Conductor sizing, terminal rating

[Casey the Electrician]

Hello all,

I believe I understand the reasoning behind sizing wire to the terminal rating but when I look at 2023 NEC for instance. Table 310.16 is the ampacity table most often used. The top of the table identifies the same insulations across 75 C to 90 C. Let's say 4/0 XHHW AL in 75 C is 180 amps, right next to it is 4/0 XHHW AL in 90 C at 205 amps. I get the different size wires and insulation type are going to operate and heat up differently but we are saying the same insulation on the same size wire is different.

Please help

Thanks in advance

Casey

 

[infinity]

Let's say 4/0 XHHW AL in 75 C is 180 amps, right next to it is 4/0 XHHW AL in 90 C at 205 amps

Doesn't the heading of the 90° C column say XHHW-2 not XHHW? They are not the same.

 

[Casey the Electrician]

It has "XHHW-2" but it specially says "XHHW" only right before that. I know some wire is "dual rated" I hear people say, obviously they write both markings on the wire. I don't think I have ever seen a "THHN" only wire. It's always "TTHN/THWN" which is my confusion about this isolation and terminal rating business lol.

 

[infinity]

It has "XHHW-2" but it specially says "XHHW" only right before that. I know some wire is "dual rated" I hear people say, obviously they write both markings on the wire. I don't think I have ever seen a "THHN" only wire. It's always "TTHN/THWN" which is my confusion about this isolation and terminal rating business lol.

Yes you're correct they're both listed. Go to Table 310.4(A) Conductor Applications and Insulation Rated 600 Volts and you'll see that the application location can change the ampacity.

 

[Casey the Electrician]

Im using the handbook. It doesn't have 310.4(A) but it does have an informational note right below 310.4 that speaks to what you are saying, also the table following that note with all of the characteristics of the insulation.

So, since most terminals are going to be rated at 75C, assuming they're dry location, which is most often the case, XHHW-2 AL 4/0 is rated for 90 C in dry and wet locations, is adequate for a 75° terminal rated at 200 amps correct?

Just to clarify, I'm not playing dumb here. I always size my conductors for things. I have to draw at the 75° column even though it upsizes my wire because of the way, I've understood the rules on terminal rating. Like when I do 400 amp services, I always put in my service conductors at parallel 250 XHHW aluminum even though technically 4/0 AL XHHW2 is rated for 205 A.

I just still feel a touch confused about this, and if I am overthinking it, which I think I am, it would make my life better to not have to go through this extra headache lol

 

[infinity]

So, since most terminals are going to be rated at 75C, assuming they're dry location, which is most often the case, XHHW-2 AL 4/0 is rated for 90 C in dry and wet locations, is adequate for a 75° terminal rated at 200 amps correct?

Yes that's correct. The 75° C terminals won't allow you to size the conductors using their 90° C ampacity. #4/0 AL is rated for 180 amps so it can be used for 200 amps due to the next size up rule when the load is 180 amps or less.

Unless you're doing a derating or other correction calculations you can pretty much just use the 75° C ampacity when sizing the conductors. Things like NM and UF cable have further restrictions.

 

[Strathead]

Im using the handbook. It doesn't have 310.4(A) but it does have an informational note right below 310.4 that speaks to what you are saying, also the table following that note with all of the characteristics of the insulation.

So, since most terminals are going to be rated at 75C, assuming they're dry location, which is most often the case, XHHW-2 AL 4/0 is rated for 90 C in dry and wet locations, is adequate for a 75° terminal rated at 200 amps correct?

Just to clarify, I'm not playing dumb here. I always size my conductors for things. I have to draw at the 75° column even though it upsizes my wire because of the way, I've understood the rules on terminal rating. Like when I do 400 amp services, I always put in my service conductors at parallel 250 XHHW aluminum even though technically 4/0 AL XHHW2 is rated for 205 A.

I just still feel a touch confused about this, and if I am overthinking it, which I think I am, it would make my life better to not have to go through this extra headache lol

Unlike some things in the code, these rules actually make common sense in my mind. Perhaps this will help. The wire itself can carry a lot of amps before the integrity of the copper or aluminum is compromised. So the limiting factors are that integrity and voltage drop since hotter and smaller wire has a higher voltage drop. (VD not important in this discussion) If the wire is insulated, the amount the wire can heat up must be limited to a heat that won't damage the insulation properties. That is why various columns and table exist in 316. When wire terminates on a piece of equipment, the amount of heat generated AT that termination must not damage the termination or the surrounding area. That is why 110.14 limits termination temperatures. 110.14 lets you know that you can use wire rated for its ampacity or only its ampacity at the terminals rating, whichever is lower. Wiring in conduits can use the insulation rating ampacity to base the calculations on, without having to use that calculation at the termination, but if you understand everything above you would realize that, if the ambient temperature AT the equipment is higher than 86 degrees Fahrenheit then you need to adjust the ampacity using the table appropriate for the termination rating.

Hope using this logic helps.

 

[Casey the Electrician]

Yes that's correct. The 75° C terminals won't allow you to size the conductors using their 90° C ampacity. #4/0 AL is rated for 180 amps so it can be used for 200 amps due to the next size up rule when the load is 180 amps or less.

Unless you're doing a derating or other correction calculations you can pretty much just use the 75° C ampacity when sizing the conductors. Things like NM and UF cable have further restrictions.

Next size up rule meaning that you round up for feeders, round down for service conductors? I remember this talk from previous code classes but don't have code references that justify it. It is permissible to run 4/0 AL for a 200A because it is a feeder and you can round up? Does that cut off at 180 amps then ?

 

[Casey the Electrician]

Unlike some things in the code, these rules actually make common sense in my mind. Perhaps this will help. The wire itself can carry a lot of amps before the integrity of the copper or aluminum is compromised. So the limiting factors are that integrity and voltage drop since hotter and smaller wire has a higher voltage drop. (VD not important in this discussion) If the wire is insulated, the amount the wire can heat up must be limited to a heat that won't damage the insulation properties. That is why various columns and table exist in 316. When wire terminates on a piece of equipment, the amount of heat generated AT that termination must not damage the termination or the surrounding area. That is why 110.14 limits termination temperatures. 110.14 lets you know that you can use wire rated for its ampacity or only its ampacity at the terminals rating, whichever is lower. Wiring in conduits can use the insulation rating ampacity to base the calculations on, without having to use that calculation at the termination, but if you understand everything above you would realize that, if the ambient temperature AT the equipment is higher than 86 degrees Fahrenheit then you need to adjust the ampacity using the table appropriate for the termination rating.

Hope using this logic helps.

That is helpful. I understand that ambient temp affects terminal and wire ampacity as well, to your point mores at the connection, if it is in a pipe down the way its away from the that possible damage, assuming that location is not like a roof top where it would be exceedingly hot and need derating anyway. Most panels are for 40 C I believe?

 

[infinity]

Next size up rule meaning that you round up for feeders, round down for service conductors? I remember this talk from previous code classes but don't have code references that justify it. It is permissible to run 4/0 AL for a 200A because it is a feeder and you can round up? Does that cut off at 180 amps then ?

Any circuit can apply the next size up rule if 800 amps or less. Dwelling services have an alternate set of rules.

240.4(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other
trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.

 

[Strathead]

Next size up rule meaning that you round up for feeders, round down for service conductors? I remember this talk from previous code classes but don't have code references that justify it. It is permissible to run 4/0 AL for a 200A because it is a feeder and you can round up? Does that cut off at 180 amps then ?

What do you mean "round down for service conductors? Also, FYI, when you are discussing the code, especially among the crowd here, for example you are best off using code language. There isn't an actual "next size up" rule for example, especially for feeders. There are rules that allow you to size a breaker up above the ampacity of the conductor it protects. So, as such, since a service conductor isn't protected by a breaker, this code section doesn't apply to it.

 

[Strathead]

It is permissible to run 4/0 AL for a 200A because it is a feeder and you can round up? Does that cut off at 180 amps then ?

It is only permissible in residences, and no, it isn't because it is a feeder and you can round up. First, in most cases it is a service entrance conductor, not a feeder. It is specifically because the code makes an exception that allows it, period. It is actually two sizes up.

To confuse the issue here. One code section that isn't superseded by any of these rules is that a conductor Can not exceed the calculated load, so if you do a calculation per 220 for a house service and it comes out higher than the 310.16 ampacity value of the conductor, you will need to go up to the next size conductor.

 

[infinity]

There isn't an actual "next size up" rule for example, especially for feeders.

The "next size up rule" refers to the next standard size of OCPD's outlined in Table 240.6(A). 240.4(B) use the term "next higher standard overcurrent device rating" I don't see an issue with using the term next size up rule.

Next size up rule meaning that you round up for feeders, round down for service conductors?

Regarding #4/0 aluminum (180 amps@75° C) for a feeder it can be protected at 200 amps as mentioned given that the calculated load is 180 amps or less. For dwelling service conductors #4/0 AL is permitted for a 200 amp service because it is within the 83% ampacity permitted for service conductors. This is one example of a conductor size that will work when applying either rule.

 

[Strathead]

The "next size up rule" refers to the next standard size of OCPD's outlined in Table 240.6(A). 240.4(B) use the term "next higher standard overcurrent device rating" I don't see an issue with using the term next size up rule.

That is fine that you don't see it, but given the entirety of the other poster's statement, it would be better to cite the rules than refer to a paraphrasing.

 

[infinity]

That is fine that you don't see it, but given the entirety of the other poster's statement, it would be better to cite the rules than refer to a paraphrasing.

I actually did in post #10.