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Code Quandaries: Stumped by the Code? December 2011

Dec 21, 2011 12:24 PM, Mike Holt, NEC Consultant

Your most pressing National Electrical Code (NEC) questions answered

All questions and answers are based on the 2011 NEC.

Q. What are the requirements in the 2011 NEC for the marking of service equipment with the available fault current?

A. Service equipment in other than dwelling units must be legibly field-marked with the maximum available fault current, including the date the fault current calculation was performed — and be of sufficient durability to withstand the environment involved [110.24(A)].

When modifications to the electrical installation affect the maximum available fault current at the service, the maximum available fault current must be recalculated to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) must be adjusted to reflect the new level of maximum available fault current [110.24(B)].

Exception: Field markings aren’t required for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.

Q. How is the photovoltaic (PV) source circuit current calculated?

A. The maximum PV source circuit current is calculated by multiplying the module nameplate short circuit current rating (Isc) by 125% [690.8(A)(1)]. The 125% current multiplier is due to the module’s ability to produce more current than its rated value based on the intensity of the sunlight. The PV source circuit consists of the circuit conductors between the PV modules and the terminals of the combiner or inverter DC input terminals if no combiner is used [690.2].

The maximum PV output circuit current is equal to the sum of parallel PV source circuit currents as calculated in 690.8(A)(1) [690.8(A)(2)]. The PV output circuit consists of circuit conductors between the PV source circuit (combiner) and the DC input terminals of the inverter or DC disconnect. Let’s run through a sample calculation to help hit this point home.

What’s the maximum PV output circuit current for two strings, each containing 23 DC modules having a nameplate Isc of 11.80A (click here to see Figure)?

Maximum PV output circuit current = (module Isc × 1.25)* × number of strings

= (11.80A × 1.25)* × 2

= (14.75A)* × 2

= 29.50A

*As per 690.8(A)(1), “the maximum current shall be the sum of parallel module rated short circuit currents multiplied by 125%.

Q. When adding a ceiling outlet for a new paddle fan in a bedroom, does the Code require this circuit addition to be AFCI protected?

A. Where branch-circuit wiring is modified, replaced, or extended in any of the areas of a dwelling unit specified in 210.12(A), the branch circuit must be protected by [210.12(B)] a listed combination AFCI located at the origin of the branch circuit or a listed outlet branch circuit AFCI located at the first receptacle outlet of the existing branch circuit.

Q. Does the NEC require that conductors be twisted together before a twist-on wire connector is installed?

A. Conductors must be spliced by a splicing device identified for the purpose or by exothermic welding [110.14(B)]. Conductors aren’t required to be twisted together prior to the installation of a twist-on wire connector, unless specified in the installation instructions.       

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