Continuous load and wire sizing

[Ponchik]

Load is a continuous 200A, size the conductors and OCPD based on 125%. So per code installation will be:
200 x 1.25 = 250A
Size of the conductor is 310.15(b)(16) is 250kcmil
OCPD is minimum 250Amps.

So far so good.

I know why the OCPD is based on 125%.

My question:

So since the conductor in table 310.15(b)(16) is based on continuous ampacity, then why does the conductor has to be sized to 125% if it will never see any load higher than 200Amps anyway.

3/0 conductor is good for 200 continuous amps, all the breaker can continuously deliver is 200Amps, all the load will ever be is 200Amps. So why size the conductors to 250 Kcmil?

Thanks.

 

[jumper]

So since the conductor in table 310.15(b)(16) is based on continuous ampacity, then why does the conductor has to be sized to 125% if it will never see any load higher than 200Amps anyway.

This statement is based on what information/reference/code section.

 

[Ponchik]

This statement is based on what information/reference/code section.

You know, after I posted my question I started thinking and searching for "if table 310.15(b)(16) is for continuous load" which I always thought it was but I can not find a section to back up my statement. However, 215.2(a)(1) exception#2 states: Grounded conductors do not have to sized to 125% of the continuous load because they are not connected to an OCPD.

Seeing 215.2(a)(1) exc#2 makes me to believe that table 310.(15)(b)(16) is for continuous load.

 

[jumper]

You know, after I posted my question I started thinking and searching for "if table 310.15(b)(16) is for continuous load" which I always thought it was but I can not find a section to back up my statement. However, 215.2(a)(1) exception#2 states: Grounded conductors do not have to sized to 125% of the continuous load because they are not connected to an OCPD.

Seeing 215.2(a)(1) exc#2 makes me to believe that table 310.(15)(b)(16) is for continuous load.

Honestly, I do not think I have ever read that exception before and makes me wonder why it exists.

The NECHB says this regarding that section:

Feeder grounded/neutral conductors that do not connect to
the terminals of an overcurrent protective device are not required
to be sized based on 125 percent of the continuous
load. For example, if the maximum unbalanced load on a
feeder neutral is calculated per 220.61 to be 200 amperes
and the load is considered to be continuous, the use of a 3/0
AWG THW conductor is permitted as long as the conductor
terminates at a neutral bus or terminal bar within the electrical
distribution equipment.

But I cannot say that I understand why. I can say that that I am pretty sure that you cannot apply this to ungrounded feeder conductors since they are connected to an OCPD.

In short, this exception has me stumped. Good thread.:thumbsup:

 

[Ponchik]

Definition of ampacity is based on continuous load. So another reason I further believe 310.15(b)16 is rated for continuous current.

 

[jumper]

Definition of ampacity is based on continuous load. So another reason I further believe 310.15(b)16 is rated for continuous current.

I can see your reasoning, but I am still not convinced. I gotta think about this for a minute or two.

The word continuous is used in the definition, yet still the minimum ampacity of the feeder is still determined by 215.2(A)(1).

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts
III, IV, and V of Article 220. The minimum feeder-circuit
conductor size, before the application of any adjustment or
correction factors, shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.

In short I guess that the code is saying that while the definition of the ampacity of a conductor is determined by its ability to carry the load continuously, the sizing of the conductor is determined by the section above.

IDK, it's late, I'm tired, and my brain is starting to hurt....will have to rethink this in morning.

 

[Smart $]

Definition of ampacity is based on continuous load. So another reason I further believe 310.15(b)16 is rated for continuous current.

It is... but that is not why we have pad continuous loads to 125%. I can't say I know the specifics with certainty, but I surmise it has to do with operating characteristics of devices and terminations thereto, and thus their ratings.
When you operate a conductor continuously at the maximum permitted level of current it generates heat. Some of that heat is transferred to the ocpd directly through the conductor termination and adds to the heat generated by the ocpd itself, thereby affecting its thermal trip characteristics.

Rather than getting into the specifics, consider the exception to 215.2(A)(1) where we are not required to pad the continuous loads to 125% because we would be using a 100%-rated assembly. This is saying the assembly, which typically includes ocpd, busing, enclosure, and ventilation thereof, is designed to handle the heat transferred through the conductor termination. A feeder neutral doesn't require the padding either in this very same sense... as there is no inline OCPD.

 

[infinity]

It is... but that is not why we have pad continuous loads to 125%. I can't say I know the specifics with certainty, but I surmise it has to do with operating characteristics of devices and terminations thereto, and thus their ratings.
When you operate a conductor continuously at the maximum permitted level of current it generates heat. Some of that heat is transferred to the ocpd directly through the conductor termination and adds to the heat generated by the ocpd itself, thereby affecting its thermal trip characteristics.

Rather than getting into the specifics, consider the exception to 215.2(A)(1) where we are not required to pad the continuous loads to 125% because we would be using a 100%-rated assembly. This is saying the assembly, which typically includes ocpd, busing, enclosure, and ventilation thereof, is designed to handle the heat transferred through the conductor termination. A feeder neutral doesn't require the padding either in this very same sense... as there is no inline OCPD.

I agree, a 100% rated assembly would account for all of the things that Smart has mentioned.

 

[Ponchik]

It is... but that is not why we have pad continuous loads to 125%. I can't say I know the specifics with certainty, but I surmise it has to do with operating characteristics of devices and terminations thereto, and thus their ratings.
When you operate a conductor continuously at the maximum permitted level of current it generates heat. Some of that heat is transferred to the ocpd directly through the conductor termination and adds to the heat generated by the ocpd itself, thereby affecting its thermal trip characteristics.

Rather than getting into the specifics, consider the exception to 215.2(A)(1) where we are not required to pad the continuous loads to 125% because we would be using a 100%-rated assembly. This is saying the assembly, which typically includes ocpd, busing, enclosure, and ventilation thereof, is designed to handle the heat transferred through the conductor termination. A feeder neutral doesn't require the padding either in this very same sense... as there is no inline OCPD.

So enlarging the conductor has nothing to do with its current carrying capacity, but rather it helps to take heat away from the OCPD.

Back to my original example, if we install 100% rated breaker then we can use 3/0.

 

[jumper]

So enlarging the conductor has nothing to do with its current carrying capacity, but rather it helps to take heat away from the OCPD.

Back to my original example, if we install 100% rated breaker then we can use 3/0.

No, as Smart said the breaker, enclosure, ventilation, etc all has to be rated for 100% continuous duty.

The same rules would apply if you wanted a 90C assembly, everything in the circuit would have to be rated for 90C.

 

[Dennis Alwon]

So enlarging the conductor has nothing to do with its current carrying capacity, but rather it helps to take heat away from the OCPD.

Back to my original example, if we install 100% rated breaker then we can use 3/0.

If you use a device rated 100% and assembly then you can use a conductors rated for the load without 25% added to the ampacity. Look at 210.19 (A)(1) exception

 

[Ponchik]

:thumbsup::thumbsup:

got it.

Thanks.

 

[Ponchik]

Question still lingers,

Is table 310.15(b)(16) based on continuous load?

Example: a 2/0 THHN based on 75 degree column will continuously carry 175Amps.

 

[iwire]

Question still lingers,

Is table 310.15(b)(16) based on continuous load?

Example: a 2/0 THHN based on 75 degree column will continuously carry 175Amps.

I don't see anything that says it can't.

 

[infinity]

If you use the definition of Ampacity from Article 100 in concert with T310.15(B)(16) the conductor shouldn't care if the load is continuous or not.

Ampacity. The maximum current, in amperes, that a con-
ductor can carry continuously under the conditions of use
without exceeding its temperature rating.

 

[kwired]

The table is just a general ampacity table and other adjustments are made as the need arises, so with no adjustments necessary it is 100% values.

The reason I was taught we need to increase ampacity by 25% for continuous load is because the overcurrent devices sink heat into the connected conductors. A 100% rated device does not sink (or at least not at the same level) heat into the conductors and that is the reason we can size conductors in those cases @ 100% of the continuous load. Remember 100% rated breakers are not the miniature breakers you find in "load centers" and may not even find in an industrial panel that has all thermal-magnetic breakers, but may find in an enclosure housing an individual breaker, or in some electronic trip type breakers, and in OP's case here 250 amps is likely about the minimum size you would find one also.

3/0 would be good for a 200 amp continuous load if connected to 100% rated devices.

A 200 amp continuous load connected to standard devices must have at least 250 amps minimum conductor ampacity, and the overcurrent device must be at least 250 amps. Other deration factors could increase conductor size but overcurrent device doesn't have to change, and really shouldn't.