Wire Derating

[NathanF]

Hi All,

I am a little confused about a few things...

1. I met with an Electrician who told me he could fill a conduit with up to 20 current carrying conductors (table 310.15(C)(1)) and still use a #12GA wire on a 20A CB because the load for the receptacles is so minimum and 12GA wire at 90C is rated at 30A (after derating 15A). Is that true? My understanding was that the derated size still needs to be greater than your receptacle / CB size (were using 20A). I am having trouble finding the concrete answer though.

2. If most 20A breakers are rated for 75C that means my wire needs to be rated for 75C as well, correct? I am having trouble finding what receptacles are rated for. However, if breakers are at 75C then based on Table 310.15(C)(1) the derating for a 12 guage wire at 50% would actually be 12.5A. My mentor taught me to always size wire at 75C... with this issue, I guess I am a little confused at when I can size wire at 75C or 90C - if anyone has a resource for that or correct code section to look at I'd appreciate it! I feel like I am missing the obvious here, Ha

Best!

 

[augie47]

20 ccc (actually 10-20) have a derating value of 50% which would, as you noted, give you an adjusted ampacity of 15 amps and could not be installed on a 20 amp over-current device.

 

[infinity]

1) Yes you could use 20 #12s CCC's in a raceway but after derating your conductor ampacity is only 15 amps so they can only be used with a 15 amp OCPD not a 20 amp.

2) The conductor insulation is almost certainly rated for 90° C so you can use the 90° C ampacity of #12 which is 30 amps.

Edit: What Augie said.

 

[tom baker]

For Q2. Derating if required starts at the max temp rating of the conductor, for THHN it’s 30 amps at 90 deg C. But the allowable ampacity is based on the terminal temp rating. I suggest a review of chapter 310 and look at mike holts videos

 

[tom baker]

For Q1 that electrician is wrong. You are being smart to question. We all learn when you ask.

 

[LarryFine]

Tell him that nine CCCc is the greatest number of #12s that you can still have on 20a breakers.

70% of 30a is 21a.

 

[Tulsa Electrician]

Hi All,

I am a little confused about a few things...

1. I met with an Electrician who told me he could fill a conduit with up to 20 current carrying conductors (table 310.15(C)(1)) and still use a #12GA wire on a 20A CB

By the term conduit, your referring to a raceway over 24" in Length.
An example being. 100' run from the panel to a pull box with 20 CCC in the raceway.

I have heard some call a nipple, conduit. Then the rule for derate changes. Then raceway fill applies.

Just wanted you to be aware depending on his wording and installation. He could be correct.

An example, a nipple between to panels.

 

[NathanF]

Hi guys,

thank you for the replies! I did not get any notifications that anyone replied so I have not responded. Thank you for answering my questions and making sure I am on the right track.

 

[charlie b]

The reason for ampacity limits is to prevent degradation, or even total failure, of the wire's insulation system. Overheating can result from heat generated by current in the wire, from high ambient temperatures, and from heat being generated by nearby wires. That is why there are correction and adjustment factors for high ambient temperatures and for multiple wires in the same conduit (I might have those two terms reversed).

But the load on any wire and the size of the conduit do not come into play, when you are working with more than three ccc's in a conduit. It's no wonder why. Can you imagine the wording of a code requirement that says if the conduit is at least this size when the currents are at this value, you don't need to derate? I can't, and how could such a rule be enforced anyway. Also, even if the conduit is way oversized, gravity will pull all the wires to the bottom, so they will still be warming each other up.

 

[charlie b]

For Q2, you can start the derating at the highest temp for which the wire's insulation is rated. For THHN, that is 90C. After you calculate the derated ampacity, you can't use a value higher than the wire's 75C rating. I think the reference is 110.14 (C), but someone will chime in if I have that wrong (I am going on memory here).

 

[Carultch]

For Q2, you can start the derating at the highest temp for which the wire's insulation is rated. For THHN, that is 90C. After you calculate the derated ampacity, you can't use a value higher than the wire's 75C rating. I think the reference is 110.14 (C), but someone will chime in if I have that wrong (I am going on memory here).

You are correct that it is 110.14(C).

This is the primary value in having 90C rated wire, despite ultimately being limited to 75C terminations most often. The 90C rating of the wire gives you headroom for these derate calculations.

 

[Carultch]

2. If most 20A breakers are rated for 75C that means my wire needs to be rated for 75C as well, correct? I am having trouble finding what receptacles are rated for. However, if breakers are at 75C then based on Table 310.15(C)(1) the derating for a 12 guage wire at 50% would actually be 12.5A. My mentor taught me to always size wire at 75C... with this issue, I guess I am a little confused at when I can size wire at 75C or 90C - if anyone has a resource for that or correct code section to look at I'd appreciate it! I feel like I am missing the obvious here, Ha

75C is the industry norm for termination ratings that you'll see most commonly in practice. For 100A and less, you have a burden of proof to confirm through product listings that they are rated for 75C if you intend to take credit for it. It's uncommon when you need to use the 60C part of this rule, since most products are listed otherwise for 75C terminations. It's more of an academic rule, than a practical rule. You'll see it when taking an exam, or when working with older equipment.

The derate factors are completely independent of termination ratings, as the wire's own rating (90C for most of today's wire) is used to start this calculation.

 

[NathanF]

75C is the industry norm for termination ratings that you'll see most commonly in practice. For 100A and less, you have a burden of proof to confirm through product listings that they are rated for 75C if you intend to take credit for it. It's uncommon when you need to use the 60C part of this rule, since most products are listed otherwise for 75C terminations. It's more of an academic rule, than a practical rule. You'll see it when taking an exam, or when working with older equipment.

The derate factors are completely independent of termination ratings, as the wire's own rating (90C for most of today's wire) is used to start this calculation.

That was super helpful I really appreciate it!