Branch Circuits, wire connections and devices Flashcards
An electrical conductor is made up of 19 strands each with a diameter of 0.0837 inches. What is the approximate circular mil area of the conductor?
- Convert inches to mils to get circular mils
- That gives the area of one strand
- Multiply that number by 19 strands
1inch = 1/1000 of a mil
0.0837 = 83.7 mil
cmil = mil squared = 1 strand
83.7mils x 83.7mils = 7005.7 cmils /per strand
19strands x 7005.7cmils = 133,108 cmil
A circuit consisting of three copper THHN wires is run in electrical metallic tubing (EMT)
and is serving a continuous load of 44 amperes. There are no other conductors in this
raceway, shown in Figure 14. If a 60 ampere overcurrent device protects the circuit, and
all terminations are 75EC rated, the minimum size wire permitted for the circuit is:
A. 8 AWG. C. 4 AWG. E. 2 AWG.
B. 6 AWG. D. 3 AWG.
B 6 AWG
The minimum size conductor permitted for a branch circuit is specified in 210.19 (A) as
125% of the continuous load plus any non-continuous load.
44 A × 1.25 = 55 A
The conductor size is found in Table 310.16, but first it must be determined which column
to use. Section 310.15(A)(2) specifies that when more than one condition for determining
ampacity of a conductor exists, the most limiting will apply to the entire circuit. The
terminations are rated 75EC and the conductor insulation is rated 90EC. In this case the
75EC column of Table 310.16 will apply and the minimum conductor size is 6 AWG. The
conductor must have an ampacity not less than the minimum calculated value which in
this case was 55 amperes, and it must also be protected with an overcurrent device not
greater than the ampacity of the conductor which in this case is 60 amperes (see 240.4).
In this case the size 6 AWG is has high enough ampacity for all requirements.
A circuit consisting of three copper THHN wires is run in electrical metallic tubing and
supplies a 44 ampere continuous load. There are six other wires with THHN insulation in
the same run of EMT for a total of nine current-carrying conductors as shown in Figure 14.
If a 60 ampere overcurrent device protects the circuit, and all terminations are 75EC rated,
the minimum size wire permitted for the circuit is:
A. 10 AWG. C. 6 AWG. E. 3 AWG.
B. 8 AWG. D. 4 AWG.
C 6 AWG
This is the same branch circuit as in the previous question except this time there are six
extra conductors in the raceway so the adjustment factors of Table 310.15(B)(2)(a) will
apply. With nine conductors in the raceway, the values of ampacity in Table 310.16 must
be reduced to 70% of their listed value for this situation. There are no 75EC terminations
in the raceway, and the conductor insulation is rated 90EC, therefore, the 90EC column of
Table 310.16 can be used for the purpose of applying the adjustment factors, which for a
size 6 AWG copper conductor is 75 ampere.
75 A × 0.7 = 53 A
The requirement in 210.19(A) that the conductor ampacity was required to be 125% of the
continuous load only applied when determining the minimum conductor size before the
adjustment factor was applied. Therefore, size 6 AWG copper conductor is acceptable in
this case because the adjusted ampacity is 53 amperes and the load is only 44 amperes.
The living area of a single family dwelling is 2680 sq. ft, and all general illumination circuits
are rated 15 amperes. The minimum number of general illumination circuits permitted is:
A. four. C. six. E. eight.
B. five. D. seven.
B five
Section 220.3(A) and 210.11(A) specify that the minimum load for general illumination in a
dwelling is not to be less than the living area of the dwelling times the volt-ampere value
found in Table 220.3(A). The divide the load by 120 volts to determine the minimum
ampacity. To determine the minimum number of circuits, divide by the rating of the circuit
which in this case is 15 amperes.
3 VA/ft2 × 2680 ft2 = 8040 VA
8040 VA / 120 V × 15 A = 4.5 circuits
A commercial building has 96 general purpose, 120 volt duplex receptacles supplied by 20
ampere branch circuits. The minimum number of circuits permitted for this load is:
A. six. C. eight. E. ten.
B. seven. D. nine.
C eight
The minimum load that is permitted to be applied to general use receptacles with no
specific load in other than dwellings is 180 VA per receptacle strap (220.3(B)(9)). A
duplex receptacle is one receptacle strap and only counts as 180 VA. Simply multiply the
180 VA by the number of receptacles, divide by 120 volts and divide by the rating of the
circuit.
min cir = 96 recept × 180 VA / 120V x 20A = 7.2
