Questions and Answers
A signal circuit means any electrical circuit that controls any other circuit through a relay or an equivalent device.
False
Class one extra low voltage power circuits are circuits which are neither remote control nor signal circuits and where the current is not limited in accordance with 16-200.
True
Equipment situated on the load side of class to current limiting devices must have no live parts accessible to unauthorized persons if operating at a peak voltage more than:
42.4 V
In a class 2 circuit, the voltage is 20 V (and the current is not supplied from batteries, transformers or current limiting devices). The rating of the overcurrent device shall not exceed:
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A power circuit which can be defined as an extra low voltage power circuit, and so meets the limitations of class one circuits may be wired with copper conductor smaller than number 14 AWG.
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A power circuit which meets the limitations of a low energy power circuit, may be wired with copper conductors smaller than number 14 AWG, if installed in a raceway.
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A class 2 circuit may be wired with aluminum conductors.
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Circuits which operate at more than 150 V cannot qualify to be classified as class two circuits.
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A class 2 circuit can be fed from a 24 V, 150 VA transformer.
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The maximum size of fuse allowed for the protection of a class one circuit where number 16 AWG conductors extend beyond the equipment enclosure is:
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Section 12 outlines the installation requirements for class two circuits.
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The 16 V door chime circuit installed in a dwelling unit is considered to be a class 2 circuit, where supplied by a current limiting transformer of 40 VA.
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A second doorbell is to be installed at a residence. It is discovered that the original class two transformer is too small to power both chime units. A second class two transformer could be installed in parallel with the original and powering both the chime units.
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A door chime circuit is known to operate from a 16 V transformer but the VA is unknown. What could this circuit be classified?
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Conductors of class two circuits shall be separated by at least 50 mm from insulated conductors of electric lighting, power or class one circuits operating at 300 V or less, if the conductors are nonmetallic sheath cable.
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A control circuit may be wired with copper conductors smaller than number 14 AWG when the control circuit meets the limitations of a class one or class two circuit.
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It is permissible to feed a class one extra low voltage power circuit from a transformer with an output of 60 V, 500 VA.
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Remote control circuits of a central vacuum system shall be considered to be class two circuits when the current is limited in accordance with rule 16-200.
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Class one circuits shall not be run in the same cable with communication circuits.
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A 24 V gas valve thermostat circuit is fed from a 20 VA transformer. This is an example of a class two low energy power circuit.
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The controls circuit of a forced air furnace, which meets the limitations of a class two circuit, may be wired with copper conductors as small as number 19 AWG.
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Study Notes
Class 1 and Class 2 Circuits Overview
- Signal circuits control other circuits via relays but are incorrectly defined as such in the context given.
- Class 1 extra low voltage power circuits are defined by their inability to limit current, thus they do not qualify as remote control or signal circuits.
Class 1 Circuit Specifications
- Equipment on the load side of current limiting devices must be designed to ensure no live parts are accessible when operating above a peak voltage of 42.4 V.
- A maximum fuse size of 10 A is allowed for protection of class 1 circuits with number 16 AWG conductors extending beyond the equipment enclosure.
- Class 1 circuits cannot share the same cable as communication circuits.
Class 2 Circuit Specifications
- Class 2 circuits operate under a maximum voltage of 20 V without sourcing from batteries, transformers, or current limiting devices, and can utilize overcurrent devices up to 5 A.
- Transfer of current through aluminum conductors is prohibited in class 2 circuits, which must be strictly copper conductors.
- Class 2 circuits cannot operate at voltages exceeding 150 V; thus, they cannot be classified as such if doing so.
- Class 2 circuits can be sourced from a transformer supplying up to 16 V and 40 VA, qualifying them under low energy requirements.
Additional Class 2 Restrictions
- Installing a second transformer in parallel to power multiple door chime units violates class 2 circuit regulations.
- Conductors in class 2 circuits must maintain a minimum separation of 50 mm from nonmetallic sheath cable conductors of higher voltage regardless of insulation type.
Low Energy Power Circuit Applications
- A 24 V gas valve thermostat circuit powered by a 20 VA transformer exemplifies a class 2 low energy power circuit.
- Control circuits, such as those for forced air furnaces meeting class 2 limitations, can use copper conductors as small as number 19 AWG.
Special Considerations
- Remote control circuits, like those in central vacuum systems, qualify as class 2 if the current is regulated in accordance with established rules.
- Incorrect definitions or setup can lead to misclassification between class 1 and class 2 circuits, stressing the importance of adhering to established guidelines and rules.
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Description
Test your knowledge on Class 1 and Class 2 circuits with this flashcard quiz. Each flashcard presents statements regarding electrical circuits, focusing on the definitions and properties of signal and power circuits. Challenge yourself and determine whether these statements are true or false.
