Derating a conductor ampacity,,,,,,,

[dicklaxt]

I understand the final ampacity and reasons for, cannot exceed the 75 degree ampacity etc etc but why can the ampacity of 90 degree wire be used as a basis along with its derating factor when derating,,,,,,,,,,I stubbed my toe again.If I must use 75 degree wire why shouldn't I use the factor for same.

I know this is a much repeated topic but humor me:grin:

dick

 

[480sparky]

You are derating the conductor, not the terminals. Derating is always done from the ?C column the conductor insulation is in. If it's a 90?C insulation, you derate from the value given in the 90?C column. If it's a 75? insulation, you derate starting from the 75?C column.

 

[charlie b]

I know this is a much repeated topic but humor me

Since it is a repeated topic, I will repeat my comment from a recent thread:

110.14(C) allows us to start the derating at the 90C column, although I will agree it is not at all clear in its wording. The reason you can start at the 90C column is that the conductor itself is rated to handle 90C above ambient. That is what the pair of "H" code letters within "THHN" is telling us. The "T" means thermoplastic, with a base temperature rating of 60C. Each "H" adds one "level of heat" at 15C. So two "H" letters means 60C + 15C + 15C, and that is why this conductor shows up in the 90C column. Since it can take 90C, without damage to its insulation, you can start with the ampacity in that column, and be confident that whatever the derated value might be, the conductor itself would not be damaged by a current at that level. Then you have to take into consideration the terminations, and that is what limits you to the value in the 75C column.

 

[Dennis Alwon]

You are derating the conductor, not the terminals.

Well said. Think of it this way. The terminal may be capable of say 100 or even 200 amps but the temp. of the terminal is only good for 75C. So if we install a wire at 90C and load it to the max that would mean the temp of that wire may reach 90C-- this is fine except that the heat would also be transmitted to the terminal which only wants 75C. Does that make sense.

I have heard there are a few terminals out there that will acccept 90C but I know of no breaker that will accept 90C. There may be but not standard.

 

[dicklaxt]

Since it is a repeated topic, I will repeat my comment from a recent thread:

That fits the question I was asking,,,,,,, thanks Charlie.

Dennis you brought up a good point of terminal rating however I would think that terminations in 2001V and above must be rated somewhat higher because of the 90 & 105 degree cables that are listed for that use,,,,am I correct in that with it being higher in SWGR,OCB's,Transformer primarys etc?

dick

 

[Dennis Alwon]

am I correct in that with it being higher in SWGR,OCB's,Transformer primarys etc?

dick

I'm a resi guy I have no idea but that would make sense.

 

[Pierre C Belarge]

Quote from Charlie B


"Originally Posted by charlie b
110.14(C) allows us to start the derating at the 90C column, although I will agree it is not at all clear in its wording. The reason you can start at the 90C column is that the conductor itself is rated to handle 90C above ambient. That is what the pair of "H" code letters within "THHN" is telling us. The "T" means thermoplastic, with a base temperature rating of 60C. Each "H" adds one "level of heat" at 15C. So two "H" letters means 60C + 15C + 15C, and that is why this conductor shows up in the 90C column. Since it can take 90C, without damage to its insulation, you can start with the ampacity in that column, and be confident that whatever the derated value might be, the conductor itself would not be damaged by a current at that level. Then you have to take into consideration the terminations, and that is what limits you to the value in the 75C column."



12 AWG Type THWN, THHN, etc, all have the same size copper in the "conductor".

It is the insulation type that permits us to use the different columns in Table 310.16

Copper melts at a much higher temperature than the insulation values we install.

 

[augie47]

Hopefully Charlie's excellent post #3 cleared up the picture for you.
If not, it may help to think of it in terms of heat...it helps me..
A #4 wire carrying 100 amps will, for practical purposes, reach a certain temperature. In time, what ever the conductor is connected to will also reach that temperature as will the insulation.
If the conductor is in a high ambient situation, between 75? and 90?, the insulation will get even warmer, but, if it's a 90? rated insulation, there will be no damage.
Maintaining the load at the 75? ampere rating will prevent the 75?termination equipment from overheating. Using a 90? insulation will protect the conductor in higher heat locations (such as in a conduit)

sorry, Pierre, I stepped on you... 1 finger typing isn't productive