Solar Wire Guide
UL 4703 Cable vs. Other Types of Cable
What makes cable that is rated UL 4703 (also known as PV cable) a better choice than some other common cable ratings, such as THHN or USE-2?
The term "PV wire" (photovoltaic wire) is often used to refer to USE-2 or THHN wire, the terms are not interchangeable. PV wire is specifically rated in accordance with UL 4703. THHN wire is used as general building wire and lacks the construction and strength of specialized cables designated as UL 4703 or USE-2. Using THHN cable in place of either UL 4703 or USE-2 will result in failures.
Though USE-2 wire is impact and crush resistant, UL 4703 rated cable is superior to USE-2 in regards to low temperature flexibility, sunlight resistance, and flame resistance. PV wire for solar panels also has a thicker jacket and insulation than USE-2 wire. USE-2 cable is used in grounded PV systems only, which UL 4703 cable can be used for both grounded and ungrounded arrays.
View the quick comparison chart below for an overview.
| Specification | UL 4703 (PV Wire) | THHN (Building Wire) | USE-2 (RHH/RHW Wire) |
| Applications | Wiring solar panels. Underground service entrance wire for both grounded and ungrounded PV arrays. |
General purpose wiring for installation in conduit. May also be used in machine tool, appliance and control circuit wiring. Cannot replace PV or USE-2 if standards require it. |
Aerial, raceway, or direct burial. For connecting PV modules. Grounded PV arrays only. |
| Min. conductor size | 18 AWG | 14 AWG | 14 AWG |
| Insulation | Cross-linked Polyethylene (XLP) | Polyvinylchloride (PVC) | Cross-linked Polyethylene (XLP) |
| Jacket | May or may not have jacket depending on requirements | Nylon | May or may not have jacket depending on requirements |
| Temp rating | 90° Wet 150°C Dry |
75°C Oil 75°C Wet 90°C Dry |
90°C Wet and Dry |
| Max voltage | 600, 1000, or 2000 V | 600 V | 600 V |
| Resistance to | Sunlight, ozone, UV, flame, moisture | Gasoline & oil | Crushing and impact, sunlight |
| Flexibility at low temperatures | Higher | Lower | Lower |
| Standards | Type USE-2 per UL 854 RHH/RHW-2 per UL 44 Type PV per UL 4703 |
UL 83 & 1063 as Type THHN Wire 90°C in dry locations. Type THWN Wire 75°C in wet locations. A-A-59544, NEMA WC-5, UL-758 |
UL 44 for rubber covered wire. UL 854 for service entrance cable. Federal Govt. Specificiation J-C-30A, ASTM-B-3(solid copper conductors), ASTM-B-3 and 6 (standard conductors) |
Features
Applications
Per the National Electrical Code, USE-2 wire is suitable for use in grounded PV arrays only. UL 4703 cable can be used within both grounded and ungrounded PV arrays. THHN cable is used as general building wire and cannot replace USE-2 or UL 4703 cable.
Temperature Rating
PV cable is rated for 90°C in wet conditions and up to 150°C in dry conditions. USE-2 is rated for 90°C in both wet and dry conditions. THHN cable is rated at 75°C in wet conditions and 90°C in dry conditions.
Voltage
PV wire can be rated for 600 V, 1000 V, or 2000 V. THHN and USE-2 wire are only rated for 600 V.
Flexibility
PV wire is made up of stranded copper conductors, which makes it flexible. USE-2 wire is usually installed in locations where it is not subject to movement or mechanical damage, so they can use either solid or stranded conductors made of copper, copper-clad aluminum, or aluminum, making it less flexible than PV wire.
Insulation and Jacket
Both PV and USE-2 cable use thermoset insulation and jacket, typically XLPE (cross-linked polyethylene) or EPCV.
Testing Requirements
USE-2 cable is usually installed underground where it will likely not encounter flames, while PV wire can be exposed in an installation, so flames tests are required only for PV wire. Overload and mechanical abuse tests (including crushing and impact resistance) are applicable only to USE-2 cable.
PV wire must pass more stringent testing requirements for sunlight resistance and low temperature flexibility. It undergoes a 720 hour weatherometer and a -40°C cold chamber conditioning. In contrast, the requirements for USE-2 cable are 300-hour weatherometer and -25°C cold chamber conditioning.