What box for my pull box and my NEMA-6-50R using #4.

[curt swartz]

You still have the issue of a 50 amp receptacle being protected by a 60 amp breaker. You need to either use 50 amp breakers or use a receptacles/plugs with a minimum 60 amp rating.

 

[curt swartz]

Thanks, I didn't know that about heater elements.

I still feel like perhaps I'm missing something. The 11,520 watts stays constant, so reducing from 240v to 208v results in increasing from 48A to roughly 55A. Sounds like the load (heater elements) would need to change as well, but it still just follows ohm's law. I could see if you switch from split phase to three phase (sharing the load on all three), you might divide by 1.73 and then use lower current and certainly the heater elements would need to change to allow the new wiring system.

The wattage does NOT stay constant. The resistance of the element stays constant. The manufacture is just providing you with the wattage of the element (resistor) at 240 volts.

 

[TheWantonElectrician]

I have to disagree. Look at the info given in Tulsa's post (#9 of this thread). Manufacturer providers the same wattage for the two voltages. It also specifies using a nema 6-50 (or direct wiring) and that the breaker should be sized to 125% of load.

 

[wwhitney]

I have to disagree. Look at the info given in Tulsa's post (#9 of this thread). Manufacturer providers the same wattage for the two voltages. It also specifies using a nema 6-50 (or direct wiring) and that the breaker should be sized to 125% of load.

Given the physics of how resistive heaters work, the only reasonable way to interpret the manufacturer's table is that they have two different models they call 10.8 kW: one model designed for 240V that will draw 45A, and one model designed for 208V that will draw 50A. If you supply the 208V model with 240V, it will draw more than 50A; and if you supply the 240V model with 208V, it will draw less than 45A.

Cheers, Wayne

 

[suemarkp]

After spending more time with the instructions and the manufacturer's website:
Page 5. "Electrical specifications are recommendations only". So not manufacturer requirements.
Page 6. They are telling you to put a 6-50 receptacle on a 60A breaker which is not permitted. NE 210.20(D), 210.21(B)(1).

It looks like you order the kiln as 208V or 240V. So it is possible they use different heating elements such that you always get the power they are advertising and the amp draw would be watts / voltage.

Normally, a fixed resistance heating element is used and designed for 240V. If you run it at 208V, it produces less heat and draws less current. This is very common on electric water heaters and you'll see "slash" ratings for the watts and amps.

 

[BarryO]

You still have the issue of a 50 amp receptacle being protected by a 60 amp breaker. You need to either use 50 amp breakers or use a receptacles/plugs with a minimum 60 amp rating.

I've seen this on other Kiln brands as well; e.g., Stutt 240V/48A kiln with a NEMA 6-50 plug, and instructions to use a 60A breaker. These are UL'd units as well. AFAIK there's no NEMA 6-60 plugs and receptacles. You could use a 14-60 if the customer is willing to cut off the 6-50p off their brand new kiln's cord and have a 14-60p installed.

As for the heating elements, R=V^2/P, so for a given power P, for a lower voltage V, lower resistance elements are used. When replacement elements are ordered (they wear out on the regular basis), the voltage must be specified.

 

[Tulsa Electrician]

Aren't these basically a resistive heater? Why is the current higher for 208 volts than 240?

I checked due I was looking at same thing. When you order you select voltage. 208 or 240.

For a given power, a different element must be used.

 

[Tulsa Electrician]

See if this help explain.

 

[Tulsa Electrician]

Looking at there chart. Is shows a 15360. If you look at red cell of 15000 and change it to 15360. You will see it falls in line with the spec sheet for 208.
I know your using 240. This is for explanation purposes for 240 on 208 and how an increase in size of 240v rated element can be sized for rated output at 208v.

 

[LarryFine]

The 11,520 watts stays constant, so reducing from 240v to 208v results in increasing from 48A to roughly 55A. Sounds like the load (heater elements) would need to change as well . . .

That's what I'm saying. To achieve the same power on a different voltage, the element MUST be changed.

 

[LarryFine]

Given the physics of how resistive heaters work, the only reasonable way to interpret the manufacturer's table is that they have two different models they call 10.8 kW: one model designed for 240V that will draw 45A, and one model designed for 208V that will draw 50A. If you supply the 208V model with 240V, it will draw more than 50A; and if you supply the 240V model with 208V, it will draw less than 45A.

What he ^ said.

 

[LarryFine]

Normally, a fixed resistance heating element is used and designed for 240V. If you run it at 208V, it produces less heat and draws less current. This is very common on electric water heaters and you'll see "slash" ratings for the watts and amps.

To add: A 240v-rated resistive load uses/produces almost exactly 75% of rated heating power when supplied with 208v.

 

[Tulsa Electrician]

What he ^ said.

That is why I had posted info. Years ago I struggled to understand the relationship.

To add: A 240v-rated resistive load uses/produces almost exactly 75% of rated heating power when supplied with 208v.

Sometimes a pic helps.
Good luck on your install. I have done a few for schools. We set disconnects and hard wired. Now no recpt rating issue as well no box fill issue.

 

[TheWantonElectrician]

Thanks again folks. I'll be thinking over this as it's not yet intuitive. Most of the more recent comments were academic for my situation as I'm not actually changing from 240V to 208V.

For those following my original question, I'll describe what I've decided to go with. I'm using an 8x6x4 J-box box with (4) #4 conductors coming into the box from 1'' EMT, (2) #4 conductors exit through a straight pull into 3/4'' EMT, and the other two go through an angled pull into 3/4'' EMT. Both 3/4'' EMT go to their respective NEMA 6-50R. I put the circuit on 50A breakers and will have the customer run a test run to see if the breaker trips. It's a solution, but I don't love it. Would rather hard wire and use 60A, but customers want to be able to unplug and move their kiln.

 

[Tulsa Electrician]

Thank you for letting us know.
The 240 /208 was more for educational purposes. You were very clear on the 240.