branch circuit protection needed for multiple heaters?

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emiller233

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pittsburgh, pa
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Controls-Automation engineer
We are having some questions about protection for multiple heaters that are going in a hydraulic oil tank.

There will be qty=4 heaters (each are 3-phase, 480VAC, 3KW) in the tank.

some guys are saying that we can get away with just a single 3-pole fuse and contactor bc all of the heaters are heating the same tank. they would run a single cable to all of the heaters and daisey chain them in parallel.

the other half of guys are saying that we need branch circuit protection, a non-fused Main Disconnect feeding qty=4 fuse blocks, and each fuse block would have a contactor for each heater. We would then have to run 4 conduit runs(1 to each heater)

while running a single setup is obviously cheaper and easier, what is the correct way to protect multiple 3-phase heaters?
 
I don't think there is anything in the code that says you have to do it one way or the other. Article 427 seems to have nothing relevant to say.

Do the heaters have a maximum overcurrent device specification? If so, and it's less than what would be required for the overall load, then you'll need to provide individual overcurrent protection.

Otherwise I think you can do it either way, but if you don't have individual protection then all wiring has to be rated for the entire load so that it would be protected by the single overcurrent device.

It might be advisable to have separate disconnecting means for each heater. But that isn't exactly what you asked and I don't think the code has an opinion.
 
I think this is an application covered by article 427.

427.55 covers required disconnecting means.

427.57 covers overcurrent protection.
 
jaggedben said:
I don't think there is anything in the code that says you have to do it one way or the other. Article 427 seems to have nothing relevant to say.

Do the heaters have a maximum overcurrent device specification? If so, and it's less than what would be required for the overall load, then you'll need to provide individual overcurrent protection.

Otherwise I think you can do it either way, but if you don't have individual protection then all wiring has to be rated for the entire load so that it would be protected by the single overcurrent device.

It might be advisable to have separate disconnecting means for each heater. But that isn't exactly what you asked and I don't think the code has an opinion.
Click to expand...

I am not sure yet, as I am still waiting on the mechanical engineers to pick out a specific part # ....?
I will double check that one they do!


petersonra said:
I think this is an application covered by article 427.

427.55 covers required disconnecting means.

427.57 covers overcurrent protection.
Click to expand...


I think we would be fine with 427.55 as we would be within eyesight of the heaters (maybe 10 feet away from the electrical box)
I checked the sections that 427.57 referred to, and I didn't see anything there that specified one way vs the other there either


we don't use heaters very often, could anyone double check that I calculated the amperages correctly?
qty=4 heaters
each heater is 3KW (3-phase)
3,000 / 480 / 1.732 = 3.61 Amps per heater

4 heaters * 3.61Amps = 14.44Amps
120% = 17.32Amps total


is THHN wire acceptable to use inside of liquidtite, or should we use some kind of high temp wire?
 
There is a slight chance that the heater manufacturer specifies a maximum OCPD for the circuit that feeds them. You can't just add those up.

Sent from my XT1585 using Tapatalk
 
GoldDigger said:
There is a slight chance that the heater manufacturer specifies a maximum OCPD for the circuit that feeds them. You can't just add those up.

Sent from my XT1585 using Tapatalk
Click to expand...


The manufacturer doesn't specify a maximum OCPD, so if the circuit is designed to handle them all, why cant we just 'add them up'?
This was our original question, that we can't find anything in the code that actually says that we cannot. can you show me where it says that we can't?
 
IMO, you put these all on one 20A circuit. If these are temperature controlled, and will likely never be on continuously for more than three hours, you do not have to multiply by 120% (actually the continuous factor is 125%)... and you could power them with a 15A circuit... but I'd go with 20A. Oh, and yes you can add the amps with 3Ø heating elements.

Using THHN/LFMC depends on the temperature in/at the terminal enclosure. You will have to ask the manufacturer about that.
 
personally, if it was me I would put a set of class CC fuses on each heater. that is not because I think it is code required, just a design decision based me being conservative and hoping that the extra $100 it would cost me would pay off in some level of added protection against some kind of mishap that might shut my process down.

I would also want some kind of high limit shut down to trip in case the heaters somehow got uncovered and were no longer in the oil.
 
Smart $ said:
IMO, you put these all on one 20A circuit. If these are temperature controlled, and will likely never be on continuously for more than three hours, you do not have to multiply by 120% (actually the continuous factor is 125%)... and you could power them with a 15A circuit... but I'd go with 20A. Oh, and yes you can add the amps with 3Ø heating elements.

Using THHN/LFMC depends on the temperature in/at the terminal enclosure. You will have to ask the manufacturer about that.
Click to expand...

These are temperature controlled via Plc, and they likely will never turn on during normal operation. They will likely only be turned on after installation at the customers sight to get the oil up to operating temperature (not even during maintenance, there is redundant everything)

We were trying to find info on the 125% rating, didn't have any luck...
I will go with 12awg wiring.

Is a 20A fused disconnect that feeds a contractor along with an Electronic overload relay(set to total amperage) an acceptable setup for heaters?
 
petersonra said:
personally, if it was me I would put a set of class CC fuses on each heater. that is not because I think it is code required, just a design decision based me being conservative and hoping that the extra $100 it would cost me would pay off in some level of added protection against some kind of mishap that might shut my process down.

I would also want some kind of high limit shut down to trip in case the heaters somehow got uncovered and were no longer in the oil.
Click to expand...

The cost is not the issue, the customer wants the smallest footprint possible. I agree that would be a better overall design, but these heaters are very rarely used(only during startup after installation). There are qty=2 150HP motors/pumps running constantly, so being able to cool the oil down is our big problem.

There is a high temp shutdown (2 of 3 setup actually) that will disable the contactor for the heater power if it's on
 
emiller233 said:
These are temperature controlled via Plc, and they likely will never turn on during normal operation. They will likely only be turned on after installation at the customers sight to get the oil up to operating temperature (not even during maintenance, there is redundant everything)

We were trying to find info on the 125% rating, didn't have any luck...
I will go with 12awg wiring.

Is a 20A fused disconnect that feeds a contractor along with an Electronic overload relay(set to total amperage) an acceptable setup for heaters?
Click to expand...
Overload is not necessary, that element will always draw same current as long as voltage stays consistent. Overloads are for motors that do vary in current as the driven load varies resistive loads won't vary unless voltage changes.
 
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