Wiring ampacity

[Btbamfan4]

When are the maximum wiring amps acceptable according to the chart? Apparently I cant run a 30 amp rv receptacle off #12 thhn, but the chart says its acceptable for 30 amps. Please answer respectfully

 

[augie47]

There are many factors involved.
True, 310.15(b)(16) shows an ampacity of 30 amps but as I pointred out in YOUR OTHER thread, you also have to take 240.4 (and 110.14) intro account among other things.

 

[roger]

Start with 110.14(C) and then the two notes under 310.15(B)(16).

Roger

 

[Btbamfan4]

Thanks guys. Soon as I get my book back I'm going to deville in

 

[Btbamfan4]

According to 110's theory, I thought most terminals were rated for 75

 

[augie47]

They are but thats only part of the story.
As Roger suggested above, look at the foot notes on 310.15(B)(16)

 

[Btbamfan4]

Oh I see. Even at 75 it's still max 25 right?

 

[Btbamfan4]

Why not make side by side charts for terminal ratings and insulation ratings?

 

[augie47]

Oh I see. Even at 75 it's still max 25 right?

If you follow the footnote it leads you to 240.4 which tells you in most cases the max is 20 amps.
(there are exceptions but the explanation is lengthy)

 

[tom baker]

Why not make side by side charts for terminal ratings and insulation ratings?

Good idea. Please submit to NPFA as public input for the 2023 NEC.

 

[Btbamfan4]

If you follow the footnote it leads you to 240.4 which tells you in most cases the max is 20 amps.
(there are exceptions but the explanation is lengthy)

Thanks boss. Exactly what I needed to know.
Last question for you or anyone. Why were we aloud to downsize ground of 50 amp services, but not this outlet? I dont mind paying more money for the equipment, I just need to know for my knowledge

 

[augie47]

Short answer is because 250.122 allows it.
In general the equipment ground has to be large enough to carry the current needed in a fault situation.
There are guys on here a bunch smarter than me that can give you a more technical answer

 

[Btbamfan4]

Short answer is because 250.122 allows it.
In general the equipment ground has to be large enough to carry the current needed in a fault situation.
There are guys on here a bunch smarter than me that can give you a more technical answer

According to B of that article it was dealing with the length of the run. I'll have a 5' max length because I'm putting the receptacle right below the panel. Does that change anything?

 

[Btbamfan4]

I'm under the impression from that article if a 30 amp circuit required, let's say #6, because of voltage "drop" then the ground must be #6 as well. But no voltage drop will be occurring in my situation. I'm so confused lol. We were sitting sub panels out in yards at 100-200 feet from the service while still using #10 grounds. Not one inspector called us on it; not even for the hots or neutral. I'm just now getting into electrical theory because I'm newly licensed and I want to do everything to code.

 

[Btbamfan4]

Here is what I found

 

[augie47]

At this stage of the game my suggestion is to simply follow 250.122. It is actually all that you need to worry about in 99% of the installs

 

[lordofthisworld]

I want to say the only time u can use the 90 degree column, in which, has the max amps on a particular gauged wire is only when u have to derate.

 

[LarryFine]

Thanks boss. Exactly what I needed to know.
Last question for you or anyone. Why were we aloud to downsize ground of 50 amp services, but not this outlet? I dont mind paying more money for the equipment, I just need to know for my knowledge

The reason larger feeders and circuits can have reduced EGCs is because a short circuit (in a properly-designed and built installation) would be of a short-enough duration as no not heat the EGC and cause insulation damage.

 

[paulengr]

Thanks guys. Soon as I get my book back I'm going to deville in

You can read it online on NFPAs web site for free.

 

[paulengr]

Good idea. Please submit to NPFA as public input for the 2023 NEC.

You also have to derate for ambient temperature and more than three current carrying conductors. And when motors arrived involved at a bare minimum you derate to 80% (upsize motor name plate current 125%). But for various intermittent duties and fire pumps and some HVAC it may be derated even further. And some cable types have one rating in dry locations and another in damp/wet areas.

And you most certainly can use the 90 C column if you buy the right lugs and say THHN-2, typically used in switchgear and industrial panels where the extra high rating has a cost incentive.

We’ve been using the same chart since Neher-McGrath came out with it in the 1950s with no issues except when green horns read it and miss all the other “gotchas”.