Mike Dolt Enterprises
Mike Dolt Enterprises
User: sirdle, Location: California
Asked on: Monday, April 4, 2011 at 1:34 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:44 am EDT
Question: I have a lot of trouble understanding NEC 110.14(C) because I don't really understand where the heat comes from and what we are trying to protect from what. Are we worried about (1) the heat generated in the conductor, (2) the heat generated by the equipment at it's terminals, or (3) the ambient heat?Example #1 - a 90 deg conductor ( #8 THHN) connected to a 75 deg equipment terminal, pulling 55 amps.The heat generated by the current flowing through the conductor will not damage the conductor insulation, but will damage the equipment terminals? So I can still use 90 deg THHN as long as I increase the wire size to #6 (75 deg rating of 65 amps)?Example #2 - a 60 deg conductor (#8 TW) connected to a 75 deg equipment terminal, pulling 40 amps.The heat generated by the current through the conductor will not damage the conductor insulation or the equipment terminals... but the heat generated by the equipment at the terminals could rise to 75 deg and that heat can damage the conductor insulation? So a 60 deg conductor cannot be used at all no matter what the actual current flow is?Example #3 - (this is what I'm leading up to) - An immersion heating element with no equipment markings, connected to some power SCRs with no termination markings. An equipment drawing from the manufacturer requires a minimum of #6 90 deg copper wire. The heating elements are rated at 41.7KW at 480VAC, 3 ph.It seems to me that the load is 50.2A ( = 41,700 watt / 480 vac / 1.73) and requires an ampacity of at least 63A (at 125%). Except when I shoot the temperature inside the heating element termination enclosure, I get a reading of 200 degF.So if the SCR had a 75 deg termination rating (I'm making this up just to understand the concept) and the conductor had at least a 75 deg rating, then the current and the SCRs aren't the source of the heat I should be worrying about - it is the ambient heat inside the heating element termination enclosure. But the temperature correction factors for THHN don't go higher than 158deg. So I can't use THHN and I would need to bump up the insulation to a Type Z (Table 310.18)?If I did, the conductor would be rated at 302deg... but not the mechanical connectors.And in any event, I can't connect the Type Z to the 75deg terminations at the SCR because Table 310.18 doesn't have a 75deg column.So at this point my head explodes.Any clarification would be greatly appreciated.
Heat Sources and Concerns in NEC 110.14(C)
NEC 110.14(C) addresses two primary sources of heat and concerns: 1. Heat Generated by Current Flow in Conductors
This heat is caused by the electrical resistance of the conductor. Excessive heat can damage the conductor insulation and eventually cause a failure.
2. Heat Generated by Equipment at Terminals
This heat is caused by the electrical connections between the conductor and the equipment. Poor connections or high currents can create a localized heat spot that can damage the equipment terminals or the conductor insulation.
Example #1 The heat generated by the current flowing through the #8 THHN conductor (rated for 90°C) is not a concern because the conductor insulation can handle it. However, the heat generated at the 75°C equipment terminals could potentially damage the terminal connection or the conductor insulation. Increasing the wire size to #6 THHN would alleviate this concern by reducing the current and heat generation at the terminals.
Example #2 The heat generated by the current flowing through the #8 TW conductor (rated for 60°C) is not a concern because the conductor insulation can handle it. However, the heat generated at the 75°C equipment terminals could potentially damage the conductor insulation if the terminal temperature exceeds 60°C. Therefore, #60°C TW conductors should not be used in this application, regardless of the actual current flow.
Example #3 The heat generated by the immersion heating element is not a direct concern for the electrical conductors or terminations. However, the ambient heat inside the heating element termination enclosure can potentially damage the conductors or terminations. In this case, the manufacturer's requirement for #6 90°C copper wire indicates that the conductors must be rated for higher temperatures than the ambient heat. Type Z conductors, rated for 302°C, could be used in this application, but they cannot be connected to 75°C terminations on the SCRs.
Conclusion
NEC 110.14(C) aims to protect conductors and terminals from damage caused by either the heat generated by current flow or by the heat generated by equipment at terminals. It requires the use of conductors and terminations rated for the expected heat conditions. In the case of high ambient heat, more heat-resistant conductors may be necessary. Flag for review