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Breaker Size worksheet

Grades 6–7 · CCSS 7.RP.A.3

Solve each problem and show your work. Round answers to 2 decimal places where needed.

1.
Problem 1.
- Load given in: Watts
- Continuous load (runs 3 hours or more): 1,500 W
- Noncontinuous load: 0 W
- Circuit voltage: 125 V
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
2.
Problem 2.
- Load given in: Watts
- Continuous load (runs 3 hours or more): 1,300 W
- Noncontinuous load: 0 W
- Circuit voltage: 96 V
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
3.
Problem 3.
- Load given in: Amps
- Continuous load (runs 3 hours or more): 15 A
- Noncontinuous load: 0 A
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
4.
Problem 4.
- Load given in: Amps
- Continuous load (runs 3 hours or more): 17 A
- Noncontinuous load: 0 A
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
5.
Problem 5.
- Load given in: Watts
- Continuous load (runs 3 hours or more): 1,500 W
- Noncontinuous load: 0 W
- Circuit voltage: 155 V
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
6.
Problem 6.
- Load given in: Watts
- Continuous load (runs 3 hours or more): 1,200 W
- Noncontinuous load: 0 W
- Circuit voltage: 120 V
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
7.
Problem 7.
- Load given in: Amps
- Continuous load (runs 3 hours or more): 16 A
- Noncontinuous load: 0 A
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
8.
Problem 8.
- Load given in: Watts
- Continuous load (runs 3 hours or more): 1,500 W
- Noncontinuous load: 0 W
- Circuit voltage: 180 V
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
9.
Problem 9.
- Load given in: Watts
- Continuous load (runs 3 hours or more): 1,300 W
- Noncontinuous load: 0 W
- Circuit voltage: 130 V
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer
10.
Problem 10.
- Load given in: Watts
- Continuous load (runs 3 hours or more): 1,700 W
- Noncontinuous load: 0 W
- Circuit voltage: 120 V
- Breaker and enclosure are listed for 100% continuous operation: No
Find: Standard breaker size
Answer

Answer key: Breaker Size worksheet

1. Standard breaker size: 15 A
1. Turn the long-running load into amps (current) by dividing watts by volts
  $\frac{1,500}{125}$ $= 12 A$
2. Do the same for the shorter-running load
  $\frac{0}{125}$ $= 0 A$
3. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 12$ $= 15 A$
4. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 15 A is 15 A$
5. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{15}{1.25}$ $= 12 A$
6. Check how much of the breaker's rating your total load uses
  $\frac{12}{15} \times 100$ $= 80%$
2. Standard breaker size: 20 A
1. Turn the long-running load into amps (current) by dividing watts by volts
  $\frac{1,300}{96}$ $= 13.5417 A$
2. Do the same for the shorter-running load
  $\frac{0}{96}$ $= 0 A$
3. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 13.5417$ $= 16.9271 A$
4. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 16.9271 A is 20 A$
5. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{20}{1.25}$ $= 16 A$
6. Check how much of the breaker's rating your total load uses
  $\frac{13.5417}{20} \times 100$ $= 67.71%$
3. Standard breaker size: 20 A
1. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 15$ $= 18.75 A$
2. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 18.75 A is 20 A$
3. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{20}{1.25}$ $= 16 A$
4. Check how much of the breaker's rating your total load uses
  $\frac{15}{20} \times 100$ $= 75%$
4. Standard breaker size: 25 A
1. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 17$ $= 21.25 A$
2. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 21.25 A is 25 A$
3. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{25}{1.25}$ $= 20 A$
4. Check how much of the breaker's rating your total load uses
  $\frac{17}{25} \times 100$ $= 68%$
5. Standard breaker size: 15 A
1. Turn the long-running load into amps (current) by dividing watts by volts
  $\frac{1,500}{155}$ $= 9.6774 A$
2. Do the same for the shorter-running load
  $\frac{0}{155}$ $= 0 A$
3. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 9.6774$ $= 12.0968 A$
4. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 12.0968 A is 15 A$
5. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{15}{1.25}$ $= 12 A$
6. Check how much of the breaker's rating your total load uses
  $\frac{9.6774}{15} \times 100$ $= 64.52%$
6. Standard breaker size: 15 A
1. Turn the long-running load into amps (current) by dividing watts by volts
  $\frac{1,200}{120}$ $= 10 A$
2. Do the same for the shorter-running load
  $\frac{0}{120}$ $= 0 A$
3. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 10$ $= 12.5 A$
4. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 12.5 A is 15 A$
5. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{15}{1.25}$ $= 12 A$
6. Check how much of the breaker's rating your total load uses
  $\frac{10}{15} \times 100$ $= 66.67%$
7. Standard breaker size: 20 A
1. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 16$ $= 20 A$
2. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 20 A is 20 A$
3. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{20}{1.25}$ $= 16 A$
4. Check how much of the breaker's rating your total load uses
  $\frac{16}{20} \times 100$ $= 80%$
8. Standard breaker size: 15 A
1. Turn the long-running load into amps (current) by dividing watts by volts
  $\frac{1,500}{180}$ $= 8.3333 A$
2. Do the same for the shorter-running load
  $\frac{0}{180}$ $= 0 A$
3. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 8.3333$ $= 10.4167 A$
4. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 10.4167 A is 15 A$
5. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{15}{1.25}$ $= 12 A$
6. Check how much of the breaker's rating your total load uses
  $\frac{8.3333}{15} \times 100$ $= 55.56%$
9. Standard breaker size: 15 A
1. Turn the long-running load into amps (current) by dividing watts by volts
  $\frac{1,300}{130}$ $= 10 A$
2. Do the same for the shorter-running load
  $\frac{0}{130}$ $= 0 A$
3. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 10$ $= 12.5 A$
4. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 12.5 A is 15 A$
5. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{15}{1.25}$ $= 12 A$
6. Check how much of the breaker's rating your total load uses
  $\frac{10}{15} \times 100$ $= 66.67%$
10. Standard breaker size: 20 A
1. Turn the long-running load into amps (current) by dividing watts by volts
  $\frac{1,700}{120}$ $= 14.1667 A$
2. Do the same for the shorter-running load
  $\frac{0}{120}$ $= 0 A$
3. Count long-running loads at 125%, as NEC 210.20(A) requires, then add the rest. This is the smallest rating allowed
  $0 + 1.25 \times 14.1667$ $= 17.7083 A$
4. Breakers only come in set sizes, so pick the first one at or above that minimum (NEC 240.6(A))
  $smallest standard size ≥ 17.7083 A is 20 A$
5. See how much long-running load this breaker may carry: its rating ÷ 1.25
  $\frac{20}{1.25}$ $= 16 A$
6. Check how much of the breaker's rating your total load uses
  $\frac{14.1667}{20} \times 100$ $= 70.83%$