Electrical FAQs

General Information

Q. What is the difference between kcmil and MCM?
A. Both “kcmil” and “MCM” are units of measurement for the area of an electrical conductor with a circular cross section. They are equal measurements and refer to thousands of circular mils. A circular mil is equal to the area of a circle with a diameter of one mil. One mil is one thousandth of an inch. Therefore, one-inch equals 1000 mils. MCM is an older designation, and kcmil is the newer reference. “M” represents 1000 in Roman numerals, and “k” represents 1000 in the SI system of measurement. The NEC^® and UL^® product standards all refer to kcmil, although you will still see some manufacturers and other reference books refer to MCM.

For conductors smaller than 250 kcmil, we use the AWG (American Wire Gauge) system to indicate size. If you need to convert from AWG to circular mils, the NEC in Chapter 9, Table 8 has cmil sizes that correspond to AWG sizes.

Q. Will the copper diameter always be exactly equal to the value in the product sheet?
A. The diameter of the copper conductor will have small variations due to normal manufacturing tolerances, so it might be slightly smaller or larger than the published value. The UL product standard and ASTM standards specify minimum and average values, and our conductors comply with those standards. We also use a test that measures the resistance of the conductors to ensure that they maintain adequate conductivity to comply with the requirements of the UL standards and the National Electrical Code^®.

Q. How should I install wire in cold weather?
A. Low temperatures can cause problems during installation due to temporary brittleness of the insulation and jacketing materials. When installing wire during cold weather, cable must be handled more carefully and should be pulled more slowly. The wire should be kept in a heated environment for at least 24 hours prior to the installation. It is not recommended that wire be installed at ambient temperatures below those shown in the table below.

Q. What is the difference between a CT (cable tray) rating and a VW-1 (vertical-wire flame test) rating?
A. Both tests are optional ratings, and they both measure the ability of the wire to resist the propagation of flame. The VW-1 test has a single specified vertical test method, while the CT rating is achieved by complying with one of two vertical tray tests found in UL 2556.

Q. Is it permissible to use pulling lube on SliPWire^® products, and is there any detrimental impact of using pulling lube on those products?
A. Although the intent of SliPWire^® is to allow pulling the wire into a conduit without the use of pulling lube, it is acceptable to use a separate pulling lube if desired. Pulling lubes that are identified for the insulation and raceway type being used will not have a detrimental effect on the installation or long-term reliability of the wire.

Q: Is it acceptable to touch insulated conductors while they are energized?
A: Unless protected by conduit, cable armor, an electrical enclosure or similar, we consider insulated conductors like THHN or XHHW to be a shock hazard when touched by a person. If the conductors cannot be deenergized, appropriate steps must be taken to protect the qualified worker within the shock boundary. This might include PPE or other means of risk control.

“Insulated” as used in NFPA 70E is understood to apply only for the conditions under which the object is normally used. Being handled or touched by humans while energized is not a normal use, so for the purposes of 70E, an energized conductor that can be touched directly is considered to be uninsulated. (See the informational note under the definition of “insulated.”)

From a practical perspective, there are many things that can affect the dielectric capability of a conductor’s insulation that could make it dangerous for a person to touch. Wet conditions, dust on the insulation that could provide an electrically conductive path, cracks in the insulation due to physical damage or age, and other conditions could make it possible for a person to receive a shock even when the conductor’s insulation is still sufficient to keep it from creating a ground fault or short circuit. When estimating the likelihood of an arc flash, NFPA 70E Table 130.5(C) indicates that an arc flash is likely to occur when the task is “Examination of insulated cable with manipulation of cable.” The table indicates that arc flash is unlikely to occur when the task is “Examination of insulated cable with no manipulation of cable.”

NM-B

Q: Can the inner insulated conductors of an NM-B cable be stripped out of the cable and used in a conduit?
A: No. Even though the conductors meet the thickness and other applicable requirements for Type THHN thermoplastic-insulated wire, they are prohibited from being surface marked with “THHN”, “-B”, or any ampacity or temperature rating. Those markings are required by the NEC for single conductors pulled into conduit.

Q. Can NM-B be installed in a building that isn’t enclosed yet?
A. Yes. Cerrowire recognizes that our NM-B cable will be exposed to weather, sunlight and temperature changes during construction in residential and commercial buildings, therefore we design our cable with that in mind. NM cable is permitted to be installed in normally dry locations in accordance with NEC Article 334. Note that the National Electrical Code specifically recognizes in the definition of a Dry Location that “A location classified as dry may be temporarily subject to dampness or wetness, as in the case of a building under construction.”

Temporary exposure to water on the exterior of the cable is not detrimental to the cable. To verify the cable is undamaged, an appropriate indicator is to check the paper wrap at termination points. If it is dry, that is evidence that the cable has not been submerged or had water wick into the cable.

UF-B

Q: Can UF-B be installed in or under concrete?
A: Type UF-B is listed and permitted for direct burial in earth, including under concrete. Type UF-B is prohibited from being installed (embedded) in concrete, but it is permitted to be pulled into conduit that runs through concrete, for example to feed a kitchen island. If Type UF-B is run underneath a building, it must be installed in a raceway.

THHN/THWN-2

Q. Can splices affect the results of an insulation resistance test after installations?
A. Yes. When splicing THHN/THWN-2 conductors, remove the nylon covering from the length of conductor that will be in contact with the splice. Avoid removing more of the nylon than is required for making the splice. If the nylon is left in place and makes contact with the splice, insulation resistance testing results may be lower than expected and the cable may fail the test. When removing the nylon, take care not to damage the insulation.

Q: Is the nylon jacket on THHN for mechanical protection only?
A: The function of the nylon jacket on any THHN/THWN product is two-fold. First, it works with the PVC insulation to provide enhanced resistance to gasoline and oil. PVC is already resistant to gasoline and oil. Secondly, it provides additional abrasion and damage resistance during installation for the PVC insulation on the product. The nylon is not intended to provide any electrical insulation for the product. If the PVC insulation is not damaged, the electrical integrity of the cable is unchanged, even if the nylon is completely removed. The PVC insulation is not compromised by contact with water, since PVC is moisture resistant.

Q: Can the words “Cable Tray Use” be applied to all sizes of THHN type products?
A: No, only sizes 1/0 AWG and larger are marked for use in cable trays. For equipment grounding conductors, conductors as small as 4 AWG are permitted, and they are not required to be marked for cable tray use.

TFFN, TFN

Q: Why can’t TFN be dual-rated as MTW?
A: The stranding of TFN is too coarse to meet the requirements for MTW. TFFN, which is more flexible, can be dual-rated.

Q: Does Cerrowire make a VW-1 version of TFFN?
A: No, it is not a popular item and the costs would be prohibitive.
 THHN, THWN, and MTW are VW-1 rated in sizes 14 AWG through 1 AWG.

XHHW-2

Q: Can XHHW-2 be made with a 2000 volt rating?
A: No, for 2000 volt insulation you would purchase an RHW-2 wire instead. Cerrowire’s XHHW-2 carries a dual 600 volt and 1000 volt rating.

Q: Why purchase a thermoset insulated XHHW when you can buy a thermoplastic insulated product having the same ampacity?
A: The main reasons are that the thermoset product has 1) better low temperature properties and 2) a higher K value, which results in lower current leakage. The short circuit current carrying capacity is also greater than THHN, providing greater protection from insulation damage during short circuit conditions.

Q: Can XHHW-2 be rated gasoline and oil resistant?
A: Yes. Cerrowire’s XHHW-2 is rated GR II for gasoline and oil resistance.

Q: Does XHHW-2 have to meet any flame test requirements?
A: At a minimum, it is required to pass a horizontal burn test or FT-2 flame test.

Q: Can XHHW-2 be rated for use in cable trays?
A: Sizes 1/0 and larger can be rated when marked for “CT Use” or “For use in Cable Trays”. Cerrowire’s 1/0 and larger XHHW-2 is marked “For CT Use”.

Q: Can XHHW-2 be rated for Sunlight Resistance?
A: Yes. Black XHHW-2 Conductors sizes 8 AWG and larger are marked “Sunlight Resistant.”

Bare Copper/SE Style U

Q: Why is the base color of both inners of Type SE Style U colored black?
A: Both the PVC insulation and the nylon jacket on the inners are required to pass a weatherometer test, and black best meets the requirement. One of the black conductors also has a red stripe for phase identification.