FAQ - 2011

Not open for further replies.

Dennis Alwon

Staff member
Frequently Asked Questions

2011 NEC.jpg

Based on the 2011 National Electrical Code

Changes to 2011 NEC

For the previous 2005 NEC FAQs, click here
For the previous 2008 NEC FAQs, click here

General Questions
Branch Circuits, Devices, and Layout Questions
Services, Feeders, Panelboards
Grounding Code Questions
Calculation Questions
The Taboo Questions that will be promptly closed, due to exhaustion of the topic (or because of Forum Rules):

If you notice a technical error, have a question you believe belongs here, or have a different opinion on a topic that you feel should be included, feel free to start a thread on it with "FAQ" in the title. Or, you can send Dennis a PM.
Last edited:

Dennis Alwon

Staff member

  • Where are smoke detectors required to be installed, and where do I find this information?
NFPA 72 and various building codes have different specific requirements. Smoke alarm installations. All homes (regardless of age) should have smoke alarms installed in the minimum siting locations prescribed in NFPA 72
  • Outside each sleeping area
  • Inside every sleeping room
  • On every level
  • All home smoke alarms should be interconnected and have two power sources.
There are also many factors that enter into the locations of these units
  • Most units can cover an 20' radius. This means if the hallway adjacent to the sleeping area is 40' then one unit in the middle of the hall would comply. Please check the manufacturers instructions of the unit that you use.
  • A 3' clearance from every paddle fan, exhaust fan, cold air return or supply vents must be maintained
  • There are clearances from kitchens and bathroom. Ionization will affect the units so cooking and steam from a shower can cause problems.
  • No closer than 4" from ceiling and no farther than 12", when wall mounted
  • No closer than 4" from wall on a flat ceiling
  • No farther than 3' from peak of vaulted ceiling
  • No closer than 4" and no farther than 3' to peak of vaulted ceiling. Smoke rolls in corners, bypassing detector
  • Ambient temperature cannot exceed 100 F, or below 40 F, as in attics and garages
  • Smokes should be replaced every ten years
Many areas now require a carbon detector in halls adjacent to bedrooms. There are combination Carbon/Smoke detectors on the market that comply nicely with the requirement. Check your local requirements on this as they vary from state to state. Here is the NC rules on carbon detectors-- Page 34 of this PDF

Many smoke detectors come with instructions that mirror NFPA 72's requirements, so they could also be considered a 110.3(B) listing issue, if NFPA 72 has not been adopted in your area.

A free online viewable copy of NFPA 72, is available by clicking on this link (Click "I agree" to see the read-only document).
Last edited:

Dennis Alwon

Staff member

The requirements for AFCIs can be found in 210.12:

210.12 Arc-Fault Circuit-Interrupter Protection.
(A) Dwelling Units. All 120-volt, single phase, 15- and 20-ampere branch circuits supplying outlets installed in dwelling unit family rooms, dining rooms, living rooms,
parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms or areas shall be protected by a listed arc-fault circuit interrupter, combination-type, installed to provide protection of the branch circuit.

Exception No. 1: If RMC, IMC, EMT, Type MC, or steel armored Type AC cables meeting the requirements of 250.118 and metal outlet and junction boxes are installed for the portion of the branch circuit between the branchcircuit overcurrent device and the first outlet, it shall be permitted to install an outlet branch-circuit type AFCI at the first outlet to provide protection for the remaining portion of the branch circuit.

Exception No. 2: Where a listed metal or nonmetallic conduit or tubing is encased in not less than 50 mm (2 in.) of concrete for the portion of the branch circuit between the branch-circuit overcurrent device and the first outlet, it shall be permitted to install an outlet branch-circuit type AFCI at the first outlet to provide protection for the remaining portion of the branch circuit.

Exception No. 3: Where an individual branch circuit to a fire alarm system installed in accordance with 760.41(B) or 760.121(B) is installed in RMC, IMC, EMT, or steelsheathed cable, Type AC or Type MC, meeting the requirements of 250.118, with metal outlet and junction boxes, AFCI protection shall be permitted to be omitted.

(B) Branch Circuit Extensions or Modifications — Dwelling Units.
In any of the areas specified in 210.12(A), where branch-circuit wiring is modified, replaced, or extended, the branch circuit shall be protected by one of the following:
(1) A listed combination-type AFCI located at the origin of the branch circuit
(2) A listed outlet branch-circuit type AFCI located at the first receptacle outlet of the existing branch circuit

This requirement covers receptacle outlets, lighting outlets, and outlets for other equipment such as smoke detectors. Bear in mind, local jurisdictions have the authority to amend the NEC as they adopt it, so local enforcement may vary.

When polled, 75 percent of respondents on a forum poll stated they believed that a switch is not an outlet. Therefore, if the AHJ in your area agrees with the majority here, then a switch controlling a load, not in an area mentioned in the article, would not require AFCI protection.

Smoke detectors are not exempt from AFCI protection unless there is a local amendment.

Old Work: In general, the NEC that is in effect when you do the work is the applicable code. So under the 2011 NEC the entire part B is applicable.

If you add a receptacle to an existing circuit then the circuit supplying this new receptacle outlet is to be AFCI protected.

Many EC's have asked if AFCI protection is required when changing out a panel. The answer is generally "NO". Unfortunately with all the changes in this section there is still some issues that are unclear and as such must be discussed with the AHJ. The problem gets tougher when you change a panel and need to extend the wire a few inches to reach the breaker location. Is AFCI required? IMO, "NO" but that is an AHJ call.

Troubleshooting: AFCI Branch/Feeder circuit breakers come equipped with 30-50mA GFPE, excluding GE brand (they did away with GFCI protection in their AFCI). What this means is that there is a device similar to a GFCI (but not as sensitive) inside the AFCI breaker, which will trip under load if the neutral is touching the EGC on the load side of the circuit breaker. Usually, a "bootleg neutral" is the cause for nuisance tripping under load, and can be detected by normal troubleshooting methods. If you have a MWBC then you must use a DP AFCI. The GE brand is different in that you can use two single pole AFCI breakers with handle ties on a MWBC. To date that is the only brand that you can use two single poles on a MWBC.

What is the difference between a "branch/feeder type" and a "combination type" AFCI?
Both types examine the waveform signature of the loads they supply, and compare the signatures supplied to signatures they are designed to interrupt. An arcing fault is usually the result of a bad connection, or damaged wiring/cords, which creates a high-resistance connection ("glowing connection") that heats up and combusts surrounding material.

The branch/feeder type AFCI was the original style of AFCI that was first introduced to the market. They look for arcing faults in loads exceeding 75 amps, which greatly limited their ability to discover and disconnect arcing faults. Most of the nuisance-tripping issues that occurred when the device was introduced was a result of the built-in ground fault protection (GFPE) detecting bootleg neutrals (an EGC carrying neutral current inadvertently).

Given the low amperage of most household appliances, and the high threshold for the branch feeder AFCI, there is virtually no protection beyond the outlet when using a branch/feeder AFCI.

The combination-type AFCI was required starting 01/01/2008, by the 2005 cycle of the NEC. It will examine all loads exceeding 5 amps for arcing faults.

The combination type will be able to offer protection to cord-and-plug connected loads even through cords, according to the manufacturers. As of yet, there are no AFCI receptacles available but they are planned to be out by 2014 based on the requirements of article 406.4(D)(4)

Article 404.6(D) Replacements
(4) Arc-Fault Circuit-Interrupter Protection. Where a receptacle outlet is supplied by a branch circuit that requires arc-fault circuit interrupter protection as specified elsewhere in this Code, a replacement receptacle at this outlet shall be one of the following:
(1) A listed outlet branch circuit type arc-fault circuit interrupter receptacle
(2) A receptacle protected by a listed outlet branch circuit type arc-fault circuit interrupter type receptacle
(3) A receptacle protected by a listed combination type arc-fault circuit interrupter type circuit breaker
This requirement becomes effective January 1, 2014.
Last edited:

Dennis Alwon

Staff member
All About GFCI’s

GFCI's are intended to protect people from electrical shock in many different situations.

The main requirements for GFCIs can be found in 210.8. There are additional requirements scattered throughout the NEC for various specific occupancies and equipment, such as elevators, swimming pools, temporary installations, etc.

Looking at 210.8, the key to note is that all receptacles in the listed locations are required to have GFCI protection, and then the exceptions allow some receptacles to go unprotected. One notable example is bathrooms in dwelling units. There are no exceptions, so therefore a single receptacle still requires GFCI protection.

One significant change in 2011 is that GFCI's must now be readily accessible.
210.8 Ground-Fault Circuit-Interrupter Protection for Personnel. Ground-fault circuit-interruption for personnel shall be provided as required in 210.8(A) through (C). The ground-fault circuit-interrupter shall be installed in a readily accessible location.
Definition of readily accessible is very important here because this article basically means that GFCI cannot be access by ladders or other means.

Accessible, Readily (Readily Accessible). Capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, and so forth.
  • When was GFCI protection required for __________ ?
Click here to see when GFCI protection was required by various cycles of the code, compiled by Jerry Peck for Mike Holt.
  • How do GFCI's work? Do they require an EGC to work?
GFCI's have coils inside them that continuously measure the current leaving the GFCI on the ungrounded (hot) conductor, and measure the current returning on the neutral conductor. If more current leaves on the ungrounded conductor than returns on the neutral conductor, then the GFCI will trip. GFCI's are set to trip when the imbalance between the conductors is from 4-6 milliamps.

They do not trip under a short circuit, because in a short circuit, all current is still travelling along the normally current-carrying conductors.

Since GFCI's monitor the ungrounded and grounded conductors of a circuit, they do not use the EGC of the circuit for any purpose. That is why GFCI's are permitted to replace two-wire non-grounding receptacles in existing structures. Click here for the FAQ discussing this.

GFPE (Ground-Fault Protection of Equipment)

GFCI is different than GFPE, in that GFPE is designed to minimize damage to equipment during a ground fault. The requirements for GFPE can be found in various locations in the NEC. Here's an excerpt from the index of the NEC:​

Ground-fault protection
Deicing and snow-melting equipment, 426.28
Emergency systems, not required, 700.26
Equipment, 215.10, 240.13
Definition, Art. 100–I
Health care facilities, 517.17
Personnel. see Ground-fault circuit interrupters
Pipeline heaters, 427.22
Service disconnecting means, 230.95
Solar photovoltaic systems, 690.5

Related links:​

Vending Machines

How GFCI's Work :)

Evolution of GFCI's from IAEI. Thanks to M. D. for the link! :)
Last edited:

Dennis Alwon

Staff member
210.52(A) – Wall Space Questions
  • Do I have to include wall space that is exclusively behind doorswings when laying out receptacles?
Yes, in dwelling units. Mobile Homes have different (but similar) rules in 550.13, and in mobiles, the space behind the doorswing is permitted to be excluded by 550.13(G).

A receptacle behind a doorswing is a natural space for a user to plug in appliances that move around the dwelling unit, such as vacuum cleaners.
  • Are foyers included in 210.52?
Foyers have generated some discussion. Some see them as a hallway, others as a habitable room. Because of this issue the 2011 NEC has added a new section called- guess what-- Foyers (210.52(I)).

(I) Foyers. Foyers that are not part of a hallway in accordance with 210.52(H) and that have an area that is greater than 5.6 m2 (60 ft2) shall have a receptacle(s) located in each wall space 900 mm (3 ft) or more in width and unbroken by doorways, floor-to-ceiling windows, and similar openings.
Can a receptacle serving a kitchen countertop also serve as the wall space receptacle for the wall space next to the countertop, sharing the same wall?
No. 210.52(A)(4) was also added and has taken care of the controversy on this subject.
(4) Countertop Receptacles. Receptacles installed for countertop surfaces as specified in 210.52(C) shall not be considered as the receptacles required by 210.52(A).
Further, CMP statements on proposals to this section confirm the idea that they are very aware of potential damage to cords that go over the edge of a counter, which they bear in mind as the amend the sections each cycle. This is explored further in the kitchen FAQ.

Related Links:
How high is normal for wall space receptacles? - Aug 2006. Started by Jaman, with 49 replies
Kitchen Wall Receptacle - Aug 2006. Started by Malachi Constant, with 36 replies. It reinforced the idea that each wall space was independent - a "counter wall space" receptacle cannot perform double duty as a "wall space" receptacle. This was argued 5 years before the change came to be.​

Island Receptacles - Dec 2004. Started by Nascar03, with 31 replies. Nascar asked the question, "Is the side of an island facing habitable rooms considered a wall space of that room?"
Receptacle by Sink with large windows -- August 2010, started by Dennis Alwon, with 40 replies. This thread, taken from an article in EC&M talks about a very large window that extends down to the countertop and the need or not of a receptacle.
Last edited:

Dennis Alwon

Staff member
210.52(B), (C) ? Kitchen, Dining and Similar Area Questions

Foreword: 210.52(B) and (C) tread deeper into design than most sections of the NEC. As a result, it can be confusing to determine the right course of action when dealing with some custom kitchens that are not designed with the electrician in mind. Designers take a form of sadistic glee in designing kitchens that are attractive to the eye and bear innovative ideas that make applying the NEC to them a somewhat arduous task. I will attempt to provide a number of links to odd kitchens presented in different threads, to give you something to work with in coming to your own conclusions. -George :)

210.52(B)(1): Requires all wall, counter, and fridge receptacles in these areas to be on the two or more required SABCs.
Exception 1 permits a receptacle installed to replace a lighting outlet, to be supplied from a lighting circuit.
Exception 2 permits the receptacle for a fridge to be on it's own individual branch circuit 15A or greater. So the fridge is either on an SABC or on it's own, two options.

210.52(B)(2): Forbids us from using the SABCs for other outlets.
Exception 1 allows an SABC to serve a receptacle for a wall clock.
Exception 2 allows an SABC to serve small loads on fixed gas cooking appliances.
These two exceptions make it clear that receptacles behind fixed (or cumbersome) appliances are not "wall receptacles". They are receptacles for specific appliances.​

210.52(B)(3): The group of receptacles serving countertops in a kitchen shall be served by two SABCs. (Each receptacle is served by one; the group of receptacles by no less than two).

210.52(C): Did you ever notice counters in dining rooms are to comply with the 2'/4' spacing rules, just like kitchens?

210.52(C)(1): "Wall Counter Space" is defined. This is different than "Wall Space".

210.52(C)(2): There is only one receptacle required on an island, unless a sink or a cooktop divides the island into two "islands". Notice that if there is a foot or more of counter space behind the sink or cooktop, then the entire area becomes one island again.

Proposals were submitted for the 2008 cycle to get rid of the island requirement completely, due to potential injury to small children pulling appliances down on themselves. The panel viewed the potential risk "to both children and adults" from extension cord use as cause to reject them.

Proposals were also submitted to increase the numbers of receptacles required to be installed on islands, and they were rejected as well. The panel indicated that one receptacle on an island is the best-fit between extention cord danger and accidental snagging of appliance cords.

210.52(C)(3): The same goes for peninsulas as islands. Only, a sink or a cooktop would create an "island" at the end of the peninsula. Most AHJs do not require a receptacle on the "peninsula" portion of such a counter space, but the wording would require a receptacle on the wall-less countertop on either side of the sink or cooktop. With an overhang for a bar top, this becomes exceedingly difficult to comply with.

210.52(C)(4): This section makes it clear that sinks, ranges and cooktops seperate counters for the purposes of this section.

Note that appliance garages are not listed. Proposals have been submitted to permit an appliance garage to seperate counter spaces, and have been rejected on the premise that there is usually counter space in front of the appliance garage large enough to facilitate food preparation.

However, in all but the same breath, they have rejected proposals to require counter-spacing requirements to be observed inside appliance garages. As in, the receptacle inside the appliance garage should not be considered serving the counter space in front of the appliance garage.

The panel has never clarified (to my knowledge or satisfaction) how exactly we are to serve this counter space.

In practice, generally the space in front of an appliance garage is considered served by receptacles with the garage. In general, GFCI protection seems to be required for these receptacles. Given the confusion, it would be good to get the AHJ's input before setting any design into stone, if it is questionable.​

In my opinion, the space in front of an appliance garage is not wall counter space, as it backs up to cabinetry and not a wall. Therefore, it should be considered a break in the counterspace and also have the 2'/4' rule enforced inside the garage as well. However, the words at this point are not there.

210.52(C)(5): This section requires all receptacles serving counter spaces to be above the counter. Only where impracticable are receptacles below the countertop allowed, due to damage that could result from the cord on the edge of the countertop.

Related links:

Appliance Garages on SABCs - Oct 2006. Started by Volt102, with 65 replies. "Are appliance garages required, permitted, or prohibited to be on the SABCs? GFCI protected?"
Can I put a microwave on a SABC - Feb 2005. Started by ethwinfir, with 31 replies.
Switched Dining Room Outlet? - Mar 2005. Started by davedottcom, with 513 replies. An exhausting odyssey through the interrelationship between 210.52 and 210.70. Pack a lunch.
Undercabinet Lights - Apr 2005. Started by Electrofelon, with 277 replies. "Can a luminaire be plugged into a countertop receptacle?" How innocent a beginning for such a bloodletting.
Since you guys are so smart... - Apr 2005. Started by Electricmanscott, with 35 replies. Addressed an odd kitchen with difficult customer requests surrounding an appliance garage.
Peninsula Receptacles - Started by Titan 1021 on May 2010- Great discussion on requirements for receptacles on peninsula. What is the connecting edge?

For more related threads, search for the term "210.52". As of this writing, it comes up with 151 threads, most of which are dealing with some aspect of complying with the NEC in kitchens and similar areas.​

If dealing with this section has you frustrated, click here for a discussion about how really, it's not easy writing code. :)

Dennis Alwon

Staff member
Detached Garages

There are many different desired designs for detached buildings, perhaps as many as people who build and wire them. For clarity, I will put some examples up and explain each example.

Example #1: A small outbuilding calls for a 15A circuit for lighting, and a 15A circuit for a receptacle. A multiwire branch circuit is selected, a 12-3 UF cable. In this case, no grounding electrode system is required to be connected to the electrical system of the detached structure. However a disconnecting means is still required. In this case a DP snap switch can be used.

According to 250.32(A), exception, no grounding electrode system is required for a single circuit, and the multiwire branch circuit is considered one circuit. 225.33 requires a disconnecting means for all ungrounded conductors supplying the building. The EGC in the UF cable is the effective ground fault clearing path required by 250.4.

Example #2: A small outbuilding calls for one 15A lighting circuit and four 20A circuits for receptacles. In this case we cannot run 5 individual uf cables as that install would violate article 225.30. A feeder would be a normal choice. The feeder would have to terminate in a panelboard and have either the Six-Handle-Rule applied or a main disconnect. A grounding electrode is also required based on 250.32(A) and an EGC must be run with the feeder.

Example #3: A customer desires a 100A panel in a garage. No other services are common between the structures. We no longer have the choice of using a feeder that does not have an egc. In the 2008 code the rule changed to require an egc in all situation. In this case, a grounding electrode system is required to be connected to the panelboard at the detached structure (250.32(A)). The panel should be a main-breaker panelboard, as explained in the Six-Handle-Rule FAQ. The neutral and egc must be isolated and the GEC gets connected to the ground bar NOT the neutral.

Also see The Six-Handle Rule FAQ and the grounding question for detached garages.
Last edited:

Dennis Alwon

Staff member
Equipotential Bonding Grid (EBG) and Wiring Methods of a Hot Tub

Is a EBG required for a hot tub? The short answer is ?YES?, however there has been a TIA issued for the 2011. Generally speaking, if the tub needs an EBG, then the easiest and most common method of bonding the tub is to lay a #8 copper conductor around the contour of the tub about 18-24? from the tub and 4?-6? below subgrade. The wire should be joined with a listed splice and then continue on to the tub motor lug. If the motor is a double insulated motor then the #8 should connect to the egc of the motor circuit.

What if???? What if my tub is on a deck? Well the NEC does not give any relief on that issue so I would check with the AHJ and discuss an appropriate means of installing the bond.

Let?s keep in mind that the rules change for a Hot Tub that is installed indoor (art. 680.43). Also remember that a Hot Tub is not a hydromassage tub, but rather a tub that is not intended to have the water drained. Article 680 Part IV covers Hot Tubs and specifically article 680.42 covers outdoor installations.

Outdoor installations are basically wired as a pool would be wired. There is one exception to this and that is 680.42(C) for one family dwellings. When wiring in the interior of the SFD you may use the wiring methods of chapter 3. What does that mean? Well you could use NM cable in the dwelling to the outdoor disconnect. From the disconnect to the outdoor tub the wiring must be done according to Part II of 680 (usually PVC). Another be careful note here?Many tub manufacturers require a full size egc. If this is the case then NM cable will not work as the egc is not sized appropriately.

Dennis Alwon

Staff member
What are the Rules for Receptacle Replacements-Non Grounding & Grounding?

There have been some major changes in the 2011 about replacement receptacles.

Art. 406(D)(1) states that if there is a grounding means at the receptacle needing replacement then we must use grounding receptacles. However there are some added requirements in articles 406.4(D)(4), (5) & (6)

As explain in article 406.4(D2) you can do the following when relacing a non grounding receptacle:
a) You can replace the non grounding receptacle with another non grounding type receptacle
b) A non grounding receptacle can be replaced with a gfci receptacle
c) If the receptacle is fed thru a gfci and is non grounded then a 3 wire receptacle may be installed without an egc, however the receptacle must be marked “No Equipment Ground”. These labels are generally found inside the GFCI box.

406.4(D)(3) requires all non grounding receptacles, that otherwise require gfci protection, shall be replaced with a gfci – either a breaker or receptacle.

Now the changes
Article 406.4(D)(4) is about AFCI. This requirement come into effect on 1/1/2014 and requires that all receptacle outlets that are replaced in areas that require AFCI shall be replaced with:
1) a listed afci receptacle
2) a receptacle protected by a afci receptacle
3) a receptacle protected by a listed afci receptacle or afci circuit breaker

The afci type receptacle, as of yet, are not on the market but they should be by 2014.

406.4(D)(5) states that tamper resistant receptacles shall be used where replacements are made in areas which require TR receptacles. What does that mean—well go back to 406.4(D)(2)(a)—since non grounding TR receptacles are not being made, we cannot use them as replacements anymore.

406.4(D)(6) states that a weather resistant receptacle must be used when replacing receptacles that require WR in the code.
Last edited:

Dennis Alwon

Staff member
Am I required to Run an EGC to a Separate Structure?

In the 2005 NEC we had options , however, since the 2008 code cycle those options were taken away and we must run an egc or we must have a raceway that is suitable as an egc. Many are confused on what to do with the grounded conductor and the egc at the panel located at the separate structure. The egc must be kept isolated from the grounded conductor. This panel in the detached structure is a subpanel and it is wired the same way as any other subpanel except that we have a GEC to install. This GEC connects to the ground bar, NOT the neutral bar, of the panel

Dennis Alwon

Staff member
The Six-Handle Rule
Services, Feeders to detached structures, etc.

The rules for the number of switches allowed to disconnect all ungrounded conductors of a structure are given in different locations depending on the application.

For services, 230.71(A) dictates that no more than six handles shall disconnect a service from the structure served. Throughout all of Part V of Article 230 are several rules dictating how large the disconnecting means must be, how to determine how large the service is, where it must be located and what may be attached to the line and load sides of a service disconnect.

For detached buildings that are fed from another building, the rules are given in Part II of Article 225. Section 225.33(A) also requires no more than six handles to a structure. Article 225.36 requires the disconnecting means to be marked as suitable for service equipment unless it is a MWBC feeding the structure. The exception allows a snap switch to be used.

Please note that the key to both 230.71(A) and 225.33(A) is the number of handles present, not the number of spaces available. Therefore, based on this fact, you can install a 40-circuit panelboard, install a single 100A breaker to feed an interior panelboard inside a structure, and comply with these rules. This is widely considered a practice that may be legal but should be avoided. However, by the NEC such an installation should be approved.

Grounding requirements for services are found in Article 250, Parts I and III.

Bonding and Grounding requirements for detached structures are found in 250.32, and are discussed in detail in this FAQ.

Related threads:
Last edited:

Dennis Alwon

Staff member
Wire Insulation Types and Letter Designations

"T" stands for thermoplastic insulated cable.

"H" means the insulation is heat resistant.

"HH" means that the insulation is heat resistant and can withstand a higher temperature. This insulation can withstand heat up to 194 degrees Fahrenheit.

"W" means that the insulation is approved for damp and wet locations. This wire is also suitable for dry locations.

"X" means the insulation is made of a synthetic polymer that is flame-retardant.

"N" is for the nylon coating that covers the wire insulation.

Thhn- thermoplastic high heat resistant nylon
Thwn- thermoplastic heat and water resistant nylon
Thw- thermoplastic heat and water resistant
Thhw- thermoplastic high heat and water resistant
Tw- thermoplastic water resistant
UF- Underground feeder
Rhw- Rubber insulated heat and water resistant
Rhh- rubber insulated and high heat resistant
Mtw- machine tool wiring
USE- underground service entrance cable
SE- service entrance
SEU- Service entrance unarmored. I have also heard it as -- uninsulated (neutral)
SER- service entrance round
Xhhw- Cross-Linked Polymer High Heat Water Resistant

It is important to note that many insulation types, such as Thhw, are listed for wet and dry location but when used in wet location the temperature rating is 75C. When used in a dry location the same wire is rated 90C. Some wires have a designation of -2, eg. Thwn-2. This means that the wire is listed for 90C in both wet and dry locations.
Last edited:
Not open for further replies.