Dakota Prep - Dakota Capacitor Guide for Red Seal Exam (Updated for CEC 2024)

What is a Capacitor?

Capacitors are like electrical batteries that charge and release power super fast. They help with power factor correction, voltage stabilization, and reducing load strain.

When we size them (and their wires, breakers, and disconnects), we need to be very specific — or risk a very cranky capacitor

And cranky capacitors can explode- literally

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Step 1: Calculate Capacitor Current

Use the capacitive reactance formula to figure out how much current the capacitor pulls:

X_C = 1 2π × f × C

Then use Ohm's Law:

I = E X_C

Example:

120V, 60Hz, 1060 μF
X_C = 1 2π × 60 × 0.00106
X_C = 2.5 Ω

I = 120 2.5

I = 48 A

That’s the Rated Current.

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Step 2: Size the Conductors (CEC Rule 26-208)

To stay safe, your wires must be 135% of the rated current:

48A×1.35=64.8A
Table 2 says: #6 AWG copper (good for 65A at 75°C)

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Step 3: Overcurrent Protection (26-210)

You must add an O/C device (breaker or fuse) rated no more than 250% of the capacitor’s rated current:

48A * 2.5 = 120A
Check Table 13 = Use **110A** (Because we can’t exceed the result!) GO DOWN!

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Step 4: Disconnect Sizing (26-214)

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A disconnect must be rated NOT MORE THAN 135% of the rated current:

140A * 1.35= 189A
Use standard Disconnect Sizes
200A Disconnect

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Multiple Capacitors? Let’s Add!

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If you’ve got 3 capacitors pulling 48A each:

Total = 48 + 48 + 48 = 144A

Step 1

O/C = 144A × 2.5 = 360A
Table 13 =350A

Step 2

Disconnect = 144A × 1.35 = 194.4A
Use 200A disconnect

Step 3

Conductors: 144A × 1.35 = 194.4A
Use #3/0 AWG (good for 200A)

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Taps to Multiple Capacitors?

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If one large branch circuit feeds individual capacitor taps, then: -

Each tap conductor must:

Be NOT MORE THAN 7.5 m
Have ampacity NOT LESS THAN 1/3 of the branch circuit supplying them,
AND meet 135% of the rated current for the capacitor it feeds

Example:

Capacitor draw = 48A (RPC) × 1.35 = 64.8A

Table 2: So each capacitor could be a #6awg

BUT:

Main Branch Circuit is = #3/0 AWG good for 200A
1/3 = 66.67A

So tap conductors must be 66.67A
Use #4 AWG (good for 85A)

**Always go with the LARGER size**

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Capacitor Review

Step 1: Use Xc = 1/2πfC and I = E/Xc

Step 2: Multiply by 1.35 to get conductor ampacity

Step 3: Multiply by 2.5 for max overcurrent device size

Step 4: Multiply by 1.35 for disconnect rating

BONUS: If using tap conductors use 7.5m max & ≥1/3 feeder size

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