Cress kiln not reaching cone 6

[Ksmith]

Hello. I have a Cress kiln- model fx1814 P, 220 volts, 19 amps. 
 

Over the years it’s struggled to reach cone 6. I haven’t used it for 2 years. Did 2 bisque firings 2 months ago and they worked. For the last 2 cone 6 firings I fired it 2-3 hours longer and it still didn’t reach temp. My thumbwheel (% power) reaches 9 but doesn’t go to 10 ( I was told it shouldn’t go past 9). In the past I’ve manually pushed it past 9 and that has allowed it to reach cone 6. 
 

New 220, with 30 amp breaker (which means it struggled with the 220 at my old and new location). 
 

Checked the coils and the cord. Ohms are supposed to be 9.6, 18.36, 18.36 and 10. 28. Mine are 11.5, 18.4, 18.4 and 12.4. Is that difference big enough to matter? 
 

The power cord tested .2 on all 3 wires. 
 

Any suggestions would be most appreciated. 

[Bill Kielb]

My thought is .2 for the power cord is too high or a meter measurement issue. Usually elements are worn and won’t make cone in a reasonable timeframe if they increase by 10% cold. So the symptoms and measurements say new elements. To know the “hot” resistance you need to accurately measure the voltage and current and do the math. BTW - North America nominal voltage these days is 120/240, 220 is virtually non existent.

Just curious if you warmed them up and made sure they are all glowing first.

Edited September 2, 2023 by Bill Kielb

[Ksmith]

Thank you for the advice. I didn’t warm anything up first. I just unplugged and tested. My last test of  the coils (2 months ago) took 30-40 minutes to glowing and each paper on each coil had burn marks. 
 

You mentioned “doing the math” after I accurately measure the voltage and current. Will you share the formula? 

 

[PeterH]

Useful reference
Amps vs. Volts vs. Watts vs. Ohms: What’s the Difference?
https://www.familyhandyman.com/article/electrical-terms-explained-watts-volts-amps-ohms-diy/
... with a good summary

 

[Ksmith]

Great. Thank you. 

[neilestrick]

30 minutes to get glowing

8 hours ago, Ksmith said:

Ohms are supposed to be 9.6, 18.36, 18.36 and 10. 28. Mine are 11.5, 18.4, 18.4 and 12.4.

Top and bottom elements are about 20% off, so that's definitely enough to be a problem since they have to work harder than the middle elements.

 

7 hours ago, Ksmith said:

My last test of  the coils (2 months ago) took 30-40 minutes to glowing and each paper on each coil had burn marks. 

Was the kiln on high for this test? If so, 30+ minutes is a really long time for the coils to glow, at least the top and bottom coils. The middle coils can take a lot longer.

[Ksmith]

Thank you. 
 

I’m guessing my thumbwheel was probably at 1, so that might be the reason it took so long. 

[Bill Kielb]

22 hours ago, Ksmith said:

Checked the coils and the cord. Ohms are supposed to be 9.6, 18.36, 18.36 and 10

@Ksmith
Not sure where these come from - closest I got was an 1814 hp. With the elements below and at todays 240v system voltage this pushed your breaker requirements to: (minimum) prox 25 amps x 1.25 = > 31 amps and maximum 25 X 1.5 = <= 40 amps. So for below, 35 amp breaker and wiring minimum. Just suggesting, after you get your new elements, double check the required breaker and wire gauge by verifying the total actual resistance and solve for the current from above. Your requirements  ought to be: equal to or  greater than 125% of the load but less than or equal to 150% If you consult an electrician this needs to be sized as a continuous load.

Edited September 3, 2023 by Bill Kielb

[Ksmith]

Thank you. Here is my kiln’s chart. I was told by Cress that I need a 20 amp breaker. We installed a 30 amp.

[Bill Kielb]

Good Find!  Yes that Truely calcs to just under 19 amps. Unfortunately a 30 amp breaker is a bit above 150%. Interesting  old unbalanced design. 

[Ksmith]

Thank you. This is all a new language for me. Guessing a 20 amp breaker would be the option I need to have installed. 

[Bill Kielb]

Technically not hard really. 19 amp kiln, minimum = 19 amps x 1.25 = 23.75 amps minimum breaker size. ….. and ….. 19 amps X 1.50 = 28.5 amps Maximum breaker. So a 25 amp breaker fits in the range best. 30 amps is a bit over. You can simply swap your 30 with a 25 and be compliant.

So under NEC: for a continuous load  not less than 125% nor over 150%

Kilns are considered continuous loads. Normal breaker sizing for non continuous load is: not to exceed 80% of the breaker rating, really just the reciprocal of 125% anyway. So non continuous allowable load on a 20 amp breaker =0.80 x 20 = 16 amps

your instincts pretty decent really, I assume you did not think good to run a 20 amp breaker maxed out.

Edited September 4, 2023 by Bill Kielb

[Ksmith]

That’s true. I’ll have it changed to a 25. Thank you for taking the time to explain.