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

Question

A panelboard for a commercial building is supplied by a [latex]250 \ ft[/latex], four-wire, [latex]480Y/277 \ V[/latex],
three-phase feeder. The panel has the following loads:
• A lighting system of a total power of [latex]55 \ kVA[/latex] with a power factor of 0.90
• A [latex]5 \ kVA[/latex] 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 [latex]35,000 \ A[/latex]. A circuit breaker is used to protect the feeder.

Accepted Answer

(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 [latex]73.29 \ kVA[/latex] with a power factor of 0.913.
The demand current is calculated:

[latex]I_L=73,290 \ VA/(1.732*480)=88.2 \ A[/latex]

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 [latex]VD_{L-N}=(0.300+0.196)/2 \ V/1000 \ A-ft[/latex] for [latex]\# 3 \ THW(Cu)[/latex],
pf = 0.90, and steel conduit. Thus, [latex]VD_{L-N}=0.248*I_L*250 \ ft/1000[/latex]. The actual load is [latex]I_L=59538/(1.732*480)=71.6 \ A[/latex]
[latex]VD_{L-N}=0.248*71.6*250 \ ft/1000=4.44 \ V[/latex] or [latex]VD(\%)=4.44/277=1.6\%[/latex]

(c) Size of feeder taking into account the three criteria (ampacity, short-circuit, and voltage drop):

Ampacity: Based on (a)[latex]I=88.2\ A[/latex] the size of the feeder should be [latex]\# 3 \ THW(Cu)[/latex], 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 [latex]Isc, asym = 1.3 * 35,000 = 45,500 A[/latex] and θ = 2
cycles, the size of the feeder should be: [latex]\# 3/0 \ THW(Cu)[/latex]
Table 6.11 indicates, for four wires, the size of the conduit is 2”

Voltage drop: For [latex]VD=1\%,VDL-N=0.01*277-V=2.77 \ V[/latex]

Since [latex]A-ft=71.6*250=17.9*10^3 \ A-ft, VD/1000 \ A-ft=2.77/17.9=0.155[/latex]

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)

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)

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)

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

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).

General Lighting Load		Occupancy Type
VA/ft²	VA/m²	Occupancy Type
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).

Question 8.6: A panelboard for a commercial building is supplied by a 250 ......

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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