Load Calculations Branch And Feeder Circuits:
Questions With Solutions
Maximum load that may be supplied by 20a, 120v circuit? Right Ans - 20a x
120v = 2400va
Maximum load that may be supplied by 20a, 3 wire, 120/240v circuit?
Right Ans - 20a x 240v = 4800va
Maximum load that may be supplied by 20a, 208v 3 phase circuit? Right
Ans - 20a x 208v x 1.732 = 7205.12va
Minimum size circuit breaker required for 9 florescent ballast rated at 1.5a at
120v, as a continuous load? Right Ans - determine branch circuit load
9 x 1.5a = 13.5a
Determine the continuous duty load
13.5a x 1.25 = 16.88a
Minimum breaker size is 20a
How many receptacle outlet devices in other than residential occupancies can
be connected to a 20a, 120v circuit? Right Ans - determine branch circuit
capacity
20a x 120v = 2400va
NEC section 220.14.i each outlet is assigned a lad of 180va so
2400va / 180va = 13.33 or 13 receptacle outlets
What is the minimum size branch circuit required to serve a 17a, 120v copy
machine? Right Ans - nec sections 210.23a1 and b, the rating of any one
cord connected utilization equipment shall not exceed 80% of the branch
circuit rating.
30a x 0.8 = 24a so the minimum size circuit is 30a
,Maximum lighting load connected to a 20a circuit supplying equipment with a
rating of 8.5a, 120v? Right Ans - 20a - 8.5a = 11.5, the equipment load is
only 50% of the circuit rating so 11.5% may be added.
minimum size branch circuit required to supply a 14.7a, 208v, 3 phase,
continuous load restaurant dishwasher? Right Ans - 14.7 x 1.25 = 18.38a,
so 20a is the minimum
derated ampacity for each of 8, no.12 thhn wire in a single conduit? Right
Ans - 310.15b16 ampacity is 30a
310.15b3a adjustment factor is 70%
70% or 30a = 30a x 0.7 = 21a
maximum load connected on each of 6, no.2 thwn wires in a single conduit?
Right Ans - 310.15b16 ampacity is 115a
310.15b3a adjustment factor 80%
80% of 115a = 155a x 0.8 = 92a
the ambient temp. in a room is 92f (33.3c) minimum size thhn wire to supply
equipment? Right Ans - 310.15b16 ampactiy is 75a
310.15b3a adjustment factor 0.96
75a x 0.96 = 72a is no.6 thhn is required
minimum size wire and over current protection device required for a 16a
circuit of continuous loads, with 12 thhn in a raceway with an ambient temp.
of 105f? Right Ans - 210.19a1 conductors
210.20a overcurrent
16a x 1.25 = 20a for a continuous load
310.15b3a adjustment factor 50%
20a / 0.50 = 40a
310.15b2a ambient temp. adjustment
40a / 0.87 = 45.98a
, 310.15b16 no.8 thhn on a 20a ocpd
will 95' of no.12, 120v, 14.5a, non-contiuous load, exceed 3% voltage drop?
Right Ans - chapter 9, table 8 no.12 resistance is 1.93 ohms
vd =2 x L x r x i / 1000
vd = 2 x 95' x 1.93 x 14.5a / 1000
vd = 5.32
divide by 120v to find percentage
5.32v / 120v = 0.0443
0.0443 x 100 = 4.43% so yes it will exceed the 3% allowed
size thhn recommended for a non-contiuous load with 130' of single phase,
23a, 240v? Right Ans - vd = 2 x L x r x i / circular mils
vd = 2 x 130' x 12.9 x 23a / 10380 no.10 wire
vd = 7.43v / 240 = 0.031
0.031 x 100 =3.10%
need to size up to no.8 wire
size thhn wire is recommended for a 240v, 3 wire circuit with 3 fixed 240v
loads of 30a at 60', 30a at 80', and 20a at 100'? Right Ans - determine load
center length
60' x 30a = 1800
80' x 30a = 2400
100' x 20a = 2000
sum of loads = 80a
sum of products = a = 77.5 load center length
310.15b16 no. 4 thhn wire for 80a is selected, now check for voltage drop
vd = 2 x L x k x I / cm
vd = 2 x 77.5' x 12.9 x 80a / 41740
vd = 3.8v
240 x 3% = 7.2v so no.4 wire is permissible
Questions With Solutions
Maximum load that may be supplied by 20a, 120v circuit? Right Ans - 20a x
120v = 2400va
Maximum load that may be supplied by 20a, 3 wire, 120/240v circuit?
Right Ans - 20a x 240v = 4800va
Maximum load that may be supplied by 20a, 208v 3 phase circuit? Right
Ans - 20a x 208v x 1.732 = 7205.12va
Minimum size circuit breaker required for 9 florescent ballast rated at 1.5a at
120v, as a continuous load? Right Ans - determine branch circuit load
9 x 1.5a = 13.5a
Determine the continuous duty load
13.5a x 1.25 = 16.88a
Minimum breaker size is 20a
How many receptacle outlet devices in other than residential occupancies can
be connected to a 20a, 120v circuit? Right Ans - determine branch circuit
capacity
20a x 120v = 2400va
NEC section 220.14.i each outlet is assigned a lad of 180va so
2400va / 180va = 13.33 or 13 receptacle outlets
What is the minimum size branch circuit required to serve a 17a, 120v copy
machine? Right Ans - nec sections 210.23a1 and b, the rating of any one
cord connected utilization equipment shall not exceed 80% of the branch
circuit rating.
30a x 0.8 = 24a so the minimum size circuit is 30a
,Maximum lighting load connected to a 20a circuit supplying equipment with a
rating of 8.5a, 120v? Right Ans - 20a - 8.5a = 11.5, the equipment load is
only 50% of the circuit rating so 11.5% may be added.
minimum size branch circuit required to supply a 14.7a, 208v, 3 phase,
continuous load restaurant dishwasher? Right Ans - 14.7 x 1.25 = 18.38a,
so 20a is the minimum
derated ampacity for each of 8, no.12 thhn wire in a single conduit? Right
Ans - 310.15b16 ampacity is 30a
310.15b3a adjustment factor is 70%
70% or 30a = 30a x 0.7 = 21a
maximum load connected on each of 6, no.2 thwn wires in a single conduit?
Right Ans - 310.15b16 ampacity is 115a
310.15b3a adjustment factor 80%
80% of 115a = 155a x 0.8 = 92a
the ambient temp. in a room is 92f (33.3c) minimum size thhn wire to supply
equipment? Right Ans - 310.15b16 ampactiy is 75a
310.15b3a adjustment factor 0.96
75a x 0.96 = 72a is no.6 thhn is required
minimum size wire and over current protection device required for a 16a
circuit of continuous loads, with 12 thhn in a raceway with an ambient temp.
of 105f? Right Ans - 210.19a1 conductors
210.20a overcurrent
16a x 1.25 = 20a for a continuous load
310.15b3a adjustment factor 50%
20a / 0.50 = 40a
310.15b2a ambient temp. adjustment
40a / 0.87 = 45.98a
, 310.15b16 no.8 thhn on a 20a ocpd
will 95' of no.12, 120v, 14.5a, non-contiuous load, exceed 3% voltage drop?
Right Ans - chapter 9, table 8 no.12 resistance is 1.93 ohms
vd =2 x L x r x i / 1000
vd = 2 x 95' x 1.93 x 14.5a / 1000
vd = 5.32
divide by 120v to find percentage
5.32v / 120v = 0.0443
0.0443 x 100 = 4.43% so yes it will exceed the 3% allowed
size thhn recommended for a non-contiuous load with 130' of single phase,
23a, 240v? Right Ans - vd = 2 x L x r x i / circular mils
vd = 2 x 130' x 12.9 x 23a / 10380 no.10 wire
vd = 7.43v / 240 = 0.031
0.031 x 100 =3.10%
need to size up to no.8 wire
size thhn wire is recommended for a 240v, 3 wire circuit with 3 fixed 240v
loads of 30a at 60', 30a at 80', and 20a at 100'? Right Ans - determine load
center length
60' x 30a = 1800
80' x 30a = 2400
100' x 20a = 2000
sum of loads = 80a
sum of products = a = 77.5 load center length
310.15b16 no. 4 thhn wire for 80a is selected, now check for voltage drop
vd = 2 x L x k x I / cm
vd = 2 x 77.5' x 12.9 x 80a / 41740
vd = 3.8v
240 x 3% = 7.2v so no.4 wire is permissible
