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Maximum Number Of Half Size Circuit Breakers In A Panel

January 9, 2006

Q: Is there a limit to the number of “piggy-back” circuit breakers that are allowed in a 24-space panel?

A: Yes, a Class CTL panelboard is marked to indicate how many circuit breaker poles can be installed. Class CTL is the designation used to identify a panelboard as ‘circuit limiting.’ In these panelboards, only certain positions in the panelboard will allow the use of tandem, half-size or piggy-back circuit breakers. For example, a 125A, 3-wire, single phase, Class CTL panelboard with 20 full-size circuit breaker spaces may be marked to allow two or more tandem circuit breakers.

Class CTL panelboards are the result of a requirement that appears in 408.15. This is the sentence that requires a limit to the number of overcurrent devices in a panelboard: “A lighting and appliance panelboard shall be provided with physical means to prevent the installation of more overcurrent devices than the number for which the panelboard was designed, rated, and approved.”

Although non-CTL tandem circuit breakers are available and will fit in any space in a CTL panelboard, they are marked “For Replacement Only, Not CTL Assemblies.” These circuit breakers are for use in existing old style non-Class CTL assemblies only.

The number of piggy-back circuit breakers that is permitted in a Class CTL lighting and appliance panelboard is marked on the product, and non-CTL tandem circuit breakers are not permitted.

Is It Necessary To Ground Dimmer Switches?

January 9, 2006

Q: I have to install dimmer switches in my home. The switchboxes are plastic and nonmetallic faceplates will be provided for the dimmer switches. Are these switches required to be grounded even though nonmetallic faceplates are installed?

A: Yes, all dimmer switches must be grounded, and they must provide a means for grounding metal faceplates, whether or not a metal faceplate is installed. (See 404.9(B))

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Wiring A Home Standby Generator

January 8, 2006

Q: Does Article 230 apply to the conductors and overcurrent protection from a standby generator located outdoors about 20 feet from the building it serves?

A:No, Article 230 applies to services, and the definition of a service, which appears in Article 100, reads: “Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.” The key words in this definition are “serving utility;” therefore, Article 230 does not apply.

Article 445 covers generators and references Article 695, 700, 701, 702 and 705, which must be complied with depending on the generator’s use. Alternating current machines must be protected from overloads by inherent design, fuses, circuit breakers or other acceptable overcurrent protection.

Supply-conductor ampacity from the generator to the overcurrent protection cannot be less than 115 percent of the nameplate current rating of the generator. These requirements appear in 445.12(A) and 445.13.

If the standby generator supplies a fire pump, additional requirements for the generator set are found in Article 695. A generator used to supply emergency power must comply with the rules in Article 700, especially 700.12(B).

Requirements for a legally required standby system are similar in some respects to those in Article 700 and are in Article 701. If the generator is connected to optional standby loads, Article 702 applies. Where the generator operates in parallel with the utility system, Article 705,Interconnected Electric Power Production Sources applies. Each Article has different requirements but they all prohibit inadvertent parallel operation of the generator with the utility system.

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Connecting 2 Ground Rods To Your Service

January 3, 2006

Q: Can I connect two ground rods spaced about 6 feet apart directly to the service equipment disconnect or should I run the ground wire from one rod to the other then to the service equipment disconnect?

A: The 6 AWG copper grounding-electrode conductor may be run from one rod to the other in an unbroken length, then to the neutral bus in the service disconnect. The grounding-electrode conductor is required to be unbroken in one continuous length or may be spliced by irreversible compression-type connectors or exothermic welding. This requirement appears in 250.62(C). Individual 6 AWG-copper conductors may also be run from each ground rod to the service disconnect; however, individual terminals in the service disconnect must be provided to terminate the two 6 AWG grounding-electrode conductors. This arrangement is permitted by 250.64(F).

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How Do I Calculate Voltage Drop?

January 1, 2006

Q: I have a barn on my property that I would like to get power to. This barn is 377 feet from the nearest power. I would like to have 50 amps @240 volts and I’m going to run my power in conduit. What size wire do I need to run?

A: The formula for voltage drop is: Vd = 2K x L x I / Cm

Vd = Voltage Drop
I = Current in Conductor (Amperes)
L = One way Length of Circuit
Cm = Cross Section Area of Conductor (Circular Mils)
K = Resistance in ohms of one circular mil foot of conductor
K = 12.9 for Copper Conductors @ 75 degrees C
K = 21.2 for Aluminum Conductors @ 75 degree C
/ = Divided by

I will assume you are going to use copper conductors and your temperature is @ 75 degrees C.

Reasonable operating efficiency is achieved if the voltage drop of a feeder or a branch circuit is limited to 3 percent. However, the total voltage drop of a branch circuit plus a feeder can reach 5% and still achieve reasonable operating efficieny (210.19(A)(1)FPN No. 4 or 215.2(A)(4)FPN No. 2).

8 AWG = 50 amps @ 75 degrees C = 16510 Cm
Vd = 2 x 12.9 x 377 x 50 / 16510 = 30 volts
30 volts / 240 volts = 0.125 = 12.5% = Not Acceptable

6 AWG = 65 amps @ 75 degrees C = 26240 Cm
Vd = 2 x 12.9 x 377 x 50 / 26240 = 19 volts
19 volts / 240 volts = 0.079 = 7.9% = Not Acceptable

4 AWG = 85 amps @ 75 degrees C = 41740 Cm
Vd = 2 x 12.9 x 377 x 50 / 41740 = 12 volts
12 volts / 240 volts = 0.05 = 5% = Not Acceptable – This is not acceptable because the 5% voltage drop is at your sub panel. If you were to run any wire beyond the sub panel, your voltage drop would exceed 5%. I’m assuming you are going to install a light and some receptacles in your barn.

3 AWG = 100 amps @ 75 degrees C = 52620 Cm
Vd = 2 x 12.9 x 377 x 50 / 52620 = 9 volts
9 volts / 240 volts = 0.038 = 3.8% = Acceptable

Your equipment grounding conductor (ground wire) is sized off of table 250.122. You need to run a 10 AWG copper ground wire for this circuit.

To summarize; you need to run 3 – 3 AWG (2-hots and 1-neutral) branch circuit conductors and 1 – 10 AWG equipment grounding conductor (ground wire).

Tip: The rule of thumb is to plan for voltage drop at 100 feet and increase one wire size for every 100 feet thereafter.

Happy New Year!