Two ampacities - 310.15(A)(2)

[cppoly]

Having a difficult time understanding the “whichever is less” concept here.

"Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."

So if the lower ampacity does not exceed 10 feet or 10%, whichever is less, the higher ampacity can be used.

Examples:

For a smaller run length of 50 feet, 4 feet of lower ampacity, allows the entire length of the run to be used at higher ampacity. In relation to the run length, this is 8 percent of the run.

Now if you have 2,000 feet run length, and 11 feet of reduced ampacity, the whole circuit must be considered reduced ampacity?? In comparison, this is just .5% run length.

So in the 1^st example, 8% of reduced ampacity allows full ampacity to be wire to be used. But in the 2^nd example, a .5% run length of reduced ampacity governs the entire run length.

Why isn't this just based on percentage?

 

[Carultch]

Having a difficult time understanding the “whichever is less” concept here.

"Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."

So if the lower ampacity does not exceed 10 feet or 10%, whichever is less, the higher ampacity can be used.

Examples:

For a smaller run length of 50 feet, 4 feet of lower ampacity, allows the entire length of the run to be used at higher ampacity. In relation to the run length, this is 8 percent of the run.

Now if you have 2,000 feet run length, and 11 feet of reduced ampacity, the whole circuit must be considered reduced ampacity?? In comparison, this is just .5% run length.

So in the 1^st example, 8% of reduced ampacity allows full ampacity to be wire to be used. But in the 2^nd example, a .5% run length of reduced ampacity governs the entire run length.

Why isn't this just based on percentage?

One place you might see this in practice, is when you have a conduit run that is mostly inside a building, and a small stub occurs above the rooftop to get to an HVAC unit. What this rule is telling you, is that as long as that "stub" is both less than 10 ft and less than 10% of the remaining length, you can forget about its increased conduit air temperature, and forget that the ampacity is lower in that short section.

The maximum total length where the 10% would govern, is 110 ft. 10 ft for the higher ampacity portion, which is 10% of the remaining length (i.e. 100 ft). Longer than that, and 10 ft is the limit for the length of the higher ampacity portion, regardless of the total length.

Below 110 ft in total, and the 10% of remaining length would be your limit.

 

[Dennis Alwon]

The code is basically saying that if your run is long enough so that the 10% of the run is less than 10' then there is no need to derate for the temp. in that small section because the heat will dissipate with the long run.

You have the right idea...If they used percentage then over 100' would be an issue. For instance 200' run would be 10% of 200 (20') in a higher temperature adjustment area and the nec says that is too much to try and cool down so you would have to use that higher temp. rating.

 

[infinity]

Having a difficult time understanding the “whichever is less” concept here.

"Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."

So if the lower ampacity does not exceed 10 feet or 10%, whichever is less, the higher ampacity can be used.

Examples:

For a smaller run length of 50 feet, 4 feet of lower ampacity, allows the entire length of the run to be used at higher ampacity. In relation to the run length, this is 8 percent of the run.

Now if you have 2,000 feet run length, and 11 feet of reduced ampacity, the whole circuit must be considered reduced ampacity?? In comparison, this is just .5% run length.

So in the 1^st example, 8% of reduced ampacity allows full ampacity to be wire to be used. But in the 2^nd example, a .5% run length of reduced ampacity governs the entire run length.

Why isn't this just based on percentage?

For whatever reason they want the length limited to 10'. When this was originally proposed the limit suggested was 15' but it ended up being 10' after it went to the CMP.

 

[cppoly]

One place you might see this in practice, is when you have a conduit run that is mostly inside a building, and a small stub occurs above the rooftop to get to an HVAC unit. What this rule is telling you, is that as long as that "stub" is both less than 10 ft and less than 10% of the remaining length, you can forget about its increased conduit air temperature, and forget that the ampacity is lower in that short section.

The maximum total length where the 10% would govern, is 110 ft. 10 ft for the higher ampacity portion, which is 10% of the remaining length (i.e. 100 ft). Longer than that, and 10 ft is the limit for the length of the higher ampacity portion, regardless of the total length.

Below 110 ft in total, and the 10% of remaining length would be your limit.

Thanks. I totally agree with this as it seems like the most likely used application to the rule. And the 10 ft part makes a lot of sense here.

Just have to accept the rule and not try to understand it. Because my original post doesn't make sense to me.

 

[cppoly]

Going through a Mike Holt example, does Mike have this wrong here?

In the example, it says the ampacity of the wire increases because of 50 degree ambient temperature. But the example shows a small wire run stubbing into a 50 degree room.

So I would think in this example the branch circuit going back to the panel would be the “reduced ampacity” portion of the circuit and the higher ampacity of the 50 degree room would be smaller portion of the circuit. But Mike increased the ampacity of the whole circuit based on the small run length of the 50 degree room.

Wouldn’t the reduced ampacity really be the whole branch circuit back to the panel which exceeds 10ft or 10%?

 

[Carultch]

Going through a Mike Holt example, does Mike have this wrong here?

In the example, it says the ampacity of the wire increases because of 50 degree ambient temperature. But the example shows a small wire run stubbing into a 50 degree room.

So I would think in this example the branch circuit going back to the panel would be the “reduced ampacity” portion of the circuit and the higher ampacity of the 50 degree room would be smaller portion of the circuit. But Mike increased the ampacity of the whole circuit based on the small run length of the 50 degree room.

Wouldn’t the reduced ampacity really be the whole branch circuit back to the panel which exceeds 10ft or 10%?

That example works to demonstrate this particular rule, however I would be skeptical of relying any ambient temperature that is artificially kept lower than the natural outside air temperature. What if the refrigeration or A/C system fails? What about when the refrigeration system is starting for the first time? You still want to keep your wiring from overheating during these periods, and if you rely on an artificially low temperature, it will overheat.

 

[cppoly]

That example works to demonstrate this particular rule, however I would be skeptical of relying any ambient temperature that is artificially kept lower than the natural outside air temperature. What if the refrigeration or A/C system fails? What about when the refrigeration system is starting for the first time? You still want to keep your wiring from overheating during these periods, and if you rely on an artificially low temperature, it will overheat.

Great point about temperature relying on certain conditions, I agree with that.

Although going back to Mike’s example. Practical application would be let’s say let's assume 50 to 100 foot branch circuit followed by what looks like an approximate 10’ section of increased ampacity in a 50 degree room. Therefore, wouldn’t the section of branch circuit outside of the 50 degree room be considered the “reduced ampacity” section, which dominates the run length. However, Mike is basically saying that since you have 10 feet of increased ampacity in this 50 degree room, the WHOLE branch circuit can use this ampacity. I don’t think this example is correct.

 

[jumper]

That ampacity could only apply to the room. I do not see where MH applies it to the whole circuit.

Besides that ampacity could only be used for off setting another correction factor. The circuit is still limited to an OCPD of 20A.