Iosegueda
New User
- Location
- California
- Occupation
- Field electrician
Can someone explain when derating. Whats the difference between using the table 310.15(B)(16). Columns of 60/75/90 degrees. I know most wires of THHN
Table 310.15(B)(16)
- Thread starter Iosegueda
- Start date
ModbusMan
Senior Member
- Location
- Cleveland, OH
- Occupation
- Building Automation Engineer
It's not uncommon to have wires of one rating, but connectors of another. Gotta go by the weakest link in the chain.
You base you derating on the type insulation.. THHN is found in the 90 deg column so you use that colums's ampacity and the muliplier in the 90 deg column in 310.15(B)(1)(1) when adjusting for ambient.
Keep in mind the result is the conductors ampacity, Termination is also based on 110.14 and Art 240 rules.
Keep in mind the result is the conductors ampacity, Termination is also based on 110.14 and Art 240 rules.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Most conductors are rated 90C so we can de-rate from that column however the final opcpd cannot be larger than the 75C col or 60C column if the conductors are part of an NM Cable.
Even though NM conductors are 90 the NM canle is rated 60C so the final overcurrent protective device cannot be larger than the 60C column but you can still de-rate from the 90C column.
Most panels and their terminations are usually rated 75C so in those cases using conduit the 90C conductors can be used at 90C for de-rating but the overcurrent protective device cnnot be larger than the 75C
Carultch
Senior Member
- Location
- Massachusetts
As similar wire insulation types have improved over the decades, there's been an evolution of the rated operating temperature rating. What started as TW, is now THWN-2. The H means heat resistant (75C), the double-H means high-heat resistant (90C), and the N means nylon jacket. The "-2" means dual-rated for both 75C and 90C, in both wet and dry locations. Previous ratings such as THHN and THWN (no -2), may still appear on the datasheet and print legend to indicate backwards compatiblility with older product standards.
The wire is tested in the factory with a controlled 30C background temperature, to confirm that, at published ampacities, it stays within a certain safety margin of the temperature rating of the insulation. There are correction factors that apply to account for higher conduit air temperatures, whether from the environment's air temperature, or from sharing the raceway with more than 3 heat-generating wires, or both factors combined.
Equipment termination ratings also evolved, but not as quickly. The majority of equipment is 75C rated, but less commonly, you may get the special case of using 90C terminations, if you can prove it on both ends of the run. For 100A and less, you have to confirm you can use 75C instead of 60C for terminations, but modern equipment without such a listing is uncommon. The primary value in having 90C rated wire despite 75C terminations, is that it gives you extra headroom for the correction factors to its ampacity. Correction factors that don't apply at terminations.