Ampacity adjustment question

[Dsg319]

This is the way I have learned and want some others insite and wether is correct or not. I will give an example say I have 4, 20amp circuits in a conduit. All #12thhn 90 degree c. I know due to termination points if under a 100 amps you still have to use the 60 degree ampacitys. BUT since I am adjusting I can use the 90 degree correct? So since there are 4, 20amp single phase circuits that leaves me with 8ccc. So therefor I have a 70percent derating to the 90 degree wire which is rated at 30amps and leaves me with a total of 21amps(30x.70=21)Which means I’m still good to go with the 4, 20 amp circuits in the conduit correct ?

 

[bwat]

Correct, you can use the 90deg as a starting point for derating your conductors if you have 90 deg conductors.

 

[AC\DC]

110.14(1)(3) allows you to use the 75 degree ampacity if the equipment your connecting to is listed and identified for 75 degree, as Long as your conductor have a insulation of at least 75 degrees even if under 100 amps

 

[ActionDave]

For twelve gauge wire and 20A circuits the magic number is 9. If you have less than 9 current carrying conductors you can use a 20A breaker. That goes away once you get into larger wire sizes and you have to pay attention to the times when you have more than three current carrying conductors in a pipe.

You are an apprentice in the era of 210.4(B) so dedicated neutrals are the norm for new construction. Back in the day we could get six circuits in a pipe with three hots and a neutral two times for three phase or two hots and a neutral three times for single phase and no derating. Now the best you are gonna do is four circuits in one pipe. It's a shame.

 

[Julius Right]

310.15 Ampacities for Conductors Rated 0–2000 Volts.(B) Tables.

The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).
That means the corrected ampacity has not to be more that ampacity permitted for termination.

 

[follybeacher]

For twelve gauge wire and 20A circuits the magic number is 9. If you have less than 9 current carrying conductors you can use a 20A breaker. That goes away once you get into larger wire sizes and you have to pay attention to the times when you have more than three current carrying conductors in a pipe.

You are an apprentice in the era of 210.4(B) so dedicated neutrals are the norm for new construction. Back in the day we could get six circuits in a pipe with three hots and a neutral two times for three phase or two hots and a neutral three times for single phase and no derating. Now the best you are gonna do is four circuits in one pipe. It's a shame.

to be clear this is with 3/4” conduit, correct?

 

[infinity]

I will give an example say I have 4, 20amp circuits in a conduit. All #12thhn 90 degree c. I know due to termination points if under a 100 amps you still have to use the 60 degree ampacitys. BUT since I am adjusting I can use the 90 degree correct? So since there are 4, 20amp single phase circuits that leaves me with 8ccc. So therefor I have a 70percent derating to the 90 degree wire which is rated at 30amps and leaves me with a total of 21amps(30x.70=21)Which means I’m still good to go with the 4, 20 amp circuits in the conduit correct ?

That's correct. Since you have separate neutrals for your 4 circuits you end up with 8 CCC's. If you had MWBC's instead the neutrals sometimes are not required to be counted as CCC's. As mentioned the magic number for #12 THHN is less than 10 CCC's so with 9 CCC's you can still use a 20 amp OCPD device. Here's a chart that shows when the neutral is counted as a CCC:

Neutral Conductors:

Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:

3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.

Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.

Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.

 

[ActionDave]

to be clear this is with 3/4” conduit, correct?

Absolutely not!! Out of the top ten stupidest requirements that show up in job specs 3/4" min conduit size has to be in the top three.

 

[Carultch]

The temperature correction and adjustment factors shall be permitted to be applied to the ampacity for the temperature rating of the conductor, if the corrected and adjusted ampacity does not exceed the ampacity for the temperature rating of the termination in accordance with the provisions of 110.14(C).
That means the corrected ampacity has not to be more that ampacity permitted for termination.

A better way to state that, is that if the termination ampacity is less than the corrected and adjusted conductor ampacity, the termination ampacity governs the ampacity of the circuit.

Think of it like a chain on hooks. The chain has a strength, and the hooks have a strength. The strength of the assembly is the weaker of the two strengths. You can put a stronger chain on the hooks, but you can't take credit for its strength being greater than the strength of the hooks, when determining what load it can support.

 

[jaggedben]

This is the way I have learned and want some others insite and wether is correct or not. I will give an example say I have 4, 20amp circuits in a conduit. All #12thhn 90 degree c. I know due to termination points if under a 100 amps you still have to use the 60 degree ampacitys. BUT since I am adjusting I can use the 90 degree correct? So since there are 4, 20amp single phase circuits that leaves me with 8ccc. So therefor I have a 70percent derating to the 90 degree wire which is rated at 30amps and leaves me with a total of 21amps(30x.70=21)Which means I’m still good to go with the 4, 20 amp circuits in the conduit correct ?

The red part is incorrect and you don't need to be thinking that, exactly. That sounds to me like an overgeneralization of the rule that requires you to use the 60C column for NM cable. NM sizes stop just short of 100A under this rule, so it kinda makes sense that way. However if you are running circuits in conduit that does not apply. Generally you can use the 75C column for basic ampacity (i.e. when you're not doing the derating calcs which are the main point of your question).

 

[jaggedben]

...
You are an apprentice in the era of 210.4(B) so dedicated neutrals are the norm for new construction. Back in the day we could get six circuits in a pipe with three hots and a neutral two times for three phase or two hots and a neutral three times for single phase and no derating. Now the best you are gonna do is four circuits in one pipe. It's a shame.

Of the many reasons that MWBCs seem to be falling out of fashion, 201.4(B) seems like the least of them to me. I'd blame arc-fault requirements sooner.

 

[infinity]

The red part is incorrect and you don't need to be thinking that, exactly. That sounds to me like an overgeneralization of the rule that requires you to use the 60C column for NM cable. NM sizes stop just short of 100A under this rule, so it kinda makes sense that way. However if you are running circuits in conduit that does not apply. Generally you can use the 75C column for basic ampacity (i.e. when you're not doing the derating calcs which are the main point of your question).

The general rule for #1 to #14 AWG and 100 amps or less is that 60° C terminals are permitted. Larger than that then 75° terminals are required. For conduit and wire if you had a 60° terminal then you still are stuck with the 60° C ampacity.

 

[Dsg319]

Anybody ever derate wire Ampacities when working with dc circuits? Or is that even a thing. I’ve never seen it done personally .

 

[bwat]

Anybody ever derate wire Ampacities when working with dc circuits? Or is that even a thing. I’ve never seen it done personally .

Sure. Same concept.

 

[jaggedben]

Anybody ever derate wire Ampacities when working with dc circuits? Or is that even a thing. I’ve never seen it done personally .

It's sometimes a major consideration with solar systems.

 

[Dsg319]

Does this exception than allow to ignore amapacity adjustment in installations like this where cables are sleeved in a conduit in a length from 18” to 10’ long?

 

[Dsg319]

Does this exception than allow to ignore amapacity adjustment in installations like this where cables are sleeved in a conduit in a length from 18” to 10’ long?

Well never mind I had just found there’s an exception for disregarding Ampacity adjustment if it’s 10ft or less and not more than 10%percent of the total circuit length..

So does this still mean I’m only limited to 10ft or could I be I still use the exception say I have them in a 20ft race way bundled but the total circuit length is 200’. Not exceeding the 10 percent rule?

 

[wwhitney]

It's the lesser of 10' or 10% of the adjoining length.

So no, not 20' in your example.

And if it's 100' total, then it would be 9', which is 10% of the adjoining 91'.

Cheers, Wayne

 

[Dsg319]

Thanks! So still yet can it be bundled in 10ft raceway even if it’s more than 10% of the entire circuit length ? Or does it gotta meet both to be used .

 

[Carultch]

Thanks! So still yet can it be bundled in 10ft raceway even if it’s more than 10% of the entire circuit length ? Or does it gotta meet both to be used .

The 10 ft/ 10% rule means that if it is both less than 10 ft AND less than 10% of the remaining length, that you can forget about the local section where ampacity is decreased. If it is either greater than 10 ft, OR greater than 10% of the remaining circuit length, you need to account for conditions of use that reduce its ampacity. 2 ft and less, is an unrelated rule, where you are allowed to neglect the bundling derate entirely for short nipples 24" and less in length.

One place where you see the 10ft/10% rule in practice in NEC2014 and prior, is in the branch circuits feeding HVAC equipment on a rooftop. Most of the circuit is inside the building, and not on the roof in direct sunlight. But there is a small stub-up that is on the roof in direct sunlight, and would otherwise be a chokepoint in the ampacity of the circuit. That small stub-up, is where the 10 ft/ 10% rule allows you to forget about the rooftop temperature adder where it is in direct sunlight, because it is considered a negligible enough portion of the length to not impact the ampacity enough to matter. The rooftop temperature adder changed significantly in NEC2017, to not require it except when the raceway is immediately nearby the roof surface (7/8" and less).