Four-pole Heat Trace Contactor

W

warraich

Member
Location
Toronto
Occupation
Electrical Engineer
Hi Everyone,

I am reviewing a record drawing and would appreciate your input on the following:

A 4-pole heat trace contactor is being fed from a 120/208V three-phase, 4-wire panelboard via a 4C (four-conductor) armored Teck90 cable. This contactor controls three separate 120V heat trace cables, each of different lengths. The breaker in the panelboard is a 3P 15A.

The contactor is controlled by a thermostat, which switches all three heat trace cables on/off simultaneously.

My Questions:​

  1. Why is a 4C cable required from the panelboard to the contactor?
    • My understanding is that three conductors are for the three phases, and the fourth is the neutral. But why is a neutral needed?
    • Could it be because the contactor coil operates at 120V and requires a neutral?
    • Or is it due to the different heat trace cable lengths, creating an unbalanced load that necessitates a neutral return path?
  2. Why is a 4-pole contactor used instead of a 3-pole contactor?
    • Would a 3-pole contactor be sufficient in this application? If not, what advantage does the extra pole provide?
I’d love to hear your thoughts! I am relatively new to working with contactors and want to understand this setup better. Am I missing any other possible reasons?
 
Last edited:
warraich said:
Hi Everyone,

I am reviewing a record drawing and would appreciate your input on the following:

A 4-pole heat trace contactor is being fed from a 120/208V three-phase, 4-wire panelboard via a 4C (four-conductor) armored Teck90 cable. This contactor controls three separate 120V heat trace cables, each of different lengths. The breaker in the panelboard is a 3P 15A.

The contactor is controlled by a thermostat, which switches all three heat trace cables on/off simultaneously.

My Questions:​

  1. Why is a 4C cable required from the panelboard to the contactor?
    • My understanding is that three conductors are for the three phases, and the fourth is the neutral. But why is a neutral needed?
    • Could it be because the contactor coil operates at 120V and requires a neutral?
    • Or is it due to the different heat trace cable lengths, creating an unbalanced load that necessitates a neutral return path?
  2. Why is a 4-pole contactor used instead of a 3-pole contactor?
    • Would a 3-pole contactor be sufficient in this application? If not, what advantage does the extra pole provide?
I’d love to hear your thoughts! I am relatively new to working with contactors and want to understand this setup better. Am I missing any other possible reasons?
Click to expand...
 
See bold text.

You stated 120v heat trace cables.

So yes, a netural is required in this case.



Now for why the netural is ran through a set of the contacts of the four pole. I'm not sure why, I would not. A poor connection there may cause a voltage imbalance on the 120v heat trace cables or damage them. Look at it as a MWBC. What if they have a different resistive value. You did mention different length.
 
Welcome to the forum.

warraich said:
  1. Why is a 4C cable required from the panelboard to the contactor?
    • My understanding is that three conductors are for the three phases, and the fourth is the neutral. But why is a neutral needed?
    • Could it be because the contactor coil operates at 120V and requires a neutral?
    • Or is it due to the different heat trace cable lengths, creating an unbalanced load that necessitates a neutral return path?
Click to expand...
Both. Even with equal loads, the neutral point of three wye-connected loads should not be left floating due to incidental differences in wiring and tolerances.

warraich said:
  1. Why is a 4-pole contactor used instead of a 3-pole contactor?
    • Would a 3-pole contactor be sufficient in this application? If not, what advantage does the extra pole provide?
Click to expand...
None that I can see, unless there is a particular reason to completely isolate the load when off.

warraich said:
I’d love to hear your thoughts! I am relatively new to working with contactors and want to understand this setup better. Am I missing any other possible reasons?
Click to expand...
You would need to ask the designer.
 
Let say for some reason they want the netural switched. I would use three two pole contactors or a six pole. The netural would have a good splice with one wire to each of the three two pole contactors or to each netural pole of the six. This way if there is an issue with one of the two pole contactors or a pole of the six there's no effect on the netural to the other loads. Then each line to neutral load back to the MWBC netural splice would be a simple series circuit ran trough a switching device, aka contactor.

As other as mentioned ask the designer and get there I put.

Also with out seeing the drawing I am just assuming the neutral is hooked to the contactor. It may just be a four pole contactors using three of the poles.
Devil is in the details, in this case the drawing.
 
warraich said:
@Tulsa Electrician Thanks! What would be an alternate to running the neutral through the fourth pole?
Click to expand...
A solid connection of incoming to all outgoing neutral conductors instead of having the contact in series with the incoming conductor.

If the heat trace cable on separate "phases" were to be individually controlled you would have to do this.

If each heat trace is identical then the load is balanced and you kind of sort of get away with not having a neutral conductor from the supply and things still work properly. That said if it is self regulating heat trace then the load will vary as conditions vary in each section of trace cable and there is no assurance of balanced load. This would result in voltage variances to each trace cable as current changes if you don't have a solid neutral. A solid neutral keeps the voltages the same regardless of any differences in current on each phase.
 
You must log in or register to reply here.
Top