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Phase converters, which are covered by Article 455 of the National Electrical Code, are defined as electrical devices that convert single-phase power to three-phase electrical power.

There are two basic types of phase converters. The first one is a static-phase converter, which consists of relays running capacitors and starting capacitors, but does not have rotating parts. Static-phase converters do not generate a true third phase. Three-phase motors used with this type of converter are operating on two single-phase lines with assistance from the running capacitors. With this condition there is a loss of output horsepower by the three-phase motor. Static converters are generally used when 100% capacity is not needed and frequent starting and reversing is not required.

The second type is a rotary-phase converter, which can operate several motors at 100% capacity plus resistance load. Rotary-phase converters come as close to real three-phase power as possible without the power company supplying it.

There are several code sections we need to discuss. The first is Section 455.4, which describes what the manufacturer is required to put on the nameplate of phase converters. Section 455-4(4) requires the nameplate to identify the part that is used for sizing conductors, disconnects and overcurrent protection for the single-phase supply.

In Section 455-6 one finds the procedure for sizing conductors supplying the phase converter. The first part of 455-6(a)(1) requires for variable loads that the conductors not be "less than 125 percent of the phase converter nameplate single-phase input full-load amperes." On fixed loads, 455-6(a)(2), when the "conductor ampacity is less than 125% of phase converter nameplate single-phase input full-load amperes, the conductors shall have an ampacity not less than 250% of the sum of the full load 3-phase current rating of all motors and other loads served." When input and output voltages are different, then "the current as determined by this section shall be multiplied by the ratio" of the input to output voltage.

I believe Section 455-6(a)(1) is clear. For example, we have 20kVA rotary-phase converter and the nameplate single-phase input is 90 amps at 230 volts (125% x 90 amps = 112.5 amps). Using this figure we would select a conductor size from Table 310.16, based on the 75-degree column, or a no. 2 copper conductor is adequate.

The second part of Section 455-6(a)(2), I believe, is a little confusing. Part two permits conductors to be sized for specific loads that may be less than the phase converter is capable of handling. As mentioned, the nameplate must identify the rated minimum load as required in Section 455-4(5). Phase converters operate better when loaded at or near full capacity. For example, using the same 20kVA rotary phase converter, let’s say the phase converter nameplate requires a minimum of five H.P. and we are using a 7½ H.P. three-phase 230-volt motor that has an ampacity of 22. The 7½ H.P. 230-volt motor is better than the minimum of the 5 H.P. required by the nameplate. Sizing the conductors would be 250% x 22 = 55 amps. Then we would select a conductor from Table 310.16 that equals 55 amps or a no. 6 TW copper conductor. If the voltages are different, we need to multiply this value by the primary to secondary voltage ratio. For example, using the 7½ H.P. three-phase motor at 460 volts three phase that has an ampacity of 11 amps—11 x 250% = 27.5 x 2 = 55. In this case, the same conductor would work in both examples.

I would like to stress that a consultation with a supplier or factory representative would be advisable before selecting a phase converter.

Hiram J. Lamb is the principal representative for IAEI on Code Making Panel 13. He is the assistant building official and chief electrical inspector for the city of Charlottesville, VA, fire department. He is also a registered master electrician with the state of Virginia.

Electrical installations in hollow spaces, vertical shafts, and ventilation or air-handling ducts shall be made so that the possible spread of fire or products of combustion will not be substantially increased. Openings around electrical penetrations through fire-resistant-rated walls, partitions, floors, or ceilings shall be firestopped, using approved methods to maintain the fire resistance rating.

FPN: Directories of electrical construction materials published by qualified testing laboratories contain many listing installation restrictions necessary to maintain the fire-resistive rating of assemblies where penetrations or openings are made. An example is the 24-in (610-mm) minimum horizontal separation that usually applies between boxes on opposite sides of the wall. Assistance in complying with Section 300-21 can be found in these directories and product listings." [Italics added by author.]

As inspectors it is not only our duty and responsibility to enforce the requirements of the National Electric Code® but to be aware also of building codes and standards regarding fire separations. As inspectors, contractors or electricians, we all need to prevent components of the electrical system from contributing to the possibility of spreading a fire. Penetrations into or through fire-resistive assemblies, fire protective membranes, thermal barriers, or construction providing a finish rating as an alternative to a fire resistive assembly shall be protected to help ensure electrical system components do not create an increased hazard. Electrical receptacle boxes, switch boxes, junction boxes and other device openings must maintain the fire integrity of the material surrounding them or in contact with them. I like to use the example of an air-filled balloon as being a complete envelope and once a hole is put in the balloon you must seal it back up to contain the air inside of it. That example demonstrates why we need to know how to properly maintain the fire rating of walls, floors, or ceilings once an electrical installation is complete and prior to covering it up. Working closely with building officials is a key as they have the primary responsibility of enforcing this aspect of the construction standard. However, every tradesman that installs electrical equipment or materials in a separation wall, floor or ceiling area is responsible for maintaining the integrity of the system. We need to inform the electrician of potential conflicts and, if need be, teach or discuss acceptable fire stopping procedures as part of our enforcement actions and procedures.

The nationally recognized testing laboratories (NRTLs) have approved methods and qualifications for installation procedures to help us out. One of the first concerns of any inspector or electrician at a project should be inquiring about any fire-resistive assemblies, "membranes penetration" or "through penetration" that may be encountered. A common definition for "membrane penetration" is an opening made through one side of an assembly that is a building element. Note: an example of a membrane penetration is a recessed outlet box penetrating the gypsum wallboard (with the gypsum wallboard being the "membrane") on one side of a stud wall construction. Whereas a "through penetration" can be defined as an opening that passes through an entire assembly that is a building element; such as, back-to-back electrical boxes in a common wall separating unit A from unit B. I like to use a highlighter or red pen on the drawings or plans to denote those areas that will need special attention during the construction phase and inspection, prior to cover up, and then discuss or determine with the electrical contractor what methods are to be used with the electrical components to maintain or sustain the fire rating of the system.

As was mentioned above in the code reference, installations in hollow spaces or vertical shafts deal with the stud cavities that our wiring and boxes must be installed in. Remember that we are dealing with separation of walls, floors, or ceiling from some other unit or use and not internal partitions within a space. Let’s discuss some common scenarios that we may encounter.

First, the installation of a wall section between living units and a 2-hour or less classification. We have a common 2" x 4" top and bottom plate with wood or nonbearing steel studs and gypsum wallboard and a metallic or nonmetallic box spaced to meet the room requirements or layout. The boxes should be examined for the classification mark and the use is based on the design of the fire resistive assembly. Often there will be a mark of the testing laboratory; hourly rating, such as, 1 hr. or 2 hr.; and a letter designator for its placement in floors (F), walls (W), or ceilings (C).

After we have looked for the mark on the product, a review of the listing criteria should be done. Many of the product listings contain specific requirements, such as, clearance between boxes and cutouts in walls shall not exceed 1/8 in. The area of openings for boxes shall not aggregate more than 100 sq. in. per 100 sq. ft. of wall or partition area, with no opening exceeding 21.02 sq. in. and a minimum horizontal separation of 24-in (610-mm) between boxes. Note: in the Fine Print Note (FPN) above, "that usually applies" was italicized to emphasize that it was an addition placed there during the 99 Code cycle and the panel accepted it. The substantiation for the change brought out the fact that there are several products on the market that are listed for outlet boxes located on opposite sides of a fire separation wall that reduce the customary 24 in. separation requirement. However, they cannot be back to back and thus create a through penetration. Listings have been developed to allow adjacent installations, provided all of the fire resistance assembly’s requirements are met.

One method is the use of firestop putty pads where the horizontal spacing of the boxes must be less than the required 24 inches. This is a particular concern with back-to-back installations such as adjoining kitchens or bathrooms located on common rated walls. Others have listings for walls containing minimum 3-1/2 in. thick minimum 2.5 pcf mineral wood bat insulation in the stud cavities, the minimum horizontal separation between outlet boxes on opposite sides of a wall may be reduced to 7 in. Some Authorities Having Jurisdiction (AHJ) have allowed the boxes to be framed in with additional studs and gypsum board to separate out the hollow space or vertical shaft so the boxes are not in the same open cavity.

Another scenario encounters the staggered-stud building method with a common top and bottom plate of 2" x 6" or 2" x 8" and the studs set on different centered dimensions for each face of a common separation wall; with an offset dimension for the studs facing into unit A and B to help overcome the noise transmittance problem encountered with a common stud framing member in the wall between the units. Thus a void is left to allow horizontal movement of products of combustion in the wall cavity. The staggered-stud wall may be assembled with two standard wall frames set back to back and an air void or gap left remaining between the two, which might be hollow or filled with a combustible sound deadening filler board or sheet. This type of construction creates a greater problem to solve for the electrician.

Check with the local inspector and discuss your options for solving the problem and maintaining the fire integrity of those common area walls and the listings of the electrical components that may be utilized. A layer of gypsum wallboard may be installed in the air void space and adhered to the backside of the studs for one wall and we have now separated out the cavity spaces. Another could incorporate the use of firestop putty pads; but this then must be applied to all boxes in the common cavity space area. Possibly another option is a listed box for that type of construction which includes mineral wool bat insulation to reduce the air movement and products of combustion. The main concern is that a wall opening protective material or installation practice for use by an approved testing laboratory protects the box opening in that fire assembly. All installations shall be made in accordance with the requirements and limitation of the listing. We have been mainly discussing wall areas but the concern also exists for the floor or ceiling penetrations as well.

It has always been stressed that the electrical code is not a design manual, but we often look to the sections and Fine Print Notes for guidance and direction to help solve our problems. The knowledge possessed by the inspector and the contractor regarding fire-stopping techniques can effectively reduce the chance of fires penetrating separation barriers and localize the effects of a fire scenario. Are we taking the time and examining the area from an informed viewpoint or just allowing the question to be answered by others? Ask the necessary questions and review the product listing and classifications to get the correct answer for the type and area where the installation occurs. Time is very precious, especially to someone in a fire scene who is hoping the separation means have been adhered to. Electrical and fire safety have to be our number one objective for code complying installations.

Raymond W. Weber is employed by the state of Wisconsin Department of Commerce Safety and Buildings Division, Electric, and is the Northern District state electrical inspector. He represents IAEI as chairman of CMP-3 of the National Electrical Code®. He is past president of the Western Section of IAEI and is 4th vice president of the Board of Directors                 

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