Licensed Electrical Contractor Electrical Contractor Electrical System Design & Installation 2

Question 1

When designing a commercial electrical system, what is the NEC-required minimum height for service entrance conductors over a public roadway?

Accepted Answer

18 feet

Answer

NEC Section 230.24(B) requires service-drop conductors to maintain a minimum clearance of 18 feet over public streets and roadways.

Question 2

In a 208/120V three-phase, four-wire wye system, what is the voltage measured between any two phase conductors?

Accepted Answer

208V

Answer

In a wye-connected system with 120V phase-to-neutral, the line-to-line voltage is 120 × 1.732 = approximately 208V.

Question 3

What is the maximum allowable voltage drop recommended by the NEC for a combination of feeder and branch circuit conductors?

Accepted Answer

5%

Answer

NEC recommends a maximum total voltage drop of 5% for the combined feeder and branch circuit, with no more than 3% on either individually.

Question 4

When installing a 400-amp service in a commercial building, what is the minimum size copper conductor required for the grounding electrode conductor?

Accepted Answer

1/0 AWG

Answer

Per NEC Table 250.66, a 400-amp service using copper conductors up to 500 kcmil requires a minimum 1/0 AWG copper grounding electrode conductor.

Question 5

What type of electrical system grounding is required by the NEC for a 480/277V, three-phase, four-wire system supplying a commercial building?

Accepted Answer

Solidly grounded

Answer

The NEC requires 480/277V wye systems supplying line-to-neutral loads to be solidly grounded per Section 250.20(B).

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