Using THHN termination

[Jpflex]

When selecting a THHN wire based on 90 degree insulation but terminating onto a 75 degree Cel rated device you must not select an ampacity higher than that for a 75 degree rated wire.

But can you select a THHN with an ampacity just under 75 degree ampacity?, I assume this depends on load ampere demand? Such as lights needing 12 amps and THHN selected after adjustment and temp correction yields 8 amperes?

But if your load requires 20 amperes and you select a THHN rated 90 a 14 AWG which allows 25 amperes before hurting insulation then since your load only uses 20 amperes which is what the 75 degree rated wire will allow and not only will the breaker be sized for 20 Amperes but the THHN wire will only see 20 amperes (temp limit of 20 amperes) then why couldn’t you use this?

All sources seem to say THHN must be limited to an ampacity of 20 which is the 75 degree column conductor. However it’s the load and somewhat breaker which determines the amperes going through a wire but not wire nor insulation itself.

This still isn’t making sense

 

[infinity]

Article 240 would limit most uses of #14 AWG conductors to 15 amps. For something like a motor or AC unit circuit it would be permitted to use the 75 degree C rating with THHN provided the terminals are also rated for 75 degrees C.

 

[augie47]

When you are looking at #12 and #14 note the " ** " in Table 310.16 which reference you back to 240.4(D) limiting the overcurrent device for those conductors.

 

[Jpflex]

When you are looking at #12 and #14 note the " ** " in Table 310.16 which reference you back to 240.4(D) limiting the overcurrent device for those conductors.

Yes I’m familiar with the NEC section regarding limiting 18 - 10 awg small wires ampacity Per required breaker ampacity. 14 to 15 amp 12 to 20 etc

However, I used the wire as Only as an example of its rated ampacity per insulation temperature and not respective of this unique code specific to small conductors.

 

[augie47]

When selecting a THHN wire based on 90 degree insulation but terminating onto a 75 degree Cel rated device you must not select an ampacity higher than that for a 75 degree rated wire. Correct

But can you select a THHN with an ampacity just under 75 degree ampacity?, I assume this depends on load ampere demand? Such as lights needing 12 amps and THHN selected after adjustment and temp correction yields 8 amperes?

The adjusted ampacity of the conductor must be rated to carry the load (NEC 210.19)
Once that conductor is selected, you can choose the OCPD to protect it. (NEC 240.4)

But if your load requires 20 amperes and you select a THHN rated 90 a 14 AWG which allows 25 amperes before hurting insulation then since your load only uses 20 amperes which is what the 75 degree rated wire will allow and not only will the breaker be sized for 20 Amperes but the THHN wire will only see 20 amperes (temp limit of 20 amperes) then why couldn’t you use this? The ** notes

All sources seem to say THHN must be limited to an ampacity of 20 which is the 75 degree column conductor. However it’s the load and somewhat breaker which determines the amperes going through a wire but not wire nor insulation itself.

This still isn’t making sense

Calculate your load..... select a conductor that with that or higher ampacity after adjustment... choose an OCPD to protect that conducior

 

[Jpflex]

You are saying to select a wire with a higher ampacity but code says it must not exceed 70 degree column ampacity.

So if a THHN 90 degree wire after ampacity adjustments goes just a few amperes over 70 degree column why does all this matter when load itself sets amperes seen in wire

A 90 degree wire with ampacity higher than rating for 70 degree column should seem ok to use but code states it cannot be this way

 

[wwhitney]

A 90 degree wire with ampacity higher than rating for 70 degree column should seem ok to use but code states it cannot be this way

110.14(C) doesn't actually say that. It's not very well worded, so it's easy to misinterpret.

What it means is that when you have conductors with 90C insulation temperature terminated on equipment with a 75C temperature rating, the overall circuit ampacity is limited to the 75C value from the table (the value without ampacity correction or adjustment). But when you need to do ampacity correction due to ambient temperature or adjustment due to conductor count, you get to start with the 90C ampacity for those computations. The circuit ampacity ends up at the lesser of the two values.

Cheers, Wayne

 

[Jpflex]

So a termination rated 60 degrees cel at 55 amp load would require minimum 6 AWG wire. (This has always been clear)

However, code incorrectly or ambiguously states that when using a conductor rated higher 90 degrees for example, you must select an ampacity at the 60 degree column. If you do this according to how it’s worded a size 8 AWG THHN 90 degrees can be selected at THE SAME AMPACITY AS THE 60 degree column for 55 amperes.

However this is wrong because under this condition the wire will be 90 degrees at this ampacity.

It would make sense to go up in THHN size but how do you know what the temperature will be on this next size up of THHN such as selecting a 6 AWG at 90 degree insulation. However, the NEC chart 310.16 only gives ampacity at 75 degrees yielding 90 degrees on wire and how will we know what this wire temperature will be at the lesser target 55 amperes?

Are you to interpret the 6 AWG THHN 90 degree insulation will be 75 degrees C at 65 amperes or 60 degrees at 55 amperes BY SLIDING YOUR FINGER TO THE LEFT OF FIRST 90 degree THHN VALUE? Is this how it’s done? Thanks

 

[roger]

However, code incorrectly or ambiguously states that when using a conductor rated higher 90 degrees for example, you must select an ampacity at the 60 degree column.

Tell us where you are reading that. You can use a 90 deg conductor at it's 90 deg rating if all parts of the chain are rated 90 deg. You can use a 90 deg conductor at a lower rating be it 75 or 60 deg.

 

[infinity]

Are you to interpret the 6 AWG THHN 90 degree insulation will be 75 degrees C at 65 amperes or 60 degrees at 55 amperes BY SLIDING YOUR FINGER TO THE LEFT OF FIRST 90 degree THHN VALUE? Is this how it’s done? Thanks

If your conductor is #6 THHN it's ampacity is @:
90° C=75 amps
75° C=65 amps
60° C=55 amps
If your terminals are 75° C rated then the conductor can be used at 65 amps. If the terminals are 60° C rated then the condcutor can be used at 55 amps. If your load required a 60 amp ampacity then you could use #6 with 75° C terminals but you could not use #6 with 60° C terminals.

 

[kwired]

When you select a conductor size you have two things to consider and must use a conductor that meets both conditions.

1. termination temperature rating

2. insulation temperature rating

And then there is the small conductor rule of 240.4(D) which will limit OCPD in most general use cases for conductors up to 10 AWG which is a main factor in those small conductor situations OP had for examples.

If you have a load of 25 amps, you could use 12 AWG conductor with 75 deg terminations and 75 or 90 deg conductor insulation, but 240.4(D) still limits you to 20 amp OCDP in general use cases - you could however use 12 AWG for say a motor load up to 25 amps as that is specifically exempted from 240.4(D) by 240.4(G).

Also don't forget minimum required conductor ampacity is often 125% of rated load in many instances so that 25 amp motor I just mentioned likely needs 31.25A minimum conductor ampacity in reality but a 20 amp rated motor would fit the 25A minimum conductor ampacity situation.

Some guys look at a 12 AWG motor or air conditioning circuit on a 30, 35 or maybe even 40 amp breaker and think something is not right, but is very possible to have this combination and be code compliant.