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Service protection higher than ampacity

Sep 1, 1995 12:00 PM, Stallcup, James G.

A lighting and appliance branch-circuit panelboard must have individual over-current protection. The overcurrent protection may be the fuses or circuit breaker protecting the service conductors, as long as the service conductors fuse or circuit breaker rating is not greater than the panel bus rating.

The following example shows how to size the main and service conductors for a lighting and appliance panelboard.

What size OCPD, phase conductors, and neutral conductor are required for a lighting and appliance panelboard with the following loads?

Loads served

* Continuous load= 145A(120/240V)

* Noncontinuous load = 68A

Sizing OCPD

Step 1: Calculating OCPD at 125%; 220-10(b), 384-16(a)

68A x 100% = 68A

145A x 125% = 181.25A

Total load = 249.25A

Step 2: Sizing OCPD

249.25A requires 250A

Solution: The size OCPD is 250A.

Sizing phase conductors

Step 1: Calculating phase conductors at 100%; 230-42(a); 220-10(a)

68A x 100% = 68A

145A x 100% = 145A

Total load = 213A

Step 2: Sizing phase conductors; Table 310-16; 384-16(a)

213A requires No. 4/0

Solution: For the 1993 NEC, the 4/0 is sufficient because its ampacity (230A) is large enough to be protected by the 250A overcurrent device, per Sec. 240-3(b). Note, however, that in the 1996 NEC the extra 25% will have to go into the conductor as well, raising this conductor to 250kcmil. In addition, some local requirements provide for the conductor to be sized no lower than the bus or the overcurrent protection for lighting and appliance panels.

Sizing neutral

Step 1: Calculating neutral, 220-22

68A x 100% = 68A

145A x 100% = 145A

Total load = 213A

Applying demand factors; 220-22

First 200A x 100 % = 200A

Next 13A x 70% = 9A

Total load = 209A

Step 2: Sizing neutral conductor

209A requires #4/0

Solution: A No. 4/0 THWN copper conductor is required.

Sizing panelboard

Solution: Panelboard must have a bus rating not less than 250A.

Power panel (Ex. 2)

A power panel is not the same as a lighting and appliance panelboard. Usually a power panel supplies individual loads such as a machine motor, heating unit, A/C unit, or large feeder loads such as to other panels, etc. It is less likely to have loads added from time to time that are uninspected or improperly supervised. This is why a power panel doesn't require individual protection, and rely on a load calculation.

The following example illustrates in detail how the main and service conductors must be sized for a power panel.

What size OCPD, phase conductors, and neutral conductor are required for the power panel with the following loads.

Loads served

* Continuous load= 145A(120/240V)

* Noncontinuous load = 68A

Sizing phase conductors

Step 1: Calculating phase conductors at 100%; 220-10(a)

68A x 100% = 68A

145A x 100% = 145A

Total load = 213A

Step 2: Sizing per Table 310-16

Solution: 213A requires No. 4/0

Sizing neutral

Step 1: Calculating neutral conductors; Sec. 220-22

68A X 100% = 68A

145A x 100% = 145A

Total load = 213A

Applying demand factors; 220-22

First 200A x 100% = 200A

Next 13A X 70% = 9A

Total load = 209A

Step 2: Sizing neutral per Table 310-16

Solution: 209A requires No. 4/0

Sizing panelboard

Solution: The panelboard and all elements must be sized to carry the loads based upon their conditions of use.

Summary: This panelboard does not require individual overcurrent protection; what is protecting the buses is the calculation. This panel could be used for a service entrance provided it did not qualify as a lighting-and-appliance branch-circuit panelboard under the definition in Sec. 384-14, and provided no more than six disconnecting means were installed. The total of these devices could exceed the ampacity of the service conductors.

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