Cabinet heaters as excepted circuits

[Fischi]

Hello everyone,

my name is Oliver. I'm an electrical engineer from Germany. My company ist starting to build machines for the USA and now I have to design the first control panel. It's quite a challenge because there are many differences between northern american and european regulations. I studied UL 508A and NFPA but I'm still not sure about a particular problem.

Our machines shall be designed for ambient temperatures of -40 to +40 °C (-40 to 104 °F). The main switch is interlocked with a thermostat because there are devices which are only suitable for temperatures greater than 0 °C (32 °F). In case of a longer blackout the cabinet has to heat up before main switch can be turned on. Therefore the heater circuit has to be derived before main switch. According NFPA this would be an excepted circuit. So I need a disconnecting means, isolating transformer, and overcurrent protection. Right?
The cabinet is supplied by WYE 208/120 V. Technically i would't need a transformer because I already have 120 V for the heaters. If I need a transformer nevertheless I would use a 208/120 V isolating transformer. Is this excepted circuit also a control circuit and therefore limited to 15 A? In this case i would need a second transformer because one doesn't have enough power. Besides the heaters there are other devices which have to be derived before main switch, e. g. lighting, receptacle, air conditioning unit. Two transformers would be pretty "unpleasant". I appreciate any advice.

 

[Jraef]

I’m not exactly sure why you think you need a transformer. All you really need is a separate disconnect and circuit protection.

But that said, one potential pitfall to your 208/120 source issue is that to attain 120V without a transformer, someone must run a neutral conductor to the panel (which also must have a terminal for it). That MIGHT become a problem in some places, because it is common for electricians to see a requirement for 3 phase and only run the 3 line conductors plus ground, they might miss the fact that it needs a neutral and not pull it in the conduit. You can of course make it VERY CLEAR that it requires a 4 wire source, I’m just warning you that in real life, that detail gets missed, a lot. Using a transformer eliminates that possibility.

Aside from the separate disconnect, what I have done in this situation is to have a line contactor that feeds power to all of the cold temperature sensitive devices, and have a thermostat near the load terminals of that contactor that is controlling the coil for it. The heater is then tapped off of the single Main breaker to keep it simple, but line power is kept off of the risky devices until the thermostat gets above the required temperature. Then it energizes the contactor coil and everyone gets power. I also put an amber (warning) light on the front door tied to a NC aux contact of that contactor, with a label that says “Freeze Protection Engaged” so that operators understand why their machine is not starting.

 

[Fischi]

I’m not exactly sure why you think you need a transformer. All you really need is a separate disconnect and circuit protection.

But that said, one potential pitfall to your 208/120 source issue is that to attain 120V without a transformer, someone must run a neutral conductor to the panel (which also must have a terminal for it). That MIGHT become a problem in some places, because it is common for electricians to see a requirement for 3 phase and only run the 3 line conductors plus ground, they might miss the fact that it needs a neutral and not pull it in the conduit. You can of course make it VERY CLEAR that it requires a 4 wire source, I’m just warning you that in real life, that detail gets missed, a lot. Using a transformer eliminates that possibility.

Aside from the separate disconnect, what I have done in this situation is to have a line contactor that feeds power to all of the cold temperature sensitive devices, and have a thermostat near the load terminals of that contactor that is controlling the coil for it. The heater is then tapped off of the single Main breaker to keep it simple, but line power is kept off of the risky devices until the thermostat gets above the required temperature. Then it energizes the contactor coil and everyone gets power. I also put an amber (warning) light on the front door tied to a NC aux contact of that contactor, with a label that says “Freeze Protection Engaged” so that operators understand why their machine is not starting.

Thank you very much for the hint! The whole plant will have an overall power consumption of more than 700 kVA. Our costumer will have to deliver the needed network types. In case of the control panel it's a wye 207/120 V, 3 phase + N + PE. This control panel itself has an power consumption of maybe 50 kVA cause it will also supply up to 10 other modules with 120 V single phase. Therefore I chose a 3 phase + N infeed. The main circuit breaker will be a 4 pole type.

Good idea to use a contactor controlled by a thermostat. Problem is that the main breaker is only suitable down to -25 °C (-13 °F). For lower temperatures I have to interlock it with a thermostat. Hence I have to derive the heater circiut before main breaker. According to NFPA 79 this would be an excepted circiut. It also says that an excepted circuit has to be provided with a disconnecting means, isolating transformer and overcurrent protection. Finding these components with approval down to -40 °F will also be a challenge.

 

[hmspe]

I would not expect to see a 4 pole main circuit breaker for North American market equipment. The grounded conductor (neutral) is not generally switched by circuit breakers here.

I would also be concerned with having anything connected before the main breaker. It is common to use the main breaker as the local disconnecting means for the equipment. If there are loads tapped before the main breaker then an external local disconnecting means would generally be required. I agree with Jraef and would use a contactor after the main breaker instead of tapping before the main breaker for the heater.

 

[Fischi]

I would not expect to see a 4 pole main circuit breaker for North American market equipment. The grounded conductor (neutral) is not generally switched by circuit breakers here.

I would also be concerned with having anything connected before the main breaker. It is common to use the main breaker as the local disconnecting means for the equipment. If there are loads tapped before the main breaker then an external local disconnecting means would generally be required. I agree with Jraef and would use a contactor after the main breaker instead of tapping before the main breaker for the heater.

Now I'm confused. UL 508A says under 75.5 that a neutral conductor shall be provided with a disconnecting means, e. g. another pole of the main disconnecting means.

Is it allowed to place a contactor in the feeder circuit directly after main breaker (molded case switch) or do I need a branch circuit protection before the contactor?