Example D3(a) Industrial Feeders in a Common Raceway

An industrial multi-building facility has its service at the rear of its main building, and then provides 480Y/277-volt feeders to additional buildings behind the main building in order to segregate certain processes. The facility supplies its remote buildings through a partially enclosed access corridor that extends from the main switchboard rearward along a path that provides convenient access to services within 15 m (50 ft) of each additional building supplied. Two building feeders share a common raceway for approximately 45 m (150 ft) and run in the access corridor along with process steam and control and communications cabling. The steam raises the ambient temperature around the power raceway to as much as 35°C. At a tee fitting, the individual building feeders then run to each of the two buildings involved. The feeder neutrals are not connected to the equipment grounding conductors in the remote buildings. All distribution equipment terminations are listed as being suitable for 75°C connections.

Each of the two buildings has the following loads:

Lighting, 11,600 VA, comprised of electric-discharge luminaires connected at 277 V

Receptacles, 22 125-volt, 20 ampere receptacles on general-purpose branch circuits, supplied by separately derived systems in each of the buildings

1-Air compressor, 460 volt, three phase, 7.5 hp

1-Grinder, 460 volt, three phase, 1.5 hp

3-Welders, AC transformer type (nameplate: 23 amperes, 480 volts, 60 percent duty cycle)

3-Industrial Process Dryers, 480 volt, three phase, 15 kW each (assume continuous use throughout certain shifts)

Determine the overcurrent protection and conductor size for the feeders in the common raceway, assuming the use of XHHW-2 insulation (90°C):

Calculated Load {Note: For reasonable precision, volt ampere calculations are carried to three significant figures only; where converted to amperes, the results are rounded to the nearest ampere [see 220.5(B)]}.

Noncontinuous Loads

 

Receptacle Load (see 220.44)

 

22 receptacles at 180 VA

3,960 VA

Welder Load [see 630.11(A), Table 630.11(A)]

 

Each welder: 480V x 23A x 0.78

= 8,610 VA

All 3 welders: [see 630.11(B)]

 

(demand factors 100%, 100%, 85% respectively)

 

8,610 VA + 8,610 VA + 7,320 VA

= 24,500 VA

Subtotal, Noncontinuous Loads

28,500 VA

Motor Loads (see 430.24, Table 430.250)

 

Air compressor: 11 A x 480 V x 3 =

9,150 VA

Grinder: 3 A x 480 V x 3 =

2,490 VA

Largest motor, additional 25%:

2,290 VA

Subtotal, Motor Loads

13,900 VA

By using 430.24, the motor loads and the noncontinuous loads can be combined for the remaining calculation.

Subtotal for load calculations,

42,400 VA

Noncontinuous Loads

 

Continuous Loads

 

General Lighting

11,600 VA

3 Industrial Process Dryers 15 kW each

45,000 VA

Subtotal, Continuous Loads:

56,600 VA

Overcurrent protection (see 215.3)

The overcurrent protective device must accommodate 125% of the continuous load, plus the noncontinuous load:

Continuous load

56,600 VA

Noncontinuous load

42,400 VA

Subtotal, actual load (actual load in amperes:

99,000 VA

99,000 VA / (480V x 3) = 119 A]

 

14,200 VA (25% of 56,400 VA) (See 215.3)

14,200 VA

Total VA

113,200 VA

Conversion to amperes using three significant figures: 113,400 VA / (480V x 3) = 136 A

 

Minimum size overcurrent protective device: 136 A

 

Minimum standard size overcurrent protective device (see 240.6): 150 amperes

 

Where the overcurrent protective device and its assembly are listed for operation at 100 percent of its rating, a 125 ampere overcurrent protective device would be permitted. However, overcurrent protective device assemblies listed for 100 percent of their rating are typically not available at the 125-ampere rating. (See 215.3 Exception.)

Ungrounded Feeder Conductors

The conductors must independently meet requirements for (1) terminations, and (2) conditions of use throughout the raceway run.

Minimum size conductor at the overcurrent device termination [see 110.14(C) and 215.2(A)(1), using 75°C ampacity column in Table 310.16]: 1/0 AWG.

Minimum size conductors in the raceway based on actual load [see Article 100, Ampacity, and 310.15(B)(2)(a) and correction factors to Table 310.16]:

99,000 VA / 0.7 / 0.96 = 147,000 VA

(70% = 310.15(B)(2)(a)) & (0.96 = Correction factors to Table 310.16)

Conversion to amperes:

147,000 VA / (480V x images/ent/U221A.GIF border=0>3) = 177A

Note that the neutral conductors are counted as current-carrying conductors [see 310.15(B)(4)(c)] in the example because the discharge lighting has substantial nonlinear content. This requires a 2/0 AWG conductor based on the 90°C column of Table 310.16. Therefore, the worst case is given by the raceway conditions, and 2/0 AWG conductors must be used. If the utility corridor was at normal temperatures [(30°C (86°F)], and if the lighting at each building were supplied from the local separately derived system (thus requiring no neutrals in the supply feeders) the raceway result (99,000 VA / 0.8 = 124,000 VA; 124,000 VA / (480V x images/ent/U221A.GIF border=0>3) = 149 A, or a 1 AWG conductor @ 90°C) could not be used because the termination result (1/0 AWG based on the 75°C column of Table 310.16) would become the worst case, requiring the larger conductor.

In every case, the overcurrent protective device shall provide overcurrent protection for the feeder conductors in accordance with their ampacity as provided by this Code (see 240.4). A 90°C 2/0 AWG conductor has a Table 310.16 ampacity of 195 amperes. Adjusting for the conditions of use (35°C ambient temperature, 8 current-carrying conductors in the common raceway),

195 amperes x 0.96 x 0.7 = 131 A

The 150-ampere circuit breaker protects the 2/0 AWG feeder conductors, because 240.4(B) permits the use of the next higher standard size overcurrent protective device. Note that the feeder layout precludes the application of 310.15(A)(2) Exception.

Feeder Neutral Conductor (see 220.61)

Because 210.11(B) does not apply to these buildings, the load cannot be assumed to be evenly distributed across phases. Therefore the maximum imbalance must be assumed to be the full lighting load in this case, or 11,600 VA. (11,600 VA / 277V = 42 amperes.) The ability of the neutral to return fault current [see 250.32(B)(2)(2)] is not a factor in this calculation.

Although the neutral runs between the main switchboard and the building panelboard, likely terminating on a busbar at both locations, the busbar connections are part of listed devices and are not "separately installed pressure devices." Therefore 110.14(C)(2) does not apply, and the normal termination temperature limits apply. In addition, the listing requirement to gain exemption from the additional sizing allowance under continuous loading (see 215.3 Exception) covers not just the overcurrent protective device, but its entire assembly as well. Therefore, since the lighting load is continuous, the minimum conductor size is based on 1.25 x (11,600 VA/277V) = 52 amperes, to be evaluated under the 75°C column of Table 310.16. The minimum size of the neutral is 6 AWG. This size is also the minimum size required by 215.2(A)(1), because the minimum size equipment grounding conductor for a 150-ampere circuit, as covered in Table 250.122, is 6 AWG.





Pocket Guide to the National Electrical Code 2005
Pocket Guide to the National Electrical Code(R), 2005 Edition (8th Edition)
ISBN: 0131480014
EAN: 2147483647
Year: 2004
Pages: 120
Flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net