Service and Feeders Flashcards
One building on the same property is supplied 120/240V single phase power from another building. There is no metallic water pipe or other metal equipment connecting the two buildings that would form a parallel path for neutral current. If the second building is supplied from a 3wire feeder (two ungrounded conductors and a neutral), the neutral conductor is:
Bond and ground the neutral at the second building.
250.32(B)(2)
The neutral conductor is required to be bonded to the disconnect enclosure in the second building and connected to a grounding electrode when the grounded conductor acts both as the neutral and the equipment grounding conductor.
One building on the same property is supplied 120/240V power from another building. If the second building is supplied from a 4 wire feeder (two ungrounded, a neutral, and an equipment grounding conductor), at the second building the neutral is:
Ground the neutral only at the supply end.
250.32(B)(1)
A feeder consists of 500Kcmil copper RHW conductors protected with a 400A CB. A tap is made to the feeder to supply a panelboard with a 100A main CB. The distance from the tap point to the circuit breaker is 22ft. The minimum size copper THWN wires permitted to be iinstalled between the tap point and the panelboard is:
1/0 AWG
25ft tap rule- 240.21(B)(2)
The 100A OC device supplied only requires 1AWG, but the tap conductor is ahead of the 100A OC device and is only protected by the 400A CB. The tap conductor is required to have an ampacity mot less than one-third that of the feeder O.C. device.
400A/3 = 133A
Then look to Table 310.16 ampacity
133 = 150A = 1/0AWG
A commercial building is served with a single-phase, 3-wire, 120/240 volt service with a
150 ampere main circuit breaker, and size 1/0 AWG copper ungrounded service
conductors. The calculated unbalanced demand load for the building is 32 amperes. If all
service wires are copper with 75EC insulation and terminations, the minimum size neutral
permitted is:
A. 6 AWG. C. 3 AWG. E. 1 AWG.
B. 4 AWG. D. 2 AWG.
A 6 AWG
There is a minimum neutral conductor size no matter the calculated unbalanced load.
The minimum neutral conductor size is specified in 230.42(C). The minimum is never
permitted to be smaller than the size of grounding electrode conductor as found in Table
250.66.
A single-family dwelling is served by a single-phase, 120/240 volt 3-wire service. The
living area of the dwelling is 2400 sq. ft, and the dwelling contains a 12 kW electric range,
a 3.5 kW, 240 volt electric water heater, a ½ horsepower, 120 volt garbage disposer, a 1.2
kVA, 120 volt dishwasher, a 5 kW clothes dryer, a 1½ horsepower, 240 volt central airconditioner,
and a a horsepower, 120 volt furnace blower motor. The minimum load that
is required to be included for general illumination, small appliances, and laundry, before applying demand factors, is:
A. 1,500 VA. C. 4,500 VA. E. 11,700 VA.
B. 3,000 VA. D. 7,200 VA.
E 11,700 VA
The general illumination load is 3 VA per square foot as found in 220.3(A) and Table
220.3(A). The small appliance load is 1500 VA for each small appliance circuit according
to 220.16(A). Section 210.11(C)(1) required a minimum of two small appliance branch
circuits. Section 220.16(B) requires 1500 VA for the laundry circuit in the dwelling.
Section 210.11(C)(2) requires one laundry circuit in a dwelling.
General illumination 2400 ft2 × 3 VA/ft2 = 7,200
Small appliance 3,000 VA
Laundry +1,500 VA
=11,700 VA
A feeder supplies 92 amperes of continuous load and 70 amperes of
noncontinuous load. Determine the minimum rating of overcurrent device permitted for this
feeder.
200A
The feeder consists of three type THHN copper current-carrying conductors in
raceway. The minimum rating of overcurrent device is required to be not smaller than
125% of the continuous load plus the noncontinuous load. The calculation for this feeder is as follows:
92A x 1.25 = 115A
115A + 70A = 185A
The overcurrent device is required to be at least 185 amperes. From 240.6, the next higher standard rating is 200 amperes.
Consider the previous example where a feeder supplies a 90 ampere continuous load and a 70 ampere noncontinuous load. The wire is THHW copper run in EMT and the termination ratings are 75EC. Determine the minimum wire size assuming no adjustment factors apply to the feeder.
3/0 AWG
90A x 1.25 = 115
115A + 70A = 185A
The wire is required to have an ampacity not less than 125% of the continuos load plus the noncontinuous load which was determined in the previous example to be 185 amperes. Since we must look at the complete circuit, the 75EC rated terminations will determine the column to use in Table 310.16. The minimum conductor size in this case is size 3/0 AWG.
Check to make sure the overcurrent rule of 240.4 is satisfied.
A set of feeder conductors is protected by a set of 400 ampere fuses and the calculated load on the feeder is 340 amperes. If the feeder conductors are copper with 75C insulation and terminations, and no derating factors apply, the minimum size conductor permitted for this feeder is:
A. 300 kcmil. B. 350 kcmil. C. 400 kcmil. D. 500 kcmil. E. 600 kcmil.
500Kcmils
The conductor is rated at 380 amperes but it is permitted to be protected at 400 amperes because the load is only 340 amperes. This is according to Section 240.4(B). Other Code sections are 240.6(A) and Table 310.16. MRC — Table E3605.1.
The table only goes up to size 4/0 AWG. For there to be an answer from the MRC there would have to be a set of two parallel 3/0 AWG.
The minimum size type THWN aluminum conductor which is permitted to be installed for a 200 ampere, 120/240 volt single-phase single-family dwelling service entrance that has a calculated demand load of 150 amperes is:
A. 250 kcmil. B. 1/0 AWG. C. 2/0 AWG. D. 3/0 AWG. E. 4/0 AWG.
4/0 AWG
The key here is residential, single-phase, 120/240 volt, 3-wire. The conductor size is found in Table 310.12(A).
83% of A rating from 310.12(A)
200 x .83 = 166A - 4/0
