Letter Designations

SOOW Letter Designations

The letter designations of SO cable come from the National Electric Code. A new edition is published every 3 years by the National Fire Protection Agency (e.g. 2008 and 2011 NEC Handbooks). SOOW cable specifically refers to cable that is rated for up to 600 volt applications, has an oil resistant jacket and insulation, and is weather and water resistant (meaning it can be used outdoors).

Since SOOW cable (now largely sold in place of SO cord) is the most common type of portable control cable people use, you may see the term being used to refer to flexible cable in general. However, keep in mind that SOOW is only a specific type of flexible cable, so be aware of the context in which the term is being used.


SOOW - Extra hard usage, oil resistant jacket and insulation, for outdoor use.

SJTO - Hard usage, thermoplastic, oil resistant jacket.

SPT-2 - Parallel cord, thermoplastic.

See the table below for a simple breakdown the UL gives of common NEC flexible cable letter designations based on NEC table 400.4.

Flexible Cable Letter Designations

Designation What this means
S Service grade. Extra hard service when not followed by J, P, or V.
SJ Junior hard service. 300 volts
SV Vacuum Service and light duty cable. Originally designed for use with vacuum cleaners but now used for a variety of light duty applications.
P Parallel cord (also known as zip cord). Light duty only.
Parallel cord types SP-1, SP-2, SP-3, SPE-1, SPE-2, SPE-3, SPT-1, SPT-1W, SPT-2, SPT-2W, SPT-3 have only one layer of insulation (see "NI")
E As first letter: Elevator cable
After first letter: Thermoplastic elastomer insulation & jacket (UL/NEC designation only)
EV Electric vehicle cable (such as for use in electric cars)
H Heater cords (cords for heating applications)
NI "Non-Integral." This designation only applies to parallel cords and means the parallel cord has two layers of insulation.
Parallel cords without an "NI" have only one layer of insulation, with no outer jacket.
Add "NI" (e.g. NISPT-2) to indicate a parallel cord with conductor insulation and a jacket covering.
O Oil resistant*
T As first letter: Tinsel cord. Single flattened 27 AWG conductor wound around insulating core, for very low current, highly flexible application.
After first letter: Thermoplastic insulation & jacket
XTW Parallel cord for decorative lighting strings.
CXTW Twisted pair cord or single conductor for decorative lighting strings.
W As last letter: Suitable for use in wet locations and sunlight resistant.
As the only letter: Portable power cable
VW-1 Flame retardant. Passes UL Vertical Wire flame test.
Flame retardant means the cable stops burning once the heat source is gone (not the same as flame proof).
-1, -2, -3 Insulation thickness for parallel cords, thinnest to thickest. Actual insulation thickness varies with cord type and AWG size.

* When one "O" is in a designation (for example, "SJOW"), only the outer jacket material is oil resistant. If there are two "O's" are in the classification (such as "SOOW"), both the outer jacket and conductor insulation are oil resistant.

Flame Retardant Designations

You may also see ratings with different letter designations regarding the resistance of insulation material to ignition or the spread of fire.

Not the same as flame proof

Flame retardant means the insulation will stop burning once the heat source is gone, but flame retardant cables will still be damaged by fire.

UL 94 and UL 746A

UL 94 describes the testing procedures and ratings for the ability of material to stop burning, within the time limit set by the test, once the heat source has been removed. A test sample fails if too much of the material gets burned or it burns for too long.

UL 746A tests for ignition resistance and is also very important since there is always the possibility of fire wherever there is electric wiring. The main concerns for potential causes of fire are overcurrent and frequent arcing between contacts and damaged or loose connections.

Some UL flame rating letter designations

Designation What this means
HB Any “HB” rating indicates the material has been tested in a horizontal position.
V-0, V-1, V-2 A “V” flammability rating indicates the material has been tested in a vertical position.
Vertical flame test for thin materials. Samples in this test would be too distorted if subjected to UL’s ordinary vertical flame test because the material is so thin.

Vertical vs. Horizontal flame tests

One important difference to note is that in vertical flame tests there is a piece of cotton underneath the material being tested to catch any flaming particles that drop off. V-0 and V-1 flame ratings indicate that flaming particles did not ignite the cotton used in the test. However, insulation material can still receive the V-2 flame rating even if the cotton ignites.

CSA Flammability Rating Examples

The Canadian Standards Association (CSA) also has flame tests and ratings for materials. FT1 and FT4 are different vertical flame tests, while FT6 is a horizontal flame and smoke test.