[Help]Single resistor out of tolerance?


Maintenance Engineer
Good day everyone,

As my previous problem on delta resistance computation has been solved. Big thanks!

We also have problem on resistance of a single heater, specifications are below:

P = 320W , V = 220V

Formula for getting its resistance is VxV/P = (220x200)/320 = 151.25ohms
We set a ±10% tolerance of resistance as a range for good heater condition.
However, upon actual reading of resistance using fluke, it gives a resistance of 182ohms which is more than ±10% of standard reading 151.25ohms.

Lowest -10% tolerance: 136.1ohms
Standard resistance: 151.25ohms
Highest +10% tolerance: 166.4ohms

This type of heater is spiral coil.

So here's my question, is our way of computing resistance if correct? Is our tolerance for judging a good heater is way to small? or is their any explanation why actual reading is far from computed resistance reading based on specs?


Senior Member
Northern illinois
Have you measured current and voltage under load?

R = E/I

Normally when you put them under load the resistance goes up a little as the temperature increases so you can't always trust the resistance readings to be exactly as expected when they are cold but they should be a little less than what you might expect from your calculation above.

You might also have some measurement error. Sometimes there is some corrosion on the surface of the screw terminals that might cause your reading to be higher than expected.


Senior Member
By the numbers..
220X220/320 = 151.25 ohms
240X240/320 = 180 ohms
So by the numbers if the current is held constant resistance increases as voltage increases.

With resistance like that when the voltage drops, so does power.
Voltage increases, so does power.

Take that reading you had, (182 ohms) plug in REAL numbers. Check the voltage with a meter. Just because you have a label that says 220 doesn’t mean that’s what the heater is getting.
P= V^2/R
P= 240X240/180 ohms
P= 320

Could be they calculated data from “nominal” and your trying to get the numbers to fit.


Maintenance Engineer
We have different heater specs, these are what's embedded: 200V, 210V, 220V & 460V

All of those specs I followed, it gives a result which is within 10% tolerance of measured resistance using fluke.
Voltage supply from main breaker of machine is 200V only, but will try to take a look on its current and voltage output when heating up.
Hot heaters may possibly affect resistance but I'm not quite sure because we measure resistance on its terminal even if its hot or cold and still gives resistance within 10% tolerance.

I will try your suggestions and give feedback.
Cheers, Enbi