Using 90 degree column for de-rating purposes

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adamscb

Senior Member
Location
USA
Occupation
EE
All,

Ever since graduation I've been using the 75 degree column of the cable ampacity table to size wire, even though I almost always use 90 degree rated THHN, due to the fact that almost all of our MCC's have 75 degree rated equipment. However, someone told me about how you can calculate using the 90 degree column for de-rating purposes. I am trying to apply it to this example: I need to size conductors for a 400A service:

Old Method I Used:

Six 250MCM wires in one conduit - 255*2*0.8*1.00 = 408A (all values taken from 75 deg column, ambient temp correction is 1.00)

New Method Someone Told Me About:

Six 4/0 wires in one conduit - 260*0.8*1.00 = 208A (less than 75 degree ampacity of 230 for 4/0), so I can use 90 degree ampacity for 4/0, so total ampacity becomes 208 x 2 = 416A

Something about this doesn't seem right to me, can someone explain am I using this method correctly? Incorrectly?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
For a 90° C conductor you can use the 90° C ampacity so for THHN you can use 290 amps for 250 kcmil and 260 amps for #4/0 for derating purposes.
 

Sahib

Senior Member
Location
India
All,


Something about this doesn't seem right to me, can someone explain am I using this method correctly? Incorrectly?

When all six 4/0 wires in conduit and 208 A flowing, temperature is 90 degrees C. But still at the termination, temperature is less than 75 degrees C because the current at termination is less than 230A for individual 4/0.

That is how it seems right to me.:)
 

adamscb

Senior Member
Location
USA
Occupation
EE
When all six 4/0 wires in conduit and 208 A flowing, temperature is 90 degrees C. But still at the termination, temperature is less than 75 degrees C because the current at termination is less than 230A for individual 4/0.

That is how it seems right to me.:)

So what I keep getting tripped up on is I'm comparing the de-rated ampacity of a single 4/0 cable with 90 degree insulation, to that of a non-derated ampacity of 75 degree insulation...but wouldn't the 75 degree insulation ampacity have to be de-rated as well? I'm still confused as to how you can compare the two numbers...it's like comparing apples to oranges in my opinion.
 

GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
So what I keep getting tripped up on is I'm comparing the de-rated ampacity of a single 4/0 cable with 90 degree insulation, to that of a non-derated ampacity of 75 degree insulation...but wouldn't the 75 degree insulation ampacity have to be de-rated as well? I'm still confused as to how you can compare the two numbers...it's like comparing apples to oranges in my opinion.
You can look at it this way for the fill derating at least: At the termination, where the 75 degree termination forces you to use the 75 degree ampacity, you are no longer bundled or in the common raceway. So no derating of the 75 degree ampacity is needed. And in the raceway you are not at the termination, so you can use the 90 degree ampacity as the starting point.

Sent from my XT1585 using Tapatalk
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
So what I keep getting tripped up on is I'm comparing the de-rated ampacity of a single 4/0 cable with 90 degree insulation, to that of a non-derated ampacity of 75 degree insulation...but wouldn't the 75 degree insulation ampacity have to be de-rated as well? I'm still confused as to how you can compare the two numbers...it's like comparing apples to oranges in my opinion.

You are making an apple to oranges comparison because you're comparing conductors with different insulation values.
 

adamscb

Senior Member
Location
USA
Occupation
EE
You can look at it this way for the fill derating at least: At the termination, where the 75 degree termination forces you to use the 75 degree ampacity, you are no longer bundled or in the common raceway. So no derating of the 75 degree ampacity is needed. And in the raceway you are not at the termination, so you can use the 90 degree ampacity as the starting point.

Sent from my XT1585 using Tapatalk

I am still bundled though at the termination...if I'm using parallel 4/0 or 250 kcmil, right?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I am still bundled though at the termination...if I'm using parallel 4/0 or 250 kcmil, right?

A 75deg termination for two 4/0s can carry 460A without exceeding the 75deg rating. Your load will be at most 400A, so the termination temperature due to load current should be well below it's rating.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
The 90 deg C cable without adjustments, is capable of carrying the rated current at 90 deg C without damage.

However, low voltage terminations are only rated for the current listed at 75 deg C without damage.

Therefore, as long as the required calculated final current of the circuit/cable, after applying any adjustments, comes out to be equal to, or less than the value listed for 75 deg C cable, than you are home free; otherwise you have to go up in cable size; or limit the cable to the 75Deg C current.

As a side note; most medium voltage cable is rated for 105 Deg C, but MV terminations are only rated for 90Deg C, so same situation.
 

Carultch

Senior Member
Location
Massachusetts
So what I keep getting tripped up on is I'm comparing the de-rated ampacity of a single 4/0 cable with 90 degree insulation, to that of a non-derated ampacity of 75 degree insulation...but wouldn't the 75 degree insulation ampacity have to be de-rated as well? I'm still confused as to how you can compare the two numbers...it's like comparing apples to oranges in my opinion.

Most wire built today will be 90C rated, except for some THHN/THWN wire that doesn't carry the "-2" rating in small sizes, and thus couldn't take credit for 90C in wet locations. Wire that is purely 75C rated is rare.

It is not usually the wire itself that is 75C rated, it is the lug/equipment to which it terminates. Even lugs marked AL9CU (which is common) are limited to 75C, when part of manufactured equipment that is rated 75C. The wire is not bundled at the termination, and as counterintuitive as it may seem, even ambient temperature derates don't have to apply there either. 110.14(C) allows you to use the wire's temperature rating for derate factors, even if the termination's temperature rating is less. As long as you consider both that, and the non-derated termination ampacity as possible "weak links"
 

kwired

Electron manager
Location
NE Nebraska
The rationale behind using 90 degree cable instead of 75 or 60 degree cable is one of economy.
Most of the time 90C conductor is all that is available these days, and you don't really have that choice when using new conductor.

Now there are some cases where we must use 60 C rating even though the conductor is 90C - such as with NM cable.
 

kwired

Electron manager
Location
NE Nebraska
All,

Ever since graduation I've been using the 75 degree column of the cable ampacity table to size wire, even though I almost always use 90 degree rated THHN, due to the fact that almost all of our MCC's have 75 degree rated equipment. However, someone told me about how you can calculate using the 90 degree column for de-rating purposes. I am trying to apply it to this example: I need to size conductors for a 400A service:

Old Method I Used:

Six 250MCM wires in one conduit - 255*2*0.8*1.00 = 408A (all values taken from 75 deg column, ambient temp correction is 1.00)

New Method Someone Told Me About:

Six 4/0 wires in one conduit - 260*0.8*1.00 = 208A (less than 75 degree ampacity of 230 for 4/0), so I can use 90 degree ampacity for 4/0, so total ampacity becomes 208 x 2 = 416A

Something about this doesn't seem right to me, can someone explain am I using this method correctly? Incorrectly?
You have two minimum requirements to fulfill, termination temperature rating and conductor insulation temperature rating.

Most terminations are 75C, though you will find some that must be 60C - usually on older equipment though. You will seldom find a 90C termination.

So for now lets assume your termination is 75C - you must have a conductor (or parallel sets) that has a minimum size from the 75C ampacity table so that the termination isn't overheated in this application.

Inside raceways/cables we don't have terminations to worry about only insulation temperature rating. This is where you have heating from multiple conductors in same raceway/cable or for ambient temp that you don't have at the termination. You must make a separate calculation for insulation temp when selecting which size of conductor to run.

After selecting both termination and insulation ampacity - whichever result yields the larger conductor is the size you must run.
 
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