National Electrical Code Basics: Overcurrent Protection Part 3

To catch up on Lorenzo Mari's series on Overcurrent Protection, please visit:

• National Electrical Code Basics: Overcurrent Protection Part 1
• National Electrical Code Basics: Overcurrent Protection Part 2

 

NEC Section 240.21 requires overcurrent-protective devices connected to each ungrounded circuit conductor at the point the conductors receive their supply, with some exceptions. If meeting a few conditions, conductors may be tapped to a feeder without overcurrent protection at the supply point.

 

Image used courtesy of IAEI

 

Location of the Overcurrent Protective Device in the Circuit

The general rule of NEC Section 240.21 is that all ungrounded conductors must have an overcurrent-protective device located where they receive their supply – within a reasonable space (a few centimeters) to allow for either circuit breakers or fuse blocks.

Example 1. One place to locate the overcurrent protective devices is the panelboard, where the branch circuit conductors receive their supply.

 

The Feeder Taps

There are exceptions to the general rule. Section 240.21(B), which contains the feeder tap rules, permits connections to the feeders without additional overcurrent protection on the supply side. Figure 1 shows a typical application of feeder tap rules.

 

Figure 1. A typical application of feeder tap rules. Image used courtesy of Lorenzo Mari

 

Feeder tap conductors cannot apply Section 240.4(B) – the round-up rule for overcurrent protective devices.

Feeder tap conductors cannot supply other conductors unless through additional overcurrent protective devices – do not tap a tap.

Figure 2 shows a hookup NOT considered a feeder tap.

 

Figure 2. A hookup that is not considered a feeder tap. Image used courtesy of Lorenzo Mari

 

NEC Table 310.16 shows an ampacity of 200 A for conductor size 3/0 THW Cu and 150 A for conductor size 1/0 THW Cu under the 75°C column. The 150 A, 3-pole circuit breaker protecting the feeder also protects the branch circuit – the branch circuit conductors do not require additional overcurrent protection at their supply or termination locations.

The branch circuit in figure 2 is not a feeder tap. Feeder tap rules would apply if the branch circuit conductors had an ampacity lower than 150 A.

 

The Five Sets of Feeder Tap Rules

1. Taps not over 3 m long. The tap conductors must comply with the following conditions:

• No longer than 3 m.

• Ampacity, not less than

• The calculated load on the circuits supplied.
• The rating of the equipment supplied or the rating of the overcurrent protective device where the tap conductor terminates.
• 1/10 of the rating of the overcurrent device protecting the feeder conductors when the tap conductors leave the enclosure or vault where the tap is made.

• Do not extend beyond the equipment it supplies.

• Enclosed in a raceway.

Example 2. Figure 3 shows a 500 kcmil THW Cu feeder protected by a 350 A circuit breaker. Feeder tap conductors supply power to a 200 A panelboard with terminals rated 75°C. Compute the size of the tap conductors.

 

Figure 3. Layout for example 2. Image used courtesy of Lorenzo Mari

 

Figure 3 shows the tap conductors leaving the enclosure where the tap is made. Then, the ampacity of the tap conductor must be equal to or higher than

1/10 x 350 = 35 A

And not less than 200 A.

NEC Table 310.16 shows an ampacity of 200 A for conductor size N° 3/0 THW Cu under the 75°C column.

200 A > 35 A

Select a conductor size N° 3/0 THW Cu.

Example 3. Repeat example 2 for tap conductors supplying a 30 A feeder disconnect.

From example 2, the ampacity of the tap conductor must be equal to or higher than 35 A. Besides, it must not be less than 30 A.

NEC Table 310.16 shows an ampacity of 35 A for conductor size N° 10 THW Cu under the 75°C column.

35 A = 35 A

Select a conductor size N° 10 THW Cu.

Example 4. What’s the maximum length of a tap conductor if its ampacity is not less than the combined calculated loads on the circuits it supplies?

 By Section 240.21(B)(1), the length of the tap conductor must not exceed 3 m.

2. Taps not over 7.5 m long. The tap conductors must comply with the following conditions:

• No longer than 7.5 m.
• The ampacity is not less than 1/3 of the rating of the overcurrent device protecting the feeder conductors.
• Terminate in a single circuit breaker or set of fuses that limit the load to the ampacity of the tap conductors. The single overcurrent protective device shall be permitted to supply any additional overcurrent devices on its load side – like terminating on the main breaker of a panelboard with breakers on the load side.
• Installed in a raceway or other approved means.

Example 5. Repeat example 2 for d = 7.5 m tap conductors.

The ampacity of the tap conductor must be equal to or higher than

1/3 x 350 = 117 A

And not less than 200 A.

NEC Table 310.16 shows an ampacity of 200 A for conductor size N° 3/0 AWG THW Cu under the 75°C column.

200 A > 117 A

Select a conductor size N° 3/0 AWG THW Cu.

Example 6. What’s the maximum rating of the overcurrent protective device at the termination of a 7.5 m long tap conductor with an ampacity of 25 A?

The tap conductor must terminate in a single circuit breaker or set of fuses that limit the load to the ampacity of the tap conductor.

Use a 25 A circuit breaker or set of fuses.

Example 7. What’s the minimum ampacity of a 6.4 m long conductor tapped to a feeder protected by a 400 A fuse?

The minimum ampacity is 1/3 of the rating of the overcurrent device protecting the feeder conductors.

400 A x 1/3 = 133.33 A

3. Taps supplying a transformer (Primary plus secondary not over 7.5 m long). The tap conductors must comply with the following conditions:

• The ampacity of primary conductors is not less than 1/3 of the rating of the overcurrent device protecting the feeder conductors.
• The ampacity of the secondary conductor is not less than Vprimary/Vsecondary x 1/3 x rating of the overcurrent device protecting the feeder conductors.
• The maximum length of the primary plus secondary conductors = 7.5 m. Excluding any portion of the primary conductor protected at its ampacity.
• Installed in a raceway or other approved means.
• Secondary conductors terminate in a single circuit breaker or set of fuses that limit the load to the ampacity of the conductors.

Example 8. Figure 4 shows a feeder tap supplying a transformer.

 

Figure 4. Feeder tap supplying a transformer. Image used courtesy of Lorenzo Mari

 

A. Compute the size of the conductors supplying the primary of the transformer.

Rated current in primary = 30 kVA/(√3 x 480 V) = 36.08 A

By Section 240.21B(3)(1) 1/3 x 200 A = 66.67 A

NEC Table 310.16 shows conductor size N° 4 AWG THW Cu ampacity = 85 A

85 A > 36.08 A

85 A > 66.67 A

Use conductor size N° 4 AWG THW Cu.

B. Compute the size of the conductors supplied by the secondary of the transformer.

Rated current in secondary = 30 kVA/(√3 x 208 V) = 83.27 A

By Section 240.21B(3)(2) 480 V/208 V x 1/3 x 200 A = 153.85 A

NEC Table 310.16 shows conductor size N° 2/0 AWG THW Cu ampacity = 175 A

175 A > 83.27

175 A > 153.85

Use conductor size N° 2/0 AWG THW Cu.

C. Compute the rating of the single circuit breaker where the secondary conductors terminate (240.21B(3)(5)).

Rated current in secondary = 83.27 A. Exceeding this current will overload the transformer. Try a circuit breaker rated 80 A.

80 A < 83.27 A

Secondary conductor ampacity = 175 A.

80 A < 175 A

Use a circuit breaker rated 80 A.

For simplicity, figure 4 does not show the transformer disconnecting means.

Note: Placing an overcurrent protective device at the point where the conductors supplying the primary of the transformer receive their supply – a set of fuses or a single circuit breaker – will convert the feeder tap conductors into feeder conductors.

 

4. Taps over 7.5 m long. This section applies to high bay manufacturing buildings.

The tap conductors must comply with the following conditions:

• System serviced by qualified persons only.
• Not over 7.5 m long horizontally and not over 30 m in total length.
• The ampacity is not less than 1/3 of the rating of the overcurrent device protecting the feeder conductors.
• Terminate in a single circuit breaker or set of fuses that limit the load to the ampacity of the conductors. The single overcurrent protective device shall be permitted to supply any number of additional overcurrent devices on its load side.
• Installed in a raceway or other approved means.
• Continuous from end to end – no splices.
• Sized N° 6 AWG Cu or N° 4 AWG Al as a minimum.
• Made at no less than 9 m in height.

Figure 5 shows a feeder tap over 7.5 m length in a high bay manufacturing building.

 

Figure 5. Feeder tap in a high bay manufacturing building. Image used courtesy of Lorenzo Mari

 

Example 9. A set of 15 m long tap conductors is connected to a 500 kcmil AWG THW feeder, protected by a 400 A circuit breaker, and terminate at a fused disconnect with fuses rated 100 A. The continuous load is 75 A, and terminations are rated 75°C. What’s the minimum required size of the tap conductor?

The ampacity of the tap conductor must be equal to or higher than

1/3 x 400 = 133.33 A

The fuses must limit the load to the ampacity of the conductors – conductors rated at least 100 A.

The continuous load requires a minimum ampacity of 125% x 75 A = 93.75 A.

NEC Table 310.16 shows an ampacity of 150 A for conductor size N° 1/0 AWG THW Cu under the 75°C column.

150 A > 133.33 A.

150 A > 100 A

150 A > 93.75 A

Select a conductor size N° 1/0 AWG THW Cu.

 

5. Outside taps of unlimited length. There is no limit on the length of outside tap conductors complying with the following:

• Protected from physical damage in an approved manner – could be protected if installed as overhead conductors.
• Terminate in a single circuit breaker or set of fuses that limit the load to the ampacity of the conductors. The single overcurrent protective device shall be permitted to supply any number of additional overcurrent devices on its load side.
• The overcurrent protective device is an integral part of a disconnecting means or is located immediately adjacent.
• Disconnecting means installed at a readily accessible location inside or outside a building or structure.

Figure 6 shows an outside feeder tap with unlimited length.

 

Figure 6. An example of an outside feeder tap. Image used courtesy of Lorenzo Mari

 

Example 10. A set of outdoor feeder tap conductors with a noncontinuous load of 110 A terminate at a single set of 125 A fuses in a fused disconnect – terminations rated 75°C. What’s the minimum required size of the tap conductor?

NEC Table 310.16 shows conductor size N° 2 AWG THW with an ampacity of 115 A in the 75°C column.

115 A > 110 A.

The next overcurrent protective device standard ampere rating is 125 A. This value meets the rating of the fuses in the fused disconnect.

Conductor size N° 2 AWG THW is correct for feeder conductors but not for feeder tap conductors. Section 240.4(B) – the round-up rule – is not permitted for tap conductors.

The fuses must limit the load to the ampacity of the conductors – the conductors must be rated at least 125 A.

NEC Table 310.16 shows conductor size N° 1 AWG THW with an ampacity of 130 A in the 75°C column.

130 A > 125 A

Use conductor size N° 1 AWG THW.

 

Key Takeaways: Overcurrent Protection

• NEC 240.21, with some exceptions, requires overcurrent protection in each ungrounded conductor where the conductors receive their supply.

• Feeder tap conductors serving downstream equipment cannot supply a second tap without additional overcurrent protective devices.

• The rule of the next higher size does not apply to tap conductors.

• There are five sets of feeder tap rules

• Not over 3 m long
• Not over 7.5 m long
• Supplying a transformer (Primary + secondary not over 7.5 m long)
• Over 7.5 m long
• Unlimited length (outside)

 

Featured image used courtesy of Adobe Stock