What is the maximum voltage to ground in a branch circuit?

(1) The system is installed only in commercial or industrial occupancies.

(2) The system's use is restricted to areas under close supervision by qualified personnel.

(3) All of the requirements in 647.4 through 647.8 are met.

647.4 Wiring Methods.

(A) Panelboards and Overcurrent Protection. Use of standard single-phase panelboards and distribution equipment with a higher voltage rating shall be permitted. The system shall be clearly marked on the face of the panel or on the inside of the panel doors. Common trip two-pole circuit breakers or a combination two-pole fused disconnecting means that are identified for use at the system voltage shall be provided for both ungrounded conductors in all feeders

and branch circuits. Branch circuits and feeders shall be provided with a means to simultaneously disconnect all ungrounded conductors.

(B) Junction Boxes. All junction box covers shall be clearly marked to indicate the distribution panel and the system voltage.

(C) Conductor Identification. All feeders and branch- circuit conductors installed under this section shall be identified as to system at all splices and terminations by color, marking, tagging, or equally effective means. The means of identification shall be posted at each branch- circuit panel-board and at the disconnecting means for the building.

(D) Voltage Drop. The voltage drop on any branch circuit shall not exceed 1.5 percent. The combined voltage drop of feeder and branch-circuit conductors shall not exceed 2.5 percent.

(1) Fixed Equipment. The voltage drop on branch circuits supplying equipment connected using wiring methods in Chapter 3 shall not exceed 1.5 percent. The combined voltage drop of feeder and branch-circuit conductors shall not exceed 2.5 percent.

(2) Cord-Connected Equipment. The voltage drop on branch circuits supplying receptacles shall not exceed 1 percent. For the purposes of making this calculation, the load connected to the receptacle outlet shall be considered to be 50 percent of the branch-circuit rating. The combined voltage drop of feeder and branch- circuit conductors shall not exceed 2.0 percent.

FPN: The purpose of this provision is to limit voltage drop to 1.5 percent where portable cords may be used as a means of connecting equipment.

647.5 Three-Phase Systems. Where 3-phase power is supplied, a separately derived 6-phase "wye" system with 60 volts to ground installed under this article shall be configured as three separately derived 120-volt single-phase systems having a combined total of no more than six disconnects.

647.6 Grounding.

(A) General. The system shall be grounded as provided in 250.30 as a separately derived single-phase, 3-wire system.

(B) Grounding Conductors Required. Permanently wired utilization equipment and receptacles shall be grounded by means of an equipment grounding conductor run with the circuit conductors to an

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equipment grounding bus prominently marked "Technical Equipment Ground" in the originating branch-circuit panelboard. The grounding bus shall be connected to the grounded conductor on the line side of the separately derived system's disconnecting means. The grounding conductor shall not be smaller than that specified in Table 250.122 and run with the feeder conductors. The technical equipment grounding bus need not be bonded to the panel-board enclosure. Other grounding methods authorized elsewhere in this Code shall be permitted where the impedance of the grounding return path does not exceed the impedance of equipment grounding conductors sized and installed in accordance with this article. FPN No. 1: See 250.122 for equipment grounding conductor sizing requirements where circuit conductors are adjusted in size to compensate for voltage drop.

FPN No. 2: These requirements limit the impedance of the ground fault path where only 60 volts apply to a fault condition instead of the usual 120 volts.

647.7 Receptacles.

(A) General. Where receptacles are used as a means of connecting equipment, the following conditions shall be met:

(1) All 15- and 20-ampere receptacles shall be GFCI

protected.

(2) All receptacle outlet strips, adapters, receptacle covers, and faceplates shall be marked with the following words or equivalent:

WARNING — TECHNICAL POWER Do not connect to lighting equipment. For electronic equipment use only. 60/120 V. l(])ac

GFCI protected

(3) A 125-volt, single-phase, 15- or 20-ampere-rated receptacle having one of its current-carrying poles connected to a grounded circuit conductor shall be located within 1.8 m (6 ft) of all permanently installed

15- or 20-ampere-rated 60/120-volt technical power- system receptacles.

(4) All 125-volt receptacles used for 60/120-volt technical power shall have a unique configuration and be identified for use with this class of system. All 125-volt, single-phase, 15- or 20-ampere-rated receptacle outlets and attachment plugs that are identified for use with grounded circuit conductors shall be permitted in machine rooms, control rooms, equipment rooms, equipment racks, and other similar locations that are restricted to use by qualified personnel.

(B) Isolated Ground Receptacles. Isolated ground recep tacles shall be permitted as described in 250.146(D); however, the branch-circuit equipment grounding conductor shall be terminated as required in 647.6(B).

647.8 Lighting Equipment. Lighting equipment installed under this article for the purpose of reducing electrical noise originating from lighting equipment shall meet the conditions of 647.8(A) through (C).

(A) Disconnecting Means. All luminaires connected to separately derived systems operating at 60 volts to ground, and associated control equipment if provided, shall have a disconnecting means that simultaneously opens all un- grounded conductors. The disconnecting means shall be located within sight of the luminaire or be capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means shall be installed on or at the switch or circuit breaker used as the disconnecting means and shall remain in place with or without the lock installed. Portable means for adding a lock to the switch or circuit breaker shall not be permitted.

(B) Luminaires. All luminaires shall be permanently in- stalled and listed for connection to a separately

Contrary to common belief, the NEC generally doesn't require you to size conductors to accommodate voltage drop. It merely suggests in the Fine Print Notes to 210.19(A), 215.2(A)(4), 230.31(C), and 310.15(A)(1) that you adjust for voltage drop when sizing conductors. It's important for you to remember that Fine Print Notes are recommendations, not requirements [90.5(C)].

The NEC recommends that the maximum combined voltage drop for both the feeder and branch circuit shouldn't exceed 5%, and the maximum on the feeder or branch circuit shouldn't exceed 3% (Fig. 1). This recommendation is a performance issue, not a safety issue.

If the NEC doesn't require you to size for voltage drop, why even think about it? Consider these reasons:

• System efficiency — If a circuit supports much of a load, a larger conductor will pay for itself many times over in energy savings alone.
• System performance — Lighting loads perform best when voltage drop is minimal. You get the light of a higher-wattage system simply by running larger wires.
• Troubleshooting — If you follow the NEC voltage drop recommendations, you don't have to guess whether your field measurements indicate a problem or if the voltage is low due to not accommodating voltage drop in the design.
• Load protection — Undervoltage for inductive loads can cause overheating, inefficiency, and a shorter life span of the equipment. When conductor resistance causes the voltage to drop below an acceptable point, increase the conductor size.

What exactly is voltage drop?

The voltage drop of a circuit is in direct proportion to the resistance of the conductor and the magnitude of the current. If you increase the length of a conductor, you increase its resistance — and thus increase its voltage drop. If you increase the current, you increase the conductor voltage drop. Thus, long runs often produce voltage drops that exceed NEC recommendations.

To test your knowledge, take the following pop quiz. What is the minimum NEC-recommended operating voltage for a 115V load connected to a 120V source (Fig. 2)?

(a) 120V(b) 115V(c) 114V

(d) 116V

The maximum conductor voltage drop recommended for both the feeder and branch circuit is 5% of the voltage source (120V). The total conductor voltage drop (feeder and branch circuit) shouldn't exceed 120V×0.05=6V. Calculate the operating voltage at the load by subtracting the conductor voltage drop from the voltage source: 120V - 6V = 114V. Therefore, the correct answer is (c), 114V.

Doing the calculations

You can determine conductor voltage drop by using the Ohm's Law method or by the formula method, but you can only use the Ohm's Law method (I × R) for single-phase systems. Regardless of which method you use, observe the following:

• For conductors 1/0 AWG and smaller, the difference in resistance between DC and AC circuits is so little that it can be ignored. In addition, you can ignore the small difference in resistance between stranded and solid wires.
• VD = Voltage drop
• I = The load in amperes at 100%, not 125%, for motors or continuous loads
• R = Conductor resistance, Chapter 9, Table 8 for DC or Chapter 9, Table 9 for AC

Let's do a sample calculation using the Ohm's Law method. What is the voltage drop of two 12 AWG THHN conductors that supply a 16A, 120V load located 100 feet from the power supply?(a) 3.2V(b) 6.4V(c) 9.6V

(d) 12.8V

The math is straightforward:I = 16A

R = 2 ohms per 1,000 feet, according to Chapter 9, Table 9: (2 ohms ÷ 1,000 ft) × 200 ft = 0.4 ohms

VD = I × R
VD = 16A × 0.4 ohms = 6.4V

Therefore, the correct answer is (b), 6.4V.

This method is slightly more involved than the Ohm's Law method, but the big advantage is you can use it for single-phase or 3-phase systems. Here are some additional items to observe:

• Single-phase VD = (2 × K × I × D) ÷ CM.
• 3-phase VD = (1.732 × K × I × D) ÷ CM.
• K = Direct-current constant. K represents the DC resistance for a 1,000-circular mils conductor that is 1,000 feet long, at an operating temperature of 75°C. K is 12.9 ohms for copper and 21.2 ohms for aluminum.
• Q = Alternating-current adjustment factor. For AC circuits with conductors 2/0 AWG and larger, you must adjust the DC resistance constant K for the effects of self-induction (eddy currents). Calculate the “Q” Adjustment Factor by dividing the AC ohms-to-neutral impedance listed in Chapter 9, Table 9 by the DC resistance listed in Chapter 9, Table 8.
• I = The load in amperes at 100% (not at 125% for motors or continuous loads)
• D = The distance between the load and the power supply. When calculating conductor distance, use this distance plus any up or down distance. An approximation is good enough. For example, the load is 140 feet from the source, but the circuit goes up 10 feet into the ceiling, then down 10 feet from the ceiling to the load. The total distance would be 160 feet.
• CM = The circular mils of the circuit conductor as listed in NEC Chapter 9, Table 8

Let's look at a 3-phase example. A 3-phase, 36kVA load rated 208V is wired to the panelboard with 80-foot lengths of 1 AWG THHN aluminum. What is the approximate voltage drop of the feeder circuit conductors?(a) 3.5V(b) 7V(c) 3%

(d) 5%

Applying the 3-phase formula, where:K = 21.2 ohms, aluminumI= [36,000VA ÷ (208V × 1.732)] = 100AD = 80 ft

CM = 83,690 (obtained from Chapter 9, Table 8)

VD = (1.732 × 21.2 × 100A × 80 ft) ÷ 83,690 CM = 3.51V

Therefore, the correct answer is (a), 3.5V.

Don't forget to verify that you haven't exceeded the recommended Code requirement of a 3% voltage drop at the end of the branch circuit or feeder.

%VD = (3.51V ÷ 208V) × 100 = 1.69%

Algebraic variations

Using basic algebra, you can apply the same basic formula to find one of the other variables if you already know the voltage drop. For example, suppose you want to know what size conductor you need to reduce the voltage drop to the desired level. Simply rearrange the formula. For 3-phase, it would look like this:

CM (3-phase) = (1.732 × K × I × D) ÷ VD

Remember, for single-phase calculations you would use 2 instead of 1.732.

Suppose you have a 3-phase, 15kVA load rated 480V and 390 feet of conductor. What size conductor will prevent the voltage drop from exceeding 3% (Fig. 3)?

K = 12.9 ohms, copperI = [15,000VA ÷ (480V × 1.732)] = 18AD = 390 ft

VD = 480V × 0.03 = 14.4V

CM = (1.732 × 12.9 × 18A × 390 ft) ÷ 14.4V = 10,892 CM (8 AWG, Chapter 9, Table 8)

You could also rearrange the formula to solve a problem like this one: What is the maximum length of 6 AWG THHN you can use to wire a 480V, 3-phase, 37.5kVA transformer to a panelboard so voltage drop doesn't exceed 3% (Fig. 4 )?

D (3-phase) = (CM × VD) ÷ (1.732 × K × I)

CM = 26,240 (6 AWG Chapter 9, Table 8)VD = 480V × 0.03 = 14.4VK = 12.9 ohms, copper

I = [37,500VA ÷ (480V × 1.732)] = 45A

D = (26,240 CM × 14.4V) ÷ (1.732 × 12.9 ohms × 45A) = 376 ft

Sometimes, the only way to limit voltage drop is to limit the load. Again, you can rearrange the basic formula algebraically: I = (CM × VD) ÷ (1.732 × K × D). Suppose an installation contains 1 AWG THHN conductors, 300 feet long in an aluminum raceway fed by a 3-phase, 460/230V power source. What is the maximum load the conductors can carry without exceeding the NEC recommendation for voltage drop (Fig. 5)? Let's walk through the required calculation:

I = (CM × VD) ÷ (1.732 × K × D)

CM = 83,690 (1 AWG, Chapter 9, Table 8)VD = 460V × 0.03 = 13.8VK = 12.9 ohms, copper

D = 300 ft

I = (83,690 CM × 13.8V) ÷ (1.732 × 12.9 ohms × 300 ft) = 172A

Note: The maximum load permitted on 1 AWG THHN at 75°C is 130A [110.14(C) and Table 310.16].

The NEC doesn't require you to do voltage drop calculations because they aren't involved in issues of safety, but that doesn't mean they aren't important. A system that meets NEC requirements may not be efficient in terms of power consumption or optimum in terms of equipment longevity and performance. As the old saying goes, “Wire is cheap, but performance loss is costly.” First, ensure that your system meets NEC requirements. Then look at it for voltage drop so you get the right cost tradeoffs between performance and cost.