Free TN-LLE Service and Feeder Sizing Questions and Answers — Questions and Answers
Question 1: What is the minimum ampacity for feeder conductors supplying a continuous load of 90 amps and a noncontinuous load of 40 amps?
- 130A
- 162.5A
- 152.5A (Correct answer)
- 175A
Correct answer: 152.5A
According to NEC 215.2(A)(1), feeder conductors must be sized to carry the noncontinuous load plus 125% of the continuous load. The calculation is (90A x 1.25) + 40A = 112.5A + 40A = 152.5A.
Question 2: Per NEC 230.71(B), what is the maximum number of service disconnects allowed to be grouped in a single location for each service?
- One
- Two
- Four
- Six (Correct answer)
Correct answer: Six
NEC 230.71(B) explicitly limits the number of service disconnecting means to a maximum of six for each service. This rule allows for a 'six-handle rule' where up to six switches or circuit breakers can serve as the service disconnect without a single main.
Question 3: According to NEC 230.70(A)(1), the service disconnecting means shall be installed at a readily accessible location:
- Inside a locked electrical room
- Either outside or inside nearest the point of entrance (Correct answer)
- Within 50 feet of the service point
- In the attic space above the garage
Correct answer: Either outside or inside nearest the point of entrance
The code requires the service disconnect to be located where it can be accessed quickly in an emergency. This must be either outside the building or inside nearest the point where the service conductors enter.
Question 4: A 120/240V single-phase service for a dwelling unit has a calculated load of 95 amps. What is the minimum standard service size that can be installed according to NEC 230.79(C)?
- 90A
- 95A
- 100A (Correct answer)
- 125A
Correct answer: 100A
NEC 230.79(C) states that for a one-family dwelling, the service disconnecting means shall have a rating of not less than 100 amperes, 3-wire. Even though the calculated load is 95A, the minimum service size permitted for a modern dwelling is 100A.
Question 5: A feeder is installed to supply a subpanel in a detached garage. Which of the following is required for the feeder?
- It must be installed in rigid metal conduit only.
- It must include an equipment grounding conductor. (Correct answer)
- It must have a disconnect within sight of the main panel.
- It must be protected by a GFCI breaker at the source.
Correct answer: It must include an equipment grounding conductor.
NEC 215.6 requires that a feeder supplying a separate building include an equipment grounding conductor for grounding the non-current-carrying metal parts of equipment. This ensures a low-impedance path for fault current back to the source.
Question 6: The ungrounded service-entrance conductors for a commercial building have a calculated load of 180A. What is the minimum standard size overcurrent device that can be used to protect this service?
- 175A
- 180A
- 200A (Correct answer)
- 225A
Correct answer: 200A
According to NEC 230.90(A), the service overcurrent device rating shall not be greater than the ampacity of the conductors. However, if a standard size does not match, you are permitted to use the next higher standard size per 240.4(B). The next standard size above 180A is 200A, as listed in 240.6(A).
Free TN-LLE Service and Feeder Sizing Questions and Answers
What is the minimum ampacity for feeder conductors supplying a continuous load of 90 amps and a noncontinuous load of 40 amps?