Code Corner

Do you have a question concerning building code or National Electrical Code® (NEC) issues? Below you will find responses to some of our most frequently asked questions. If you have a code-related question, we have an expert who can help! Submit your question today, and we will have the appropriate member expert contact you with a response.

IMC vs. GRC for Service Masts

Question: I have an issue with our power company regarding the use of IMC (intermediate metal conduit) for service masts. Several contractors have installed IMC in this application only to be rejected by the power company. The power company says that GRC means galvanized steel RMC. I asked the power company to refer me to the NEC for clarification on how they prove that GRC and galvanized steel RMC are the same thing. My problem, of course, is that GRC is not found in the NEC. Ironically, nor is GRC defined in the power company install guide, but they are emphatic that GRC is clearly mentioned in the NEC under RMC. My question is:

1. Is it an understood fact that GRC and galvanized steel RMC are one and the same?
2. Is it possible to prove that IMC has the same strength as GSRMC when it comes to the weight stress of an overhead service on the mast?

Please also let me know whether or not you wish to be quoted. Thanks for your time.

Answer: Thank you for your questions. GRC is a common industry acronym used for galvanized rigid conduit. GRC is the same product as UL-6 electrical rigid metal conduit — steel (ERMC-S) and rigid metal conduit (RMC) covered in NEC Article 344, which also covers aluminum RMC, stainless RMC and brass RMC. IMC is covered in Article 342 and is a steel galvanized conduit just like GRC, but it is made of a different type of steel with a thinner wall thickness. The wording in Article 342 and Article 344 is identical; therefore, both products are permitted in exactly the same installation applications. IMC is slightly stronger due to the different type of steel. However, for some reason, there are utilities that only permit the Article 344 rigid metal conduit for service masts. Generally, they will change their installation specification when they are made aware of these facts. You may quote me, and I will be happy to answer any additional questions you have.

Supplementary Equipment Grounding Conductors and Corrosion

Question: Does a supplementary equipment grounding conductor (EGC) solve corrosion concerns in metal raceways?

Answer: The belief that a supplementary equipment grounding conductor will solve corrosion concerns is unfounded. However, the National Electrical Code (NEC) and Underwriters Laboratories are quite clear on how to deal with corrosive atmospheres. None of the requirements or recommendations suggests a supplementary equipment grounding conductor (EGC) as a means to solve corrosion.

The NEC and the UL White Book “GUIDE INFORMATION FOR ELECTRICAL EQUIPMENT- THE WHITE BOOK 2010” clearly require supplementary corrosion protection where the atmosphere (soil) has severely corrosive effects. “In the absence of specific local experience, soils producing severe corrosive effects are generally characterized by low resistivity (less than 2000 ohm centimeters).” Corrosive effects are very easy to determine in all local jurisdictions. Generally the corrosive conditions will vary throughout most states, but they do exist in jurisdictions nationwide. For example, in Maricopa County, AZ, the corrosive conditions described above are an issue, and supplementary protection on all metal in contact with the earth is required. Contractors do a very effective job dealing with this in the greater Phoenix area at minimal additional cost.

This is a design issue.  To determine if corrosion is an issue in your jurisdiction, you can check with local sources such as the local utilities: electric, gas and water.  Generally, the general contractor will have this information long before the electrical installation begins. Soil conditions will vary some locales, or they may be similar in a whole jurisdiction.  It is up to the Authority Having Jurisdiction (AHJ) to determine this and to enforce section 300.6 (NEC) “Protection against Corrosion and Deterioration”.

Grounding Parallel Feeders

Question: When running feeders in parallel, for example a 400 ampere motor control center, how do you size the ground (EGC) in the raceways? What if you are paralleling MC Cable?

Answer: If the parallel raceways are rigid steel conduit, intermediate metal conduit or electrical metallic tubing, an additional EGC is not required.

If you plan to install a copper, aluminum or copper clad aluminum equipment grounding conductor, you would be required to install a 3 AWG copper or a 1 AWG aluminum or copper clad aluminum in each parallel path based on the 400 ampere overcurrent device.

If Type MC Cable is used, the manufacturer must be told the overcurrent device rating at the time of the order.  This allows the manufacturer to provide the proper size equipment grounding conductor, since MC Cable is manufactured based on the maximum overcurrent device permitted to protect a single cable run.

References: 310.4(A) through (E), 300.5(I) Exception 1 and 2, 250.118, 250.122(A) and (F) and Table 250.122

Watertight Conduit Systems

Question: In reviewing a spec there was a clause “provide a watertight conduit system”. Can GRC be assembled into a “watertight conduit system”?

Answer: The NEC does not recognize any conduit system as watertight, since normally raceways will collect condensation. The 2008 NEC Sections 300.5(B) and 300.9 cover raceways in underground and above ground locations.  These sections state that these locations are considered “wet locations,” and that conductors and cables installed in the raceways are to be listed for use in wet locations. Please see question 5 in the FAQs section of this website for more information.

Welding Steel Conduit

Question: Can you provide information about the welding of steel conduit and about welding things to it?

Answer: Galvanized steel conduit and tubing can be successfully welded.  However, the zinc coating that provides corrosion protection will likely be damaged around the area of the weld.  This may shorten the life of the raceway.  Once electrical conduit or tubing has something welded to it, it is no longer in conformance with UL and ANSI C80 standards.  In addition, Section 300.18(B) of the National Electrical Code prohibits metal raceway from being supported, terminated, or connected by welding to the raceway.

If the conduit or tubing is not being used in an electrical application (the intended use of conduit and EMT) and the installer decides to weld the product, proper precautions must be taken, since welding naturally produces smoke and fumes.  Refer to MSDS sheets for steel tubular products for more information.

NEC Conduit Fill Tables

Question: Please explain how to use the Tables in the National Electrical Code (NEC) to figure conduit fill.I have 3-12 AWG THWN, 4-10 AWG THWN, and 8-8 AWG THWN conductors. I plan to install them in EMT. What size EMT is required?

Answer: Each raceway article in .22 (ie: 358.22) in the NEC indicates that the number of conductors shall not exceed that permitted based on the percentage of fill specified in Table 1, Chapter 9.

Table 1 shows that for over 2 conductors the percentage of fill permitted for all conductor types is 40 percent.

Step 1. Use Table 5 for area of conductors: This table is based on conductor insulation and size. Find the table section that includes THWN, go down the left hand column to 12 AWG and move across the table to approximate area in square inches.

12 AWG Type THWN = .0133 Sq. inches.
10 AWG Type THWN = .0211 Sq. inches
8 AWG Type THNN = .0366 Sq. inches

Step 2. Total the square inch areas:
3 – 12 THWN X .0133 = .0399
4 – 10 THWN X .0211 = .0844
8 – 8 THWN X .0366 = .2928

Total .4171 Sq. inches

Step 3. Use Table 4 for areas of conduit permitted: Find the table section that includes electrical metallic tubing (EMT). Go down the right hand column (over 2 wires – 40%) to find area for .4171.

1” = .346
1 ¼” = .598

The answer, then, is that trade size 1¼ EMT would be required.

EMT Voltage Permissions

Question: I have six four-inch runs inside the building supported overhead to feed two unit substations. Is it permissible to run these 13,800 volt circuits in EMT?

Answer: Yes, NEC Section 300.37 specifically permits EMT for this purpose. Underwriters Laboratories in the listing states: “EMT is suitable for enclosing circuits above and below 600 volts.”

Unsupported Drops

Question: Could virtually any electrical machinery or equipment be supplied from an exposed 20-foot vertical riser of rigid metal conduit or intermediate metal conduit where there is no intermediate supporting means readily available?

Answer: Unsupported drops are permitted if it is to supply fixed equipment, such as a machine tool or a checkout stand in a big box store. It is not the intent of NEC Sections 342.30(B)(3) and 344.30(B)(3) that moveable stands or equipment be connected in this manner.

The drop must be supported at the top and bottom and all couplings must be the threaded type.

THHN Conductors and RMC

Question: How many #12 THHN conductors can I install in a 3/4″ rigid metal conduit?

Answer: The simple way to obtain this information is to refer to Annex C, Table C8 in the NEC. Find “THHN” in the left-hand column, and find trade size 3/4 across the top of the Table. The answer is “16”.

NEC Article 344 covers rigid metal conduit. Section 344.22 states: “The number of conductors shall not exceed that permitted by the percentage fill specified in Table 1, Chapter 9. (40%).”

You can make your own calculations using Tables 4 and 5 in Chapter 9. However, where all of the conductors are the same size, it is easier to use Annex C; Annex C Tables are based on Table 1, Chapter 9 and incorporate the calculations.

Outdoor EMT Installation

Question: Can electrical metallic tubing (EMT) be installed outdoors?

Answer: Yes. Section 358.10(B) allows the use of EMT “in concrete, in direct contact with the earth or in areas subject to severe corrosive influences where protected by corrosion protection and judged suitable for the condition.”

The galvanizing on EMT provides corrosion protection. If necessary, supplementary corrosion protection can be added by taping or painting the tubing. Appropriate fittings must be used.

Use of Type NM Cable in Conduit Outdoors

Question: I would like to run Type NM cable through my outside wall and run along the outside of the house to another location of the house and re-enter the home. If this Type Nm cable is installed in EMT (electrical metallic tubing) properly filled with compression coupling and connectors, is this code-compliant?

Answer: No, Type NM cable cannot be installed outside, regardless of the wiring method enclosing it. The installation is a damp or wet location.

Reference: 334.12(B)(4) not permitted in a wet or damp location, 300.9 Raceways in Wet Locations Above Grade. Where raceways are installed in wet locations above grade, the interior of these raceways shall be considered to be a wet location. Insulated conductors and cables installed in raceways in wet locations above-grade shall comply with 310.8(C). Also see 358.42, which requires the EMT couplings and connectors comply with 314.15, which requires that in a wet or damp location they be listed for wet locations.