Load Calculations Branch And Feeder Circuits Exam
Questions And Correct Answers
what is the maximum load that may be supplied by 20a, 120v circuit? - Answer 20a x 120v
= 2400va
what is the maximum load that may be supplied by 20a, 3 wire, 120/240v circuit? - Answer
20a x 240v = 4800va
what is the maximum load that may be supplied by 20a, 208v 3 phase circuit? - Answer
20a x 208v x 1.732 = 7205.12va
What is the minimum size circuit breaker required for 9 florescent ballast rated at 1.5a at
120v, as a continuous load? - Answer 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? - Answer determine branch circuit capacity
20a x 120v = 2400va
NEC Section 220.14.I
Each outlet is to be allocated a LAD of 180VA so
2400VA / 180VA = 13.33 or 13 Receptacle Outlets
,What is the smallest size branch circuit needed to feed a 17A, 120V copy machine? -
Answer
NEC Sections 210.23A1 and B says, 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? - Answer 20a - 8.5a = 11.5, the equipment load is only 50% of the circuit rating
so 11.5% may be added.
What is the minimum size branch circuit required to supply a 14.7A, 208V, 3 phase,
continuous load restaurant dishwasher? Answer 14.7 x 1.25 = 18.38A, so 20A is the
minimum
rated ampacity for each of 8, No.12 THHN wire in a single conduit? Answer 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? - Answer
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? -
Answer 310.15b16 ampactiy is 75a
310.15b3a adjustment factor 0.96
75a x 0.96 = 72a is no.6 thhn is required
, What size minimum wire and OCPD is required for a 16A continuous load circuit, when
there are 12 THHN conductors in a raceway at an ambient temperature of 105F? - Solution
210.19A1 Conductors
210.20A Overcurrent
16A x 1.25 = 20A Continuous load
310.15B3A Adjustment Factor - 50%
20A / 0.50 = 40A
310.15B2A Ambient Temperature 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? - Answer
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
Questions And Correct Answers
what is the maximum load that may be supplied by 20a, 120v circuit? - Answer 20a x 120v
= 2400va
what is the maximum load that may be supplied by 20a, 3 wire, 120/240v circuit? - Answer
20a x 240v = 4800va
what is the maximum load that may be supplied by 20a, 208v 3 phase circuit? - Answer
20a x 208v x 1.732 = 7205.12va
What is the minimum size circuit breaker required for 9 florescent ballast rated at 1.5a at
120v, as a continuous load? - Answer 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? - Answer determine branch circuit capacity
20a x 120v = 2400va
NEC Section 220.14.I
Each outlet is to be allocated a LAD of 180VA so
2400VA / 180VA = 13.33 or 13 Receptacle Outlets
,What is the smallest size branch circuit needed to feed a 17A, 120V copy machine? -
Answer
NEC Sections 210.23A1 and B says, 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? - Answer 20a - 8.5a = 11.5, the equipment load is only 50% of the circuit rating
so 11.5% may be added.
What is the minimum size branch circuit required to supply a 14.7A, 208V, 3 phase,
continuous load restaurant dishwasher? Answer 14.7 x 1.25 = 18.38A, so 20A is the
minimum
rated ampacity for each of 8, No.12 THHN wire in a single conduit? Answer 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? - Answer
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? -
Answer 310.15b16 ampactiy is 75a
310.15b3a adjustment factor 0.96
75a x 0.96 = 72a is no.6 thhn is required
, What size minimum wire and OCPD is required for a 16A continuous load circuit, when
there are 12 THHN conductors in a raceway at an ambient temperature of 105F? - Solution
210.19A1 Conductors
210.20A Overcurrent
16A x 1.25 = 20A Continuous load
310.15B3A Adjustment Factor - 50%
20A / 0.50 = 40A
310.15B2A Ambient Temperature 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? - Answer
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
