Wire Ampacity

A

Alwayslearningelec

Senior Member
Location
NJ
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Estimator
I was listening to the 2023 fundamentals NEC calculation chapter 2 starting at 21:16.

It was referring to 4 wires in raceway. He was doing the adjustments but giving a different ampacity to the wire in the conduit and the wire in the box @ the terminals.

That’s correct to have two different ampacities for same wire at different points?

When using adjustment and correction and it's a continuous load you don't use 125% for the portion in the raceway?
 
Yes, a run can have more than one ampacity assigned for different portions of a run. That can enable you to use different wire sizes for different portions of the run.

The concepts of continuous loads and adjustments/corrections are unrelated. You calculate the load taking continuous and non-continuous loads into account. Then you select wire sizes with sufficient ampacity for that load. The value of ampacity will depend on conditions of installation, including the use of conduit. Then you select a breaker capable of protecting the wires.
 
Alwayslearningelec said:
It was referring to 4 wires in raceway. He was doing the adjustments but giving a different ampacity to the wire in the conduit and the wire in the box @ the terminals.
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Since you mentioned "4 wires in a raceway" one would assume that there were 4 CCC's, otherwise there would be no derating required.
 
Alwayslearningelec said:
I was listening to the 2023 fundamentals NEC calculation chapter 2 starting at 21:16.

It was referring to 4 wires in raceway. He was doing the adjustments but giving a different ampacity to the wire in the conduit and the wire in the box @ the terminals.

That’s correct to have two different ampacities for same wire at different points?

When using adjustment and correction and it's a continuous load you don't use 125% for the portion in the raceway?
Click to expand...
Very often the ampacity of the conductors after the application of any required adjustment and/or corrections will exceed the 75°C ampacity as shown in Table 310.16. However, most equipment terminations limit you to the 75°C or sometimes the 60°C ampacity.
 
charlie b said:
The concepts of continuous loads and adjustments/corrections are unrelated.
Click to expand...
They are actually related. In the portions of the circuit where ampacity adjustment/correction apply (the run of the wire), there is no continuous load factor. The wire ampacity is itself a continuous rating. In the portions of the circuit where you need to use a 125% continuous load factor (terminations that are not 100% rated), you do not need to apply ampacity adjustment/correction.

Really "wire ampacity" and "termination ampacity" are two different but related concepts, and the NEC should use two different terms for them.

Cheers, Wayne
 
If you have a 50A continuous load with 4 CCC's in EMT.

55A X 80% then use 90 degree column for wire ampacity in the raceway?
50A X 125% then use 75 degree column for wire ampacity in panel/terminals?
 
Depends on if you're calculating ampacity of a specific wire or the needed ampacity based on load. If you need to support 50A and need to run 4 current carrying conductors, you need a wire ampacity of 50/.8 = 62.5A which could be referenced to the 90C rating if its 90C wire. Or, if you're starting with a wire that has a 70A ampacity at 90C, its ampacity will be 70*.8 = 56A when there a 4 current conductors together in a raceway. You need this number to be 50A or larger.

To some people, starting with a specific wire size and calculating the deratings makes sense to see if it works. Others start with the load and work the deratings to determine the needed conductor. I usually start with the wire size based on termination temp. Then check to see if that wire size still works based on deratings from conduit fill and ambient temperature deratings.
 
Alwayslearningelec said:
If you have a 50A continuous load with 4 CCC's in EMT.

55A X (Product of both applicable derate factors, based on environment of wire) then use 90 degree column for wire ampacity in the raceway?
Max continuous current * 125%, then use 75 degree column for wire ampacity in panel/terminals?
Click to expand...
Generally speaking, yes, that is the most common case, with the edits to your quote.

60C terminations are the default for equipment 100A and less, but most commonly, equipment is listed otherwise for 75C.
90C terminations exist as well, when products have special listings. You have a burden of proof on both ends of the wire.
Continuous duty rated equipment is also a special product rating to consider, that allows you to forgo the 125% continuous load factor for that particular device. Uncommon, but it is a product listing that may work to your advantage.

Wire less than 90C-rated also exists, but is uncommon for most wire made today. Places you can expect to see ratings <90C:
1. Smaller sizes of some versions of THHN/THWN, that only carry the -2 rating in large enough sizes, e.g. #8 and larger. This designation without -2, means it's 90C dry and 75C wet.
2. Manufactured cable assemblies, e.g. UF cable.
3. Special environments where you need petrol/oil resistance.
4. Older wire
 
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