Welding Cable ampacity in RMC

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aelectricalman

aelectricalman

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In two recents installations, a subcontractor used welding cable in separate capacitor bank projects due to the flexibility of the cable. For the 200kVAr bank he choose to go with 4/0 for the 3Ph Delta, on a 400 amp breaker. We are getting some increased temperatures in the wiring and conduit as a result. All seemed to be sized close enough to where it should not be heating up warm to the touch (excessively warm). It appears the wire is acceptable for the feeders but not the overcurrent protection (which doesnt explain the heat. Either way, does welding cable present an issue when used in this application? Does it typically heat up more so than THHN? I appreciate any help understanding your experiences. Thank you.
 
aelectricalman said:
In two recents installations, a subcontractor used welding cable in separate capacitor bank projects due to the flexibility of the cable. For the 200kVAr bank he choose to go with 4/0 for the 3Ph Delta, on a 400 amp breaker. We are getting some increased temperatures in the wiring and conduit as a result. All seemed to be sized close enough to where it should not be heating up warm to the touch (excessively warm). It appears the wire is acceptable for the feeders but not the overcurrent protection (which doesnt explain the heat. Either way, does welding cable present an issue when used in this application? Does it typically heat up more so than THHN? I appreciate any help understanding your experiences. Thank you.
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Welding cable is a generic term. Since most welding is done at low voltages, I would be suspect of the insulation properties. What type of cable is it, and is it marked with a voltage rating?
 
The code does not permit the use of welding cable for power circuit applications. There are some welding cables that are also listed as RHH or RHW or some other NEC conductor type. For those conductors, you would use the standard NEC ampacity tables. 4/0 is not rated for use at 400 amps at any reasonable temperature.
 
don_resqcapt19 said:
The code does not permit the use of welding cable for power circuit applications. There are some welding cables that are also listed as RHH or RHW or some other NEC conductor type. For those conductors, you would use the standard NEC ampacity tables. 4/0 is not rated for use at 400 amps at any reasonable temperature.
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On the southwire website and a few other places, it listed welding cable type ( RHH or RHW) at 4/0 for 403 Amps @75C up to 480 volt open air. In RMC im sure its not 400 amp rated. That seems a far cry from THHN which off the cuff i think is rated about 230 amps @ 75C in a raceway. Why is Welding cable rated so much higher?
 
aelectricalman said:
On the southwire website and a few other places, it listed welding cable type ( RHH or RHW) at 4/0 for 403 Amps @75C up to 480 volt open air. In RMC im sure its not 400 amp rated. That seems a far cry from THHN which off the cuff i think is rated about 230 amps @ 75C in a raceway. Why is Welding cable rated so much higher?
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NEC shows both THHW and RHW being 230 amps at 75 C in a raceway. So, the NEC doesn't rate RHW any higher.

The difference is that you are comparing apples to oranges. Look at 310.17 and you will see that 4/0 RHW is good for 360 amps. RHW-2 is good for 405 and so is THHN.
 
K8MHZ said:
NEC shows both THHW and RHW being 230 amps at 75 C in a raceway. So, the NEC doesn't rate RHW any higher.

The difference is that you are comparing apples to oranges. Look at 310.17 and you will see that 4/0 RHW is good for 360 amps. RHW-2 is good for 405 and so is THHN.
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I see. The website is using single insulated conductors ratings hence what welders would typically use. makes sence.

On a similar note, the temperature of the RHH was 59 C when measured with my IR gun. I dont like that as my normal operating temperature since the heat is from inrush and it will always be that temp. Is there an industry standard (other than appropriate wire sizing of course) that defines acceptable normal temp ratings for such an application.
 
Another interesting issue is that the heat tends to be worse around the bends (which are not kinked) and in the conduits. It seems as if there is "Overheating and an Inductive effect" going on. Article 408.3 (B) is short and sweet!
 
aelectricalman said:
Another interesting issue is that the heat tends to be worse around the bends (which are not kinked) and in the conduits. It seems as if there is "Overheating and an Inductive effect" going on. Article 408.3 (B) is short and sweet!
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I can see a pretty good chance that whoever designed the capacitor bank did not think about keeping the return conductor from the bank in the same conduit as the line conductor. (If he thought about a return conductor at all.) For a three-phase bank, the three phases would have to be kept together to avoid inductive heating.
 
aelectricalman said:
On the southwire website and a few other places, it listed welding cable type ( RHH or RHW) at 4/0 for 403 Amps @75C up to 480 volt open air. In RMC im sure its not 400 amp rated. That seems a far cry from THHN which off the cuff i think is rated about 230 amps @ 75C in a raceway. Why is Welding cable rated so much higher?
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The manufacturer's ratings cannot change an NEC rule. In most cases this type of cable is rated by the manufacturer at 90?C in free air. It would be a very rare installation where you could use that.
 
Stay way away from "welding" cable. Sure code violation and likely inadequate insulation level.

If you need super flexibility, use DLO cable which is available TC rated in 1/0 and larger. Insulation is 2000V. Cable sizes are slightly different, 350 MCM becomes 373 MCM, etc. Overall outside diameter is larger as well.

Be careful to use crimp lugs listed for DLO cable, which are available. Be sure to use those rated at 90degC, which is also available. REgular lugs will NOT work. Because of the fine stranding it's bigger in diameter and special lugs are required.

We have used DLO between a transformer secondary and the switchgear in retrofit situations where space and accessibility was tight. Obviously it costs more than THHN, etc. but works where you have no room or where you are working against a short fused shutdown. Just don't forget to run the green EGC which typically doesn't need to be super flexible since it's much smaller.

RC
 

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Ragin Cajun said:
Stay way away from "welding" cable. Sure code violation and likely inadequate insulation level.

If you need super flexibility, use DLO cable which is available TC rated in 1/0 and larger. Insulation is 2000V. Cable sizes are slightly different, 350 MCM becomes 373 MCM, etc. Overall outside diameter is larger as well.

Be careful to use crimp lugs listed for DLO cable, which are available. Be sure to use those rated at 90degC, which is also available. REgular lugs will NOT work. Because of the fine stranding it's bigger in diameter and special lugs are required.

We have used DLO between a transformer secondary and the switchgear in retrofit situations where space and accessibility was tight. Obviously it costs more than THHN, etc. but works where you have no room or where you are working against a short fused shutdown. Just don't forget to run the green EGC which typically doesn't need to be super flexible since it's much smaller.

RC
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I don't see DLO mentioned in the NEC. Is it cross rated as a Chapter 3 conductor?
 
Just like welding cable, DLO can only be used for an NEC application if it is dual listed as a type found in Table 310.13 (2008) or 310.104 (2011).
 
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