current per phase in electric heaters 1

[robbm]

I'm trying to figure out the amount of current on each phase in an electric heater. The heater is 24kW @ 460VAC, which calculates to about 30A. The dual-phase contactor we are looking at using is rated for 25A per phase, and the engineer with the company said that the 30A load is divided equally between the 3 phases (i.e. 10A per phase). We don't have a unit to test in-house, but everyone here seems to think that isn't the case, that all 3 phases draw 30A when it's running. Can anyone lead me to some literature or provide a rationale as to who is right? I know we can just size it for 30A per phase to be safe, but obviously we don't want to do that all the time, we'd like to know what's correct.

thanks in advance.

 

[electricpete]

You want to know if heaters are sometimes unbalanced?
Certainly some are. Heaters can develope open circuits for one thing. You don’t really know unless you’ve measured, especially if the unit has been in service.

If one goes open, do the others increase their current? Depends upon the control.


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[edison123]

Line current = Power in watts/(sqrt 3 * Line Voltage)

This is the cuurent in each line, which in your case is 30 Amps

Muthu

http://www.edison.co.in

 

[rbulsara]

Neither seems right. 30A would be for a 3 phase heater and supply. What is dual phase? I take it as single phase 460V, connected between the two phases. In that case the min. current will be 24000/460 = 52A.

In either case, the calculated current will flow in each of the lines used. You also need to apply at least 25% margin for continuous loads, a requirement in NEC, but also a good engineering practice.

Rafiq Bulsara

http://www.srengineersct.com

 

[robbm]

thanks, that's what i thought. so the damn salesman trying to sell me something that won't work, go figure...

 

[waross]

What is a dual phase contactor?
Are the heaters single phase or three phase?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[robbm]

the heater is three phase, but we only turn on/off 2phases. The third phase is ran thru a fuse and straight to the heater.

 

[waross]

This must be heating and air conditioning. That is the only industry that does not follow established practice and use three pole contactors for three phase loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[robbm]

actually, its for heating up cleaning solution for food/bev industry. to eliminate the excess heat caused by switching all 3 legs, we only switch 2 legs. all motor loads we use 3phase contactors or VFDs.

 

[ScottyUK]

Maybe to eliminate the excess heat caused by overloading a 25A contactor with a 30A load.

Joking aside, it's a truly horrible practice and the only reason to do it is cost. You can get the heat load down by putting a slightly larger contactor in, but is it really a problem if a properly-sized contactor is used? The heat rejection from a 30A contactor shouldn't be more than a few tens of watts.


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If we learn from our mistakes I'm getting a great education!

 

[robbm]

The installation docs show wiring for a resistive load by only switching 2phases (see link), so i dont think we're the only ones to do this.
Regarding the heat, the few tens of watts difference in a small enclosure thats only 12x12" makes a big difference and would require a cooling device of some sort. Just using a larger contactor doesn't eliminate that issue, because we're switching the same ampload and thus dissipating the same heat.
The end result is that we're using a contactor that's rated for 40A/leg, instead of one rated for 10A/leg (30A total) as was suggested to me initially by the sales rep.

 

[robbm]

i think some confusion in my post is that i was basically looking for some definition on a heater load. a 24kW heater should run about 30A on each leg, not 30A total (10/leg). the sales rep tried saying that the FLA of the heater can be divided by 3 to find the amount on each leg, which i didn't buy, and the 2nd response at the top confirmed that for me.

 

[ScottyUK]

Sounds like you got it sorted, at least you picked up that the salesman was talking crap.

BTW, a bigger contactor almost always has a lower resistance than a smaller one so the I[sup]2[/sup]R losses are lower for a given current. Somewhere down the line it comes down to cost and size.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[robbm]

Power Dissipation
Size [A] Power Dissipation [W/A]
20 1.10
30 0.95
50 0.85
70 0.90

to me it looks like using a 50A contactor for a 30A load as you suggest will dissipate slightly less heat (2-legs*.85*30=51W vs. 2-legs*.95*30=57W), but i guess in our case upsizing it isn't worth the savings of the 6W considering space requirements for the larger one.

 

[waross]

If you ever have a contact weld shut or break internally so that it doesn't open, a three contact scheme will still interrupt the current. A two contact scheme will leave one phase connected and the current may increase by 50%.
And, if one contact fails to close, the current through the other one may increase by 50%.
The current for incandescent loads will increase by more than 50%.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottEE45]

the sales rep tried saying that the FLA of the heater can be divided by 3 to find the amount on each leg, which i didn't buy,

You are correct in not accepting the Sales Person's example here!

24 KVA @ 480V 3 Phase Electric Heater connected to a Wye System equates to 8 KVA per Line (Phase); which equates to apx. 28.88 Amperes per Line.

Suggest to use a 3 Pole Contactor, with 40 Amp Resistive / Tungsten rated Contacts.
Coil Voltage rating to match the Control Circuit's Voltage.

BTW, if this Heater is to run no longer than 179 Minutes (< 3 Hours), it would not be considered a Continuous Load, so the Branch Circuit could be 30 Amp Circuit breaker, and 3 #10 THHN CU.

If the Heater _MAY_ run for 3 Hours or more, then add an additional 25% of the FLA to the Branch Circuit's Load - which is referred to as "LCL Adder"
With the LCL Adder, the Circuit's FLA is now figured as 36.00 Amps, so the Circuit breaker would be 40 Amp 3 Pole, and the Conductors would be 3 #8 THHN CU.

Just my 2¢
Good luck
Scott

 

[ScottEE45]

Robbm,

What's the status of this Contactor issue???

Scott

Scott Thompson

http://setelectric.electrical-contractor.net

 

[robbm]

We used a 2phase contactor rated for the full load on each leg.

 

[jraef]

BTW for those unfamiliar with the "2 phase control" issue, this is a very common practice in the resistive heating industry. Contact welding, in the absence of inductive loads, is uncommon. However in the HVAC world, they even do that for motors!

This issue of "30A means 10A per phase" is a really common error made by non-electrical people. But to his very "uncalledfor" defense, if he thinks he has 3 1-phase heaters, each being 8kW at 480V* the 3 together, from a heating standpoint, would be 24kW and then each one would be 17A and the 25A contactor rating would have been fine. That is probably how he arrived at that initial value in the first place

* What he failed on in that scenario is that for it to work that way, the primary phase-phase voltage would have needed to be 831V, something that doesn't exist. When your primary is 480V Wye, the single phase - neutral voltage is 277V, so then his 8kW heaters are 28.8A each as ScottEE45 indicated. I run into non-electrical people doing that all the time as well, especially heater people.


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