General Breaker Sizing Question

[_Sam]

I'm a little confused on if this is allowed or not.

I'm installing a heater in a large enclosure. A 415V-3Ø, 42/21 kW heater was purchased, with those numbers being the high & low heater settings. 42kW is way too much for this enclosure and 21kW is the amount we need. Are we allowed to size our branch circuit breaker to the 21kW knowing that we will never turn it up to the 42kW setting? If we did, it will trip the breaker, stopping any additional issues. A sign will be placed on the heater to state its never to be changed to 42kW.

So in this case, installing a 40A breaker to limit it to the 21kW. In my opinion this is ok according to code because you are sizing the breaker to the continuous load current. It may not be the continuous full load of the equipment, but it is for our application on the low setting.

 

[ptonsparky]

A 415 volt 21KW unit will be roughly 28KW @ 480 and about 34 amps or a 133.8% increase. A 40 amp breaker may hold.
IDK how well you heaters will hold up under that increase. Do you have a step down transformer ahead of it?

IMO signage would not matter if someone can easily select the higher wattage with a simple Hi/Lo switch.

 

[_Sam]

A 415 volt 21KW unit will be roughly 28KW @ 480 and about 34 amps or a 133.8% increase. A 40 amp breaker may hold.
IDK how well you heaters will hold up under that increase. Do you have a step down transformer ahead of it?

IMO signage would not matter if someone can easily select the higher wattage with a simple Hi/Lo switch.

Supplied power where its being installed is (240 -1Ø) / (415-3Ø) volts, so no 480V involved and no step down transformer needed. A 40 amp breaker will hold but is that allowed under the NEC? To size according to the lower wattage or are you required to size a breaker to the rating of the equipment? In this case, the 42kW rating. If you select the higher wattage, the breaker trips and now you know to follow the posted sign in my opinion.

 

[garbo]

I would bring it up to the AHU. I have removed wires from one stage when customer purchased too high of a wattage unit heater. P touched outside of heater cover & interior space stating this and not to attempt reconnecting the unused stage unless feed and circuit breaker is increased. Had small buildings that only had a 100 amp single phase service and a large unit heater that pulled over 70 amps that tripped the 100 amp main breaker until some load was removed from heater.

 

[ptonsparky]

OP, I am curious as to how you are getting three phase from a single phase source

 

[_Sam]

OP, I am curious as to how you are getting three phase from a single phase source

I'm not sure what you mean. I'm only using three phase.

 

[_Sam]

I would bring it up to the AHU. I have removed wires from one stage when customer purchased too high of a wattage unit heater. P touched outside of heater cover & interior space stating this and not to attempt reconnecting the unused stage unless feed and circuit breaker is increased. Had small buildings that only had a 100 amp single phase service and a large unit heater that pulled over 70 amps that tripped the 100 amp main breaker until some load was removed from heater.

Interesting. This heater is properly coordinated, I don't think it will trip and upstream breaker. If we remove wires from one stage, then we have to deal with voided warranties etc. I don't think anyone wants to go that route here haha

 

[Dennis Alwon]

IMO, you need to wire the unit for the max current that it can draw and not just say we won't turn it past 21kw.

 

[ptonsparky]

IMO, you need to wire the unit for the max current that it can draw and not just say we won't turn it past 21kw.

Not much different than putting a 2 hp motor in place of a 1 and saying it only needs 3/4 to operate.

 

[wwhitney]

What about modifying the unit to disable the 42 kW setting, relabeling it as a 21 kW unit, and then sizing the circuit based on 21 kW? A viable strategy, or a problem?

Cheers, Wayne

 

[ptonsparky]

What about modifying the unit to disable the 42 kW setting, relabeling it as a 21 kW unit, and then sizing the circuit based on 21 kW? A viable strategy, or a problem?

Cheers, Wayne

He is worried about voiding warranties.

 

[wwhitney]

He is worried about voiding warranties.

Yes, missed that, thanks. But it seems like if the disabling is done in a reversible way (remove a wire and cap it), there would be no warranty issue.

A related question: say the selection is done by turning a knob. And you externally mechanically prevent the knob from turning, without modifying the equipment at all. Effectively locking out the higher wattage setting. Would that be sufficient for wiring the equipment based on only the lower setting?

Cheers, Wayne

 

[ptonsparky]

The instructions usually say MOP and MCA.

The maximum is a suggestion. Minimum is not.

So, run full sized wires and protect at whatever.

 

[don_resqcapt19]

The instructions usually say MOP and MCA.

The maximum is a suggestion. Minimum is not.

So, run full sized wires and protect at whatever.

Have never seen that on resistive heating equipment.

 

[don_resqcapt19]

While I don't think the code would permit this, I see no hazard, so I would not have an issue with the proposed installation.

 

[Jraef]

Supplied power where its being installed is (240 -1Ø) / (415-3Ø) volts, so no 480V involved and no step down transformer needed. A 40 amp breaker will hold but is that allowed under the NEC? To size according to the lower wattage or are you required to size a breaker to the rating of the equipment? In this case, the 42kW rating. If you select the higher wattage, the breaker trips and now you know to follow the posted sign in my opinion.

Where are you installing something that runs on 415Y240V that would need to comply with the US NEC? That's not a normal voltage here.

Be that as it may, the NEC is not concerned with what you SAY you will be doing, it is concerned for what a piece of equipment is RATED for. If it is rated for 40kW, it is what it is.

 

[petersonra]

It kind of depends on how you get the lower amount of heat. Some heaters you can wire up either in wire or Delta configuration and you get a different amount of heat. If that was the case, then there's no reason why you can't provide conductors and circuit protection for the way the heater is actually wired.

 

[_Sam]

Where are you installing something that runs on 415Y240V that would need to comply with the US NEC? That's not a normal voltage here.

Be that as it may, the NEC is not concerned with what you SAY you will be doing, it is concerned for what a piece of equipment is RATED for. If it is rated for 40kW, it is what it is.

Where in code does it say it has to be for what its RATED for? It only says it should be sized for the continuous load. 21kW is the continuous load.

 

[_Sam]

The instructions usually say MOP and MCA.

The maximum is a suggestion. Minimum is not.

So, run full sized wires and protect at whatever.

The plan is to run full sized wire and protect at the continuous load of 21kW

 

[_Sam]

Yes, missed that, thanks. But it seems like if the disabling is done in a reversible way (remove a wire and cap it), there would be no warranty issue.

A related question: say the selection is done by turning a knob. And you externally mechanically prevent the knob from turning, without modifying the equipment at all. Effectively locking out the higher wattage setting. Would that be sufficient for wiring the equipment based on only the lower setting?

Cheers, Wayne

Looking into a way to lock out the higher setting