1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA PRP 4-2009 (R2016)Expansion Fittings for PVC Rigid Nonmetallic ConduitNEMA PRP 4-2009 (R2016) Page 1 A NEMA Polymer Raceway Products Document PRP 4-2009 (R2016) Expansion Fittings for PVC Rigid Nonmetallic Conduit Published
2、by National Electrical Manufacturers Association 1300 North 17thStreet, Suite 900 Rosslyn, Virginia 22209 www.nema.org 2016 National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention f
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13、 4-2009 (R2016) Page 1 2016 National Electrical Manufacturers Association Expansion Fittings for PVC Rigid Nonmetallic Conduit All construction materials expand and contract with fluctuations in temperature. These temperature changes affect Polyvinyl Chloride (PVC) more than traditional materials su
14、ch as aluminum and steel. For example, the coefficient of thermal expansion for PVC (3.38 x 10-5in./in./F) is almost three times that of aluminum and six times that of steel. Thermal expansion and contraction have a significant effect on long straight runs of conduit. For this application, expansion
15、 fittings are used to accommodate changes in length. This prevents potentially harmful thermal stresses in the conduit itself or in any supporting structures. Expansion Fittings Required Section 352.44 of the National Electrical Code(NEC) requires that expansion fittings be installed for PVC rigid n
16、onmetallic conduit to compensate for thermal expansion and contraction where the length change is anticipated to be 6 mm (1/4 inch) or greater, in accordance with Table 352.44 of the NEC, in a straight run between securely mounted items. Securely mounted items are boxes, cabinets, elbows, or other c
17、onduit terminations. When Are Expansion Fittings Used? The importance of knowing when to use an expansion fitting is as important as knowing how to use an expansion fitting. Expansion fittings are used in aboveground installations where temperature changes are anticipated to exceed 25F (14C). Rigid
18、PVC conduit has a coefficient of expansion of 3.38 x 105in./in./F. A 10 foot length will change in length about 3/8” with a change in temperature of 100F. This equates to more than 4 inches over a 100 foot run. Changes in conduit temperature outdoors can exceed 140F, considering the heat of summer a
19、nd cold of winter. This means a 100 foot run of conduit could change approximately 6 inches in length between summer and winter. If not compensated for with expansion fittings, this change in length can cause unsightly conduit bowing, distorted boxes, or broken couplings or fittings. The anticipated
20、 expansion and contraction for various lengths of runs and temperature changes are given in Table 352.44 of the NEC (See table below.) BarrelPistonMuzzleNEMA PRP 4-2009 (R2016) Page 2 2016 National Electrical Manufacturers Association Expansion Characteristics of PVC Rigid Nonmetallic Conduit Coeffi
21、cient of Thermal Expansion = 3.38 x 10-5in./in./F Temperature Change Degrees F Length Change in Inches per 100 ft of PVC Conduit Temperature Change in Degrees F Length Change in Inches per 100 ft of PVC Conduit Temperature Change in Degrees F Length Change in Inches per 100 ft of PVC Conduit Tempera
22、ture Change in Degrees F Length Change in Inches per 100 ft of PVC Conduit 5 0.2 55 2.2 105 4.2 155 6.3 10 0.4 60 2.4 110 4.5 160 6.5 15 0.6 65 2.6 115 4.7 165 6.7 20 0.8 70 2.8 120 4.9 170 6.9 25 1.0 75 3.0 125 5.1 175 7.1 30 1.2 80 3.2 130 5.3 180 7.3 35 1.4 85 3.4 135 5.5 185 7.5 40 1.6 90 3.6 14
23、0 5.7 190 7.7 45 1.8 95 3.8 145 5.9 195 7.9 50 2.0 100 4.1 150 6.1 200 8.1 For Conduit Installed Outdoors, the maximum and minimum air temperature expected can be used to calculate the range of expansion and contraction. Conduit installed outdoors in direct sunlight will be considerably hotter than
24、air temperature during the day because of the absorption of the suns heat. Expansion fittings should be installed to allow for the anticipated temperature change. For direct sunlight exposure, add 30F to ambient air temperature change. Where Installed Inside Buildings, the conduit temperature is the
25、 ambient air temperature plus the heat contributed by the conductors inside the conduit. Generally, expansion fittings are not required inside unless conduit is located in an area of widely varying temperatures. Attics of buildings are areas where temperature swings exceeding 100F are common and exp
26、ansion fittings are required. Proper Position Expansion fittings are usually installed in a horizontal position. However, if they must be put in a vertical application, the open end should be securely fastened in the down position. The expansion coupling must be installed close to the top of the run
27、 with the barrel pointing down, so that rain water does not enter into the opening. The lower end of the conduit run should be secured at the bottom. This will result in an upward movement because of the length change caused by temperature variations. Length of Run Inches of Expansion Allowance Need
28、ed for Temperature Changes of: feet 60F 80F 100F 120F 140F 25 1 1 1 50 1 1 2 2 2 75 1 2 3 3 4 100 2 3 4 4 5 125 3 4 5 6 7 150 3 4 6 7 8 Add 30F to temperature change for direct sunlight exposure. NEMA PRP 4-2009 (R2016) Page 3 2016 National Electrical Manufacturers Association Piston Opening Pipe st
29、rapmounted looselyto allow movementPipe-strapshould be anchoredtightly to barrelSetting Position of Piston If an expansion fitting was installed completely closed with the piston bottomed on a cool day, there would be no travel available when the conduit warmed up and expanded. If it was installed i
30、n a maximum open position on a hot day, the expansion fitting would pull apart when cooled. Thus, “setting” the expansion fitting is very important. In addition to the proper number and sizes of expansion fittings chosen, they must also be installed so they can accommodate the expected length change
31、. This requires proper “setting” of the expansion fitting. For example, the expansion fitting is installed at 65F, halfway between the anticipated maximum temperature of 125F and minimum temperature of 5F. In this case, the expansion fitting would be installed or set with the inner piston halfway ou
32、t. Expansion fittings are marked with a mid-point ring. If it was a colder day, for example 35F, with the same expected temperature change range, the inner piston would be set with of its maximum travel out. If the temperature was hot (95F), it would be set with the inner piston only way out. Settin
33、g Expansion Fitting for Temperature A. For medium temperature (65 to 75F) set piston way in barrel. 3/4 1/2 1/41/4 1/2 3/4 MAXB. For cold temperature (20 to 30F) set piston of the way in barrel. 3/4 1/2 1/43/41/21/4 MAXC. For hot temperature (95 to 105 F) set piston of the way in barrel. 3/43/41/2 1
34、/41/4 1/2 MAXsolvent weld solvent weld mid-point ring NEMA PRP 4-2009 (R2016) Page 4 2016 National Electrical Manufacturers Association When a more precise calculation for expansion is needed, it can be calculated as follows: 1. Determine the maximum temperature range. This is done by subtracting th
35、e lowest temperature you anticipate from the highest temperature you anticipate. Example: Lets say in winter, we expect the temperature to get down to 10F and in the summer, we expect the temperature to get as high as 110F. The maximum temperature range is 110 (10) = 120F. 2. Determine the length of
36、 run between fixed points. Lets assume 90 feet for the above example. 3. Calculate the total expansion change: Total expansion change in inches = (coefficient of thermal expansion) (maximum temperature range) (length of run in inches). For the above example: Total expansion change = (3.38 10-5in./in
37、./F) (120F) (90 ft 12 in./ft) = 4.38 in. Thus, travel can be covered with one 6-inch expansion fitting. 4. Determine the “set position” as shown in the following example. For the above example: If the temperature at installation was 65F, the difference from maximum is 110F 65F = 45F. Use ratio: (dif
38、ference from maximum)/(total expected range) = 45/120 = .375 in. (.375) x 6-inch travel = 2.25 in. The piston at 65F would retract 2.25 inches. NEMA PRP 4-2009 (R2016) Page 5 2016 National Electrical Manufacturers Association Installation 1. The expansion fitting should be mounted so that the piston
39、 can travel in a straight line with the barrel. If alignment is not straight, the piston will bind and prevent the fitting from working properly. 2. The expansion fitting barrel must be clamped securely whereas the conduit is mounted loosely so that it can slide freely during expansion and contracti
40、on. 3. Nonmetallic conduit straps suitable for this purpose, must be used to allow the conduit to move freely during expansion and contraction while properly securing it. Box1Box2Expansion joint barrel fixed rigidRun conduit through loose supportLocation Proper placement and installation of expansio
41、n fittings and conduit allow for correct functioning of the expansion fittings. For example, if one expansion fitting is needed between two boxes, the barrel of the fitting must be securely fastened close to one box. The conduit must be fastened loosely so as to allow for movement during expansion a
42、nd contraction. If there is more than one expansion fitting used, then fittings are installed at the boxes with the center of the conduit run rigidly fixed. With two expansion fittings utilized, the center of the conduit must be fixed so as to properly proportion expansion or contraction between the
43、 two fittings. Otherwise, the expansion or contraction would only occur at the free fitting, while the tighter fitting would not move. In a very long run, two or more fittings should be placed in series with one another. The barrels of each expansion fitting must be anchored so as to allow each leng
44、th of conduit to expand or contract freely. Each section acts independently. Wire Inside the Conduit The coefficient of expansion for the wire inside the conduit is much lower than that of the nonmetallic conduit; therefore, its length change due to expansion and contraction is very little. If the e
45、xpansion fittings are mounted correctly, there should be little concern as to the effect of thermal expansion on the wires inside. Use of Expansion Fittings Where Concrete Encased or Direct Buried In normal circumstances, expansion fittings are not needed with concrete-encased nonmetallic conduit si
46、nce the movement of the conduit will coincide with the expansion and contraction of the concrete. Two Expansion Fittings One Expansion Fitting Box1Box2Expansion joint barrel fixed rigidRun conduit through loose supportNEMA PRP 4-2009 (R2016) Page 6 2016 National Electrical Manufacturers Association
47、In direct burial situations, expansion fittings are not needed because the ground has relatively constant temperatures. In cold areas, buried lines must be below the frost line to prevent buckling during freezing and thawing cycles. In warmer areas, the depth of cover protects against temperature ex
48、tremes. Expansion/deflection fittings have been used in structural construction joints in building, bridges, parking garages, and other structures. These applications are unique and require the approval of the professional design engineers, and building and electrical inspectors. Most Common Mistake
49、 The most common error is not using enough expansion fittings. When in doubt, use an additional expansion fitting. It is much more difficult and costly to insert an expansion fitting once wiring has been set up and is in service. NATIONAL ELECTRICAL MANUFACTURERS ASSOCIATION 1300 NORTH 17TH STREET, SUITE 900 ROSSLYN. VA 22209www.NEMA.orgTO ORDER ADDITIONAL NEMA STANDARDS VISITWWW.GLOBAL.IHS.COM OR CALL 1-800-854-7179/1-303-397-79565612_0514TB
