Low Voltage Systems (C-7) · Lesson 2 of 4

Class 2 Power Source Limits: How Big a Supply Can One Circuit Use

"Class 2 Power Source Limits: How Big a Supply Can One Circuit Use" is lesson 2 of 4 in the Low Voltage Systems (C-7) Learning Hub. It opens with "What the Classes Mean" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. What the Classes Mean
  2. Marking You Must Be Able to Read
  3. Read Table 11(B) for Direct-Current Sources
  4. Turn the Table Into a Supply Check
  5. Alternating Current and Fire Alarm Sources Follow the Same Pattern

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Find the total load" through "Convert to volt-amperes".

What you'll be able to do

  • Explain the main idea of "Class 2 Power Source Limits: How Big a Supply Can One Circuit Use" in plain language you can use on the exam.
  • Apply this step with confidence: "Read Table 11(B) for Direct-Current Sources".
  • Apply this step with confidence: "Turn the Table Into a Supply Check".
  • Apply this step with confidence: "Alternating Current and Fire Alarm Sources Follow the Same Pattern".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Class 2 DC Source Limits (CEC Table 11(B)) Class 2, not inherently limited 2025 CEC Chapter 9 Table 11(B), direct current 0 through 20 V Nameplate max = 5.0 x Vmax VA Overcurrent max = 5.0 A Over 20 through 60 V Nameplate max = 100 VA Current max = 100 / Vmax amperes At 24 V: 100 / 24 = 4.17 A At 12 V: 5.0 x 12 = 60 VA, 5.0 A 24 VDC load check (VA = V x A) 100 VA limit 162 VA 90 VA 72 VA 6.75 A 3.75 A 3.00 A one supply supply 1 supply 2 9 x 0.75 A = 6.75 A = 162 VA, over 100 VA: one Class 2 supply FAILS Split: 5 groups = 90 VA and 4 groups = 72 VA, both under 100 VA Source: 2025 CEC Chapter 9 Table 11(B); 8 CCR §2585.1 and §2585.2 CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: What the Classes Mean

Low-voltage circuits are grouped by how much power the source can deliver. 8 CCR §2585.1 classifies them: a Class 1 power-limited circuit is supplied from a source rated not more than 30 volts and 1000 volt-amperes; a Class 1 remote control or signaling circuit may run up to 600 volts with no limit on source power; and the source of a Class 2 or Class 3 circuit must be listed equipment marked as a Class 2 or Class 3 power source. Only two exceptions are printed: thermocouples need no Class 2 listing, and a dry cell battery of 30 volts or less (no more capacity than series-connected No. 6 carbon zinc cells) counts as an inherently limited Class 2 source.

Step 2: Marking You Must Be Able to Read

8 CCR §2585.2 requires a Class 2 or Class 3 power supply unit to be durably marked where plainly visible with the class of supply and its electrical rating. 2025 CEC Chapter 9 Tables 11(A) and 11(B) add that a Class 2 source not suitable for wet locations must be marked that way. If the nameplate does not state Class 2 or Class 3, treat it as an ordinary power supply and do not wire it as a Class 2 circuit.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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