# Peak Current vs RMS Current when calculating Reactance

I am having a hard time getting a grip on AC circuit analysis. I have been taking my peak voltage/current and converting it to RMS current and RMS voltage so I can use ohms law and things like that to calculate values. I just got introduced to Inductive Reactance and Capacitive Reactance but am confused as to how it relates to RMS and Peak values. Should I still make sure I am using RMS values with these equations or do I use peak? For instance: What inductance draws a 160mA RMS current when connected to a 110V, 25Hz source? Assuming I want RMS values I first solve for the resistance of the circuit.

R=V/I, R=110V/160mA, R=687.5 OHMS, R=X

So my Inductive Reactance will be 687.5. I can sub this into the equation and solve for Inductance

X=2πfL, 687.5=2π(25Hz)L, L=4.3H

So my answer would be 4.3 Henries. However I began to second guess myself and wonder if I should be using peak current instead. That makes the current from 160mA to around 226mA.

Calculating resistance with peak values.

R=V/I, R=110V/226mA, R=486.7 OHMS

Calculate Reactance with 486.7 OHMS

X=2πfL, 486.7=2π(25Hz)L, L=3.1H

Just wondering if anyone could provide some insight into this, been looking all over for answers. Most places don't go into great detail.

• RMS voltage use RMS current to get X(f) or use Vpp(f) and Ipp(f) to get same X(f) , If you assume Vrms and measure Ipp then you must convert to the same units. – Tony Stewart Sunnyskyguy EE75 Jan 18 '19 at 22:32
• for more insight electronics.stackexchange.com/questions/417666/… – Tony Stewart Sunnyskyguy EE75 Jan 18 '19 at 22:33
• i.stack.imgur.com/0BMsQ.png – Tony Stewart Sunnyskyguy EE75 Jan 18 '19 at 22:45
• Be sure to differentiate between Resistance (R) and impedance ($Z=\sqrt{R^2+X^2}$). Getting in the habit now will save you confusion and effort later. – K H Jan 18 '19 at 23:23
• @KH The formula you give is for the magnitude of the impedance: $|Z| = \sqrt{R^2+X^2}$. Impedance is a complex value. Your formula does not differentiate between capacitive reactance and inductive reactance, where in reality their impedances are different. – Elliot Alderson Jan 18 '19 at 23:49

Sorry type on graph meant 688 not 488 