Question 8.6: A panelboard for a commercial building is supplied by a 250 ......
A panelboard for a commercial building is supplied by a 250 \ ft, four-wire, 480Y/277 \ V,
three-phase feeder. The panel has the following loads:
• A lighting system of a total power of 55 \ kVA with a power factor of 0.90
• A 5 \ kVA resistive heater with a power factor of 1.0
Determine:
(a) The size of the feeder and conduit using THW copper conductors (use the ampacity criterion only).
(b) The voltage drop in the feeder. (Assume a steel conduit.)
(c) The size of the feeder and conduit if a limit for the voltage drop is set to be 1% and the feeder should be able to handle a symmetrical short-circuit current is 35,000 \ A. A circuit breaker is used to protect the feeder.
(a) Size of feeder based on ampacity:
The calculations are summarized in Table 8.3 using the complex power (power triangle) calculation method to account for the effect of power factors for all the loads.
The total demand load is then 73.29 \ kVA with a power factor of 0.913.
The demand current is calculated:
so use #3 THW (Cu) (refer to Table 6.1).
For the conduit, use 11/4-in (refer to Table 6.11).
(b) From Table 6.7, the VD_{L-N}=(0.300+0.196)/2 \ V/1000 \ A-ft for \# 3 \ THW(Cu),
pf = 0.90, and steel conduit. Thus, VD_{L-N}=0.248*I_L*250 \ ft/1000. The actual load is I_L=59538/(1.732*480)=71.6 \ A
VD_{L-N}=0.248*71.6*250 \ ft/1000=4.44 \ V or VD(\%)=4.44/277=1.6\%
(c) Size of feeder taking into account the three criteria (ampacity, short-circuit, and voltage drop):
Ampacity: Based on (a)I=88.2\ A the size of the feeder should be \# 3 \ THW(Cu), as noted in Table 6.1
Table 6.11 indicates, for four wires, the size of the conduit is 1.25”
Short-circuit: Figure 6.7 indicates for Isc, asym = 1.3 * 35,000 = 45,500 A and θ = 2
cycles, the size of the feeder should be: \# 3/0 \ THW(Cu)
Table 6.11 indicates, for four wires, the size of the conduit is 2”
Voltage drop: For VD=1\%,VDL-N=0.01*277-V=2.77 \ V
Since A-ft=71.6*250=17.9*10^3 \ A-ft, VD/1000 \ A-ft=2.77/17.9=0.155
From Table 6.7, for the steel conduit, pf = 0.9, and copper wires,
the size of the feeder should be #1/0 THW (Cu)
Table 6.11 indicates, for 4 wires, the size of the conduit is 2”
Thus, the size of the feeder is 2”C- 4 # 3/0 THW (Cu)
TABLE 6.1
Ampacities for Insulated Conductors Rated 0–2000 V in Free Air
| Tamperature Rating of Conductor | |||||||
| 60°C (140°F) | 75°C (167°F) | 90°C (194°F) | 60°C (140°F) | 75°C (167°F) | 90°C (194°F) | ||
| Size AWG or kcmil | Types TW,UF | Types RHW, THHW, THW, THWN, XHHW, USE, ZW | Types TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2,THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2 | Types TW,UF | Types RHW, THHW, THW, THWN, XHHW, USE | Types TBS, SA, SIS, THHN, THHW, THW-2, THWN-2, RHH, RHW-2, USE-2, XHH, XHHW, XHHW-2, ZW-2 | Size AWG or kcmil |
| Copper
|
Aluminum or Copperclad Alum | ||||||
| 18 | – | – | 14 | – | – | – | – |
| 16 | – | – | 18 | – | – | – | – |
| 14^a | 20 | 20 | 25 | – | – | – | – |
| 12^a | 25 | 25 | 30 | 20 | 20 | 25 | 12^a |
| 10^a | 30 | 35 | 40 | 25 | 30 | 35 | 10^a |
| 8 | 40 | 50 | 55 | 30 | 40 | 45 | 8 |
| 6 | 55 | 65 | 75 | 40 | 50 | 60 | 6 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 | 4 |
| 3 | 85 | 100 | 110 | 65 | 75 | 85 | 3 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 | 2 |
| 1 | 110 | 130 | 150 | 85 | 100 | 115 | 1 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 | 1/0 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 | 2/0 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 | 3/0 |
| (Continued) | |||||||
TABLE 6.1 (Continued)
Ampacities for Insulated Conductors Rated 0–2000 V in Free Air
| Tamperature Rating of Conductor | |||||||
| 60°C (140°F) | 75°C (167°F) | 90°C (194°F) | 60°C (140°F) | 75°C (167°F) | 90°C (194°F) | ||
| Size AWG or kcmil | Types TW,UF | Types RHW, THHW, THW, THWN, XHHW, USE, ZW | Types TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2,THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2 | Types TW,UF | Types RHW, THHW, THW, THWN, XHHW, USE | Types TBS, SA, SIS, THHN, THHW, THW-2, THWN-2, RHH, RHW-2, USE-2, XHH, XHHW, XHHW-2, ZW-2 | Size AWG or kcmil |
| Copper | Aluminum or Copperclad Alum | ||||||
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 | 4/0 |
| 250 | 215 | 255 | 290 | 170 | 205 | 230 | 250 |
| 300 | 240 | 285 | 320 | 190 | 230 | 255 | 300 |
| 350 | 260 | 310 | 350 | 210 | 250 | 280 | 350 |
| 400 | 280 | 335 | 380 | 225 | 270 | 305 | 400 |
| 500 | 320 | 380 | 430 | 260 | 310 | 350 | 500 |
| 600 | 355 | 420 | 475 | 285 | 340 | 385 | 600 |
| 700 | 385 | 460 | 520 | 310 | 375 | 420 | 700 |
| 750 | 400 | 475 | 535 | 320 | 385 | 435 | 750 |
| 800 | 410 | 490 | 555 | 330 | 395 | 450 | 800 |
| 900 | 435 | 520 | 585 | 355 | 425 | 480 | 900 |
| 1000 | 455 | 545 | 615 | 375 | 445 | 500 | 1000 |
| 1250 | 495 | 590 | 665 | 403 | 485 | 545 | 1250 |
| 1500 | 520 | 625 | 703 | 435 | 520 | 585 | 1500 |
| 1750 | 545 | 650 | 735 | 455 | 545 | 615 | 1750 |
| 2000 | 560 | 665 | 750 | 470 | 560 | 630 | 2000 |
| (Continued) | |||||||
TABLE 6.1 (Continued)
Ampacities for Insulated Conductors Rated 0–2000 V in Free Air
| Tamperature Rating of Conductor | |||||||
| 90°C (194°F) | 75°C (167°F) | 60°C (140°F) | 90°C (194°F) | 75°C (167°F) | 60°C (140°F) | ||
| Size AWG or kcmil | Types TBS, SA, SIS, THHN, THHW, THW-2, THWN-2, RHH, RHW-2, USE-2, XHH, XHHW, XHHW-2, ZW-2 | Types RHW, THHW, THW, THWN, XHHW, USE | Types RHW, THHW, THW, THWN, XHHW, USE, ZW | Types TBS, SA, SIS, FEP, FEPB, MI, RHH, RHW-2, THHN, THHW, THW-2,THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2 | Types TW,UF | Types TW,UF | Size AWG or kcmil |
| Aluminum or Copperclad Alum | Copper | ||||||
| Correction Factors | |||||||
| Ambient Temp (°F) | For Ambient Temperatures Other Than 30°C (86°F),Multiply the Allowable Ampacities Shown Above by the Appropriate Factor Shown Below | Ambient Temp.(°C) | |||||
| 70-77 | 1.04 | 1.05 | 1.08 | 1.04 | 1.05 | 1.08 | 21-25 |
| 78-86 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 26-30 |
| 87-95 | 0.96 | 0.94 | 0.91 | 0.96 | 0.94 | 0.91 | 31-35 |
| 96-104 | 0.91 | 0.88 | 0.82 | 0.91 | 0.88 | 0.82 | 36-40 |
| 105-113 | 0.87 | 0.82 | 0.71 | 0.87 | 0.82 | 0.71 | 41-45 |
| 114-122 | 0.82 | 0.75 | 0.58 | 0.82 | 0.75 | 0.58 | 46-50 |
| 123-131 | 0.76 | 0.67 | 0.41 | 0.76 | 0.67 | 0.41 | 51-55 |
| 132-140 | 0.71 | 0.58 | – | 0.71 | 0.58 | – | 56-60 |
| 141-158 | 0.58 | 0.33 | – | 0.58 | 0.33 | – | 61-70 |
| 159-176 | 0.41 | – | – | 0.41 | – | – | 71-80 |
| Note: This table is adapted from NEC-2014 Table 310.15(B)(16)(NEC,2014) | |||||||
| ^a For branch circuits rated 15-A,20-A,and 30-A, conductors of at least respectively ,14, 12, and 10 AWG should be used | |||||||
TABLE 6.7
Cross-Sectional Areas for Conduits Associated with 40% Fill
| PVC in.² (mm²) | IMT Conduit in.² (mm²) | EMT Conduit in.²(mm²) | Trade Conduit Size |
| 0.087 (56) | 0.137 (89) | 0.122 (78) | ½ |
| 0.164 (105) | 0.235 (151) | 0.213 (137) | ¾ |
| 0.275 (178) | 0.384 (248) | 0.346 (222) | 1 |
| 0.495 (320) | 0.659(425) | 0.598 (387) | 1^1/4 |
| 0.684 (442) | 0.890 (573) | 0.814 (526) | 1½ |
| 1.150 (742) | 1.452 (937) | 1.342 (866) | 2 |
| 1.647 (1064) | 2.054 (1323) | 2.343 (1513) | 2½ |
| 2.577 (1660) | 3.169 (2046) | 3.538 (2280) | 3 |
| 3.475 (2243) | 4.234 (2729) | 4.618 (2980) | 3½ |
| 4.503 (2907) | 5.452 (3490) | 5.901 (3808) | 4 |
| Note: This table is adapted from NEC-2014 Table 4, Chapter 9 (NEC, 2014). | |||
TABLE 6.11
NEC Minimum Loads for General Lighting for Select Occupancy Types
| General Lighting Load | Occupancy Type | |
| VA/ft² | VA/m² | |
| 1.0 | 11.0 | Auditoriums |
| 3.5 | 39.0 | Banks^a |
| 1.0 | 11.0 | Churches |
| 2.0 | 22.0 | Court rooms |
| 3.0 | 33.0 | Dwelling units^b |
| 2.0 | 22.0 | Hospitals |
| 2.0 | 22.0 | Hotels and motels (rooms without cooking appliances) |
| 1.5 | 17.0 | Lodge rooms |
| 3.5 | 39.0 | Office buildings^a |
| 2.0 | 22.0 | Restaurants |
| 3.0 | 33.0 | Schools |
| 3.0 | 33.0 | Storesc |
| 0.25 | 3.0 | Warehouses for storage |
| 0.50 | 6.0 | Other spaces such as halls, corridors, stairways, and closets |
| Note: This table is adapted from NEC-2014 Table 220.12 (NEC, 2014). ^a For banks and office buildings, the larger loads estimated the values in this table and those estimated values based on actual connected lighting fixtures and recep tacles should be utilized in the design specifications.
^b For dwellings, refer to Chapter 5 for more details on the design specifications for branch circuits and service entrance feeders. ^c For stores, additional show windows loads can be estimated using 200 VA/0.30 m (1 ft). |
||
TABLE 8.3
Calculation of the Demand Load for Panel Feeder of Example 8.6
| Demand Load | DF | Connected Load | PF | kVA | Loads |
| 61.88 + j29.97 | 1.25 | 49.5 + j23.97 | 0.90 | 55 | Lighting |
| 5.0 + j0.0 | 1.0 | 5.0 + j0.0 | 1.00 | 5.0 | Heater |
| 66.88 + j29.97 | 54.5 + j23.97 | Total |
