Fixed electric space heaters

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tlsa98

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I work in an industrial facility, in one of the buildings we have a 20kw 480v 3ph heater in 2 corners. The nameplate states 24.4 full load amp. They share a 60amp breaker. 3 number 6 wires leave the breaker to a junction box perhaps 50 feet away. Then the number 6 wire is spliced to number 10 wire that then feeds the heaters perhaps 30 feet away. Near the heaters are 30amp fused disconnects. I happened to look inside one of the disconnects the other day and noticed that the top of the disconnect has been very hot causing some plastic to melt and the insulation on the number 10 wires on the line side of the disconnect was charred away for an inch or 2 and the tops of the fuses were very black. The wires on the bottom of the fuses looked normal. So seeing this I decided to look at the other heater disconnect and it was in just as bad a shape as the first one. The heaters have been in service like this for 20 years. Looking at article 424 I'm not sure if the wiring method is to code or not. Can 2 heaters this size be fed from 1 60 amp breaker? Was it ok for the number 10 to be spliced to the number 6 and ran to the disconnects?
I know this was long winded but I'm hoping with this much detail someone will know exactly what I should do.
Thanks,
Tony L Sikes
My email address is Tony.Sikes@Linde.com
 
augie47

augie47

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"Near" could make a difference as far as Code compliance. Barring some special circumstance, 240.21 tap rules would limit the #10 taps to 25 ft.
IMO, the conditions you relate would most likely be from loose connections although a 30 amp circuit is actually a hair under the 125% requirement of Art 424.
 
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tlsa98

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iwire said:
I agree with Gus, the taps could be fine.
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Ok, just to clarify, It is ok by code to have 2 fixed electric space heaters fed by one 60 amp breaker with number 6 spiced to number 10's as long as the number 10's don't exceed the 25ft tap rules? I agree loose connections could be the cause. I forgot to mention the number 10's are solid wire not stranded.
 
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tlsa98

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tlsa98 said:
Ok, just to clarify, It is ok by code to have 2 fixed electric space heaters fed by one 60 amp breaker with number 6 spiced to number 10's as long as the number 10's don't exceed the 25ft tap rules? I agree loose connections could be the cause. I forgot to mention the number 10's are solid wire not stranded.
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By the way Gus and Iwire, Happy Thanksgiving! Thanks for responding so quickly. Especially on a holiday! I just know that the managers aregoing to want this addressed first thing Monday morning and I'm not sure what to tell them. Should I just replace the damaged disconnects and repull the number 10's or do I redo the entire job putting each heater on its own separate 30 or 40 amp breaker?
 
cadpoint

cadpoint

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In 2011 your held to 25 Amps for #10 via Table 310.15(B)(16) ((note ** )) via Article 240.4(D) - this article only point forward into 240.4(E).

I don't like the layout, if you think of it as a 3 line power drawing and hang 2 units of three wires
off of each, one gets the correct size to a continious load and one does not.

Sorry I mis read it's #10 - 30 amp CU - 25 for al and Cu/Al...

In any case you not sure of the 25 feet, but your heating up the wires of the #10 and the heat is running through the circuit.
 
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tlsa98

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cadpoint said:
In 2011 your held to 25 Amps for #10 via Table 310.15(B)(16) ((note ** )) via Article 240.4(D) - this article only point forward into 240.4(E).

I don't like the layout, if you think of it as a 3 line power drawing and hang 2 units of three wires
off of each, one gets the correct size to a continious load and one does not.
Click to expand...

I don't like the layout either. The funny thing is, on the other 2 corners of the building there are also electric heaters but they each have their own separate 480 breaker feeding them. One of the old timers there told me that those 2 heaters were the originals, the other 2(the ones that share a breaker) were installed around 3 years later.
 
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GoldDigger

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tlsa98 said:
Ok, just to clarify, It is ok by code to have 2 fixed electric space heaters fed by one 60 amp breaker with number 6 spiced to number 10's as long as the number 10's don't exceed the 25ft tap rules? I agree loose connections could be the cause. I forgot to mention the number 10's are solid wire not stranded.
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The key points in the tap rule that need to be observed are that the actual load not exceed the capacity of the tap wires and that the overcurrent device at the load end of the tap wires be sized to protect the wires at or below their ampacity.
The second point is only one which is in doubt, depending on the type of wire used and whether any derating factors are applied. Article 240.4(D)(7) appears to permit this for #10 if all other conditions are met.
The additional limitation on the tap conductor amperage relative to the 60A upstream OCPD is clearly met.

I would be less comfortable with this for a motor load than for a resistive load.
 
augie47

augie47

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I don't have my 2011 Code handy but have no doubt of the post, however, since the OP states the install is 20 years old I would tend to think we can use the 35 amp ampacity & 30 amp OCP allowance from Codes prior to '11.
That said, I think the point concerning load distribution is worth looking into.
24.4 amps 3 phase 480 would calculate to the 20kw but the possibility of the load not being evenly distributed and the question about any blower load raises questions as to the actual load per phase.
With it being in operation that long, I still lean toward connections or the inherent possibility of loose components in the disconnects as the main problem.
I would suggest verifying the load on each phase to assure no one phase exceeds the 24.4, but before I reinvented the wheel I'd lean toward replacement disconnects.
 
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tlsa98

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GoldDigger said:
The key points in the tap rule that need to be observed are that the actual load not exceed the capacity of the tap wires and that the overcurrent device at the load end of the tap wires be sized to protect the wires at or below their ampacity.
The second point is only one which is in doubt, depending on the type of wire used and whether any derating factors are applied. Article 240.4(D)(7) appears to permit this for #10 if all other conditions are met.
The additional limitation on the tap conductor amperage relative to the 60A upstream OCPD is clearly met.

I would be less comfortable with this for a motor load than for a resistive load.
Click to expand...

Thanks, by the way, the fuses in the disconnects are 30 amp FRS-R-30 These disconnects are mounted within 4 feet of the heaters.
 
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tlsa98

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augie47 said:
I don't have my 2011 Code handy but have no doubt of the post, however, since the OP states the install is 20 years old I would tend to think we can use the 35 amp ampacity & 30 amp OCP allowance from Codes prior to '11.
That said, I think the point concerning load distribution is worth looking into.
24.4 amps 3 phase 480 would calculate to the 20kw but the possibility of the load not being evenly distributed and the question about any blower load raises questions as to the actual load per phase.
With it being in operation that long, I still lean toward connections or the inherent possibility of loose components in the disconnects as the main problem.
I would suggest verifying the load on each phase to assure no one phase exceeds the 24.4, but before I reinvented the wheel I'd lean toward replacement disconnects.
Click to expand...

Sounds good. I agree. I think I will purchase 2 new disconnects. Replace the old solid #10's with some good THHN stranded 10's. Make sure everything is good and tight. Turn the heaters on and get some amp readings. Thank you all and goodnight, it's getting late and I ate too much today as usual on Thanksgiving!
 
kwired

kwired

Electron manager
Location
NE Nebraska
If unit FLA is 24.4 it should have had a 30.5 minimum condctor ampacity and a 35 amp overcurrent device - even 20-30 years ago to meet code.

That said 10AWG and 30 amp overcurrent device is still likely to give little troubles even though it is not quite code. I would guess the disconnect(s) maybe failed more so than the conductor had failed.

If we assume for a moment that 10 AWG is ok, I still think there is probably tap rule violations in this install unless the taps are outside or less than 25 feet.
 
iwire

iwire

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kwired said:
If unit FLA is 24.4 it should have had a 30.5 minimum condctor ampacity and a 35 amp overcurrent device - even 20-30 years ago to meet code.

That said 10AWG and 30 amp overcurrent device is still likely to give little troubles even though it is not quite code. I would guess the disconnect(s) maybe failed more so than the conductor had failed.

If we assume for a moment that 10 AWG is ok, I still think there is probably tap rule violations in this install unless the taps are outside or less than 25 feet.
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Why would 10 be insufficient for 30.5 amps?
 
iwire

iwire

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kwired said:
It is not. But if you increase overcurrent protection to 35 (as you really should for this application) then you have a violation of 240.4(D)
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I don't think it would be, they are taps. I don't believe 240.4(D) applies to taps.

I guess it might be an issue between the disco and the heater.
 
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