wet locations including outdoor conduit runs. If the wire carries only a THHN marking (no THWN-2), it should not be used where exposed to moisture. Always check the print legend on the wire to confirm the ratings before installation.
Can THHN wire be used for direct burial?
No. THHN and THWN-2 are not rated for direct burial. For underground installations, use cable specifically listed for direct burial such as USE-2 or UF-B, or install conductors inside approved underground conduit. See our Direct Burial Cable Guide for approved cable types and NEC burial depth requirements.
What is medium voltage cable?
Medium voltage (MV) cable is rated for systems operating between 2,001V and 35 kV. These cables feature insulation systems (typically EPR or XLPE), semiconducting shields, and a metallic shield or concentric neutral for voltage stress control. MV cable is used for utility distribution, industrial plant feeders, substation connections, and renewable energy collection systems. Common ratings include 5 kV, 8 kV, 15 kV, 25 kV, and 35 kV. See our MV-105 Medium Voltage Cable Guide for construction details and selection guidance.
What is the difference between tray cable and individual conductors in conduit?
Tray cable runs exposed in cable trays without conduit, reducing material and labor costs in large commercial and industrial installations. Individual conductors in conduit offer better physical protection and easier future maintenance (you can pull new conductors without replacing the entire cable). Tray cable is standard in industrial plants, refineries, and data centers; conduit systems are more common in commercial buildings and areas requiring frequent wiring changes.
What types of electrical cable are used in data centers?
Data centers typically use a mix of electrical cable types: building wire (THHN/THWN-2) in conduit for branch circuits, tray cable for cable tray runs, and medium-voltage cable for primary power feeds. Power whips and portable cord (SOOW) are used for connections to PDUs and server cabinets. See our Data Centers industry page for more on cable selection for data center environments.
What cable do I need for a construction job site?
Temporary power at construction sites typically uses portable cord (SOOW or SJOOW) rated for hard or extra-hard usage, connected to temporary power panels. Permanent wiring installed during construction uses building wire (THHN/THWN-2 in conduit) or tray cable, depending on the project specification. All temporary wiring must comply with NEC Article 590 and OSHA requirements. See our Construction & Infrastructure page for recommended cable types.
NEC Code & Ratings
What does the NEC require for conductor sizing?
The NEC (National Electrical Code) requires that conductors be sized to carry the expected load without exceeding their rated ampacity after all derating factors are applied. Key references include Table 310.16 (ampacity tables), Section 310.15 (derating for temperature and conduit fill), and Section 110.14(C) (termination temperature limitations). The conductor must also be protected by an overcurrent device that does not exceed its ampacity.
What is the 75°C termination rule?
NEC 110.14(C) requires that conductor ampacity be evaluated at the lowest temperature rating in the circuit, which is usually the termination point (breaker, lug, or device). Most breakers and equipment terminals rated over 100A are listed for 75°C. This means even though THHN is rated for 90°C, its ampacity for circuit sizing is typically taken from the 75°C column. The 90°C rating can be used as a starting point for derating calculations only.
What is a UL listing and why does it matter for wire and cable?
UL (Underwriters Laboratories) listing means the wire has been tested and certified to meet specific safety and performance standards. NEC Section 110.2 requires that electrical equipment be “approved” — in practice, this means UL-listed or listed by a recognized NRTL (Nationally Recognized Testing Laboratory). Using non-listed wire can result in code violations, failed inspections, and insurance issues. See our UL Listings Cable Guide for an overview of common UL wire ratings.
What do the markings on wire insulation mean?
The print legend on wire insulation includes the wire type (e.g., THHN/THWN-2), AWG size, voltage rating, temperature rating, UL listing marks, manufacturer name, and date or footage markers. These markings are required by UL and NEC for identification and inspection. See our Cable Print Legend Guide for a breakdown of every element in a typical cable print legend.
What does “plenum” vs. “riser” mean for electrical cable?
Plenum-rated cable has a fire-retardant jacket that produces low smoke and limited flame spread when burned, and is required in air-handling spaces (above drop ceilings and below raised floors used for air return). Riser-rated cable is designed to prevent fire from spreading between floors and is required in vertical shafts. These ratings primarily apply to low-voltage and communication cables, but the concepts also affect routing of electrical cable through plenum and riser spaces. See our Plenum vs. Riser Cable Guide for a full comparison.
Specialty & High Temperature
What is high temperature cable?
High temperature cable is engineered with insulation materials — such as PTFE, FEP, silicone rubber, fiberglass, or mica tape — that maintain performance at sustained temperatures from 150°C up to 450°C or higher. These cables are used in steel mills, glass plants, kilns, furnaces, engine compartments, and other environments where standard PVC or XLPE insulation would degrade. See our High Temperature Cable Guide for insulation comparisons and temperature ratings.
What is VFD cable and when is it required?
VFD (variable frequency drive) cable is designed to withstand the high-frequency voltage spikes and reflected wave phenomena generated by VFD motor controllers. It features symmetrical construction, a continuous ground conductor, and shielding that reduces electromagnetic interference and bearing currents. VFD cable is recommended for any motor circuit powered by an inverter or variable speed drive, especially on runs longer than 50 feet. See our VFD Cable Guide for selection criteria and installation best practices.
Buying Electrical Cable from Ramcorp
Can I buy electrical cable by the foot?
Yes. We sell most building wire and electrical cable by the foot with no minimum length requirement. Order the exact footage you need for your project, whether it’s 50 feet or 50,000 feet. Custom cuts ship within 24 business hours. For very large orders, we can ship full manufacturer reels for faster handling and lower per-foot pricing.
Do you carry both solid and stranded building wire?
Yes. We stock both solid and stranded copper building wire. Solid wire (typically 14–10 AWG) is used for permanent branch circuits in conduit and boxes where the wire is not flexed after installation. Stranded wire (typically 8 AWG and larger, though also available in 14–10 AWG) is easier to pull through conduit and terminate in larger sizes. Stranded wire is required in most sizes above 10 AWG.
What brands of electrical cable does Ramcorp carry?
We carry building wire and electrical cable from major U.S. manufacturers. Brand availability varies by product type and gauge. If you need a specific brand or manufacturer for your project specification, contact our sales team and we’ll confirm availability and pricing.
Can Ramcorp supply wire for large commercial or industrial projects?
Yes. We supply electrical cable for projects of all sizes, from single-circuit residential jobs to multi-million-foot commercial and industrial builds. For large projects, we offer volume pricing, phased delivery schedules, and dedicated account support. We can also coordinate job-site delivery directly to your project location anywhere in the United States.
Do you ship electrical cable internationally?
Yes. We ship wire and cable to businesses in countries permitted by U.S. export laws. Most electrical cable is classified EAR99, which allows export to most destinations without a license. See our International Orders & Shipping page and EAR99 Export Guide for details on export documentation and compliance.
Have a question not answered here? Contact our sales team — we respond within one business day.
Disclaimer: This information is for general reference only. Always consult a licensed electrician and your local authority having jurisdiction (AHJ) for installation and code compliance decisions.