Sizing Conductors for Continuous Loading

[rlee]

I just purchased the calculations DVD set and I've come across an issue I'm failing to understand. That issue relates to sizing conductors for continuous loads. On page 246 of the Electrical NEC Exam Preparation text book under the heading "Overcurrent Protection of Conductors" there is a question asking me to size a conductor for a 104A continuous load. The 104A load is then multiplied by 125% to arrive at 130A for a 1 THHN conductor on 75 degree terminals.

This all makes perfect sense accept for the fact that table 310.16 states in its title the following: Allowable ampacities of Insulated Conductors...

According to article 100, the definition of ampacity is stated as follows: The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

So, my question is this; Why am I increasing the ampacity of a conductor for continuous loading if table 310.16 already takes continuous loading into account?

Richard Lee

 

[infinity]

Re: Sizing Conductors for Continuous Loading

So, my question is this; Why am I increasing the ampacity of a conductor for continuous loading if table 310.16 already takes continuous loading into account?

Because it doesn't. The problem is that if many circuits were to operate continuosly at 100% the amount of heat generated would be problematic. Some equipment is designed to operate continuously at 100% and is listed for that application.

 

[bob]

Re: Sizing Conductors for Continuous Loading

rlee
The 125% factor is not for the conductor but for the breaker. The rating on the breaker is a rating for non-continuous loads. A 100 amps breaker will carry 80 amps continuous loads.

 

[rlee]

Re: Sizing Conductors for Continuous Loading

Infinity and Bob, you have not satisfactorly answered my question. Infinity, read the title to table 310.16. The first two words are "Allowable ampacity...". Look up the definition of ampacity in article 100 of the definitions and you will learn that the table 310.16 ampaticities apply to continuous loads. Bob, table 310.16 in no way references ground fault or overload protection devices; it simply lists allowable ampacities for various conductors under specific conditions. Unless someone with creditable input can prove otherwise, I have concluded that there is a conflict between 210.19 (A)(1) and table 310.16. As the 2005 reads, it is not necesary to multiply a continuous load by 125% to select a conductor size. Table 310.16 already takes into account continuous loads.

 

[WCR]

Re: Sizing Conductors for Continuous Loading

rlee you should take in count the voltage drop caused in a conductor also the derated factor if a raceway is to contain more than three current carrying conductors the maximum allowable ampacity must be reduced,also there is the correction factors due to locations where the temperature is higher than 86 grades f. Is need use the 125% factor to the protection of the breaker.

 

[iwire]

Re: Sizing Conductors for Continuous Loading

Originally posted by rlee:
Infinity and Bob, you have not satisfactorly answered my question.

That is an 'intresting' way to ask for free help. :roll:

 

[infinity]

Re: Sizing Conductors for Continuous Loading

Thanks Bob,

Rlee,

I'd suggest that you read 210.19(A)(1), its exception and FPN No. 1 and then let me know if we've answered your question in a satisfactorily manner.

 

[augie47]

Re: Sizing Conductors for Continuous Loading

B&I I'm impressed..grafceful even under fire....and no mention of any kissing

 

[iwire]

Re: Sizing Conductors for Continuous Loading

Originally posted by augie47:
B&I I'm impressed..grafceful even under fire....and no mention of any kissing

We have to wait for the original 'Bob' to respond.

 

[rlee]

Re: Sizing Conductors for Continuous Loading

Again, those ampacities listed in Table 310.16 are good all day long, seven days a week, 52 weeks a year. If I had a continuous load of 50 amps in an ambient temperature of 86 F with a raceway with not more than three current-carrying conductors, terminated to terminals rated at 75 C and voltage drop is of no concern, I can use an 8 THWN landed on a continuous rated breaker of 50 amps and be good to go, cased closed, end of story.

In this case, increasing the conductor ampacity by 125% is pointless.

 

[infinity]

Re: Sizing Conductors for Continuous Loading

In this case, increasing the conductor ampacity by 125% is pointless.

I think that you've missed the point. By increasing the conductor size to 125% it will lower the amount of heat it generates at the OCPD. If the system is designed for continuos loading than the ampacity in table 310.16 will be sufficent. But if a load is used with a standard OCPD that is not 100% rated and the load is on continuously for more than 2 hours, 59 minutes and 59 seconds than the conductor size must be increased to 125%. Your argument about 24/7/52 only applies if it meets the criteria of the exception to 210.19(A)(1).

 

[bob]

Re: Sizing Conductors for Continuous Loading

I can use an 8 THWN landed on a continuous rated breaker of 50 amps and be good to go, cased closed, end of story.

Absolutely correct. You did not state in your post that you were using 100% rated OC device. If you look at 210.19 exception, You will see that you statement is correct. However, If the equipment is not rated 100% then you need to increase the continuous by 25%.

 

[bphgravity]

Re: Sizing Conductors for Continuous Loading

I don't think there is a 50A continuous rated breaker available. I know that is not the point of the discusson, but still a point.

 

[infinity]

Re: Sizing Conductors for Continuous Loading

You did not state in your post that you were using 100% rated OC device.

Thanks Bob for pointing that out. Had the OP mentioned 100% rated breakers this could have been summed up in one or two posts.

 

[rlee]

Re: Sizing Conductors for Continuous Loading

Actually the continuous rated breaker still doesn't affect the fact that T310.16 already takes into account continuous loading of a circuit.

Let's say I have a 16A 120V continuous load and all the stipulations of T310.16 have been met (3 or less CCC's, temp @ 86F, etc). The reason you multiply the continous load, in this case 16A, by 125%, which would equal 20, is so you don't load the non-continuous rated breaker to more than 80% of its rating. So the point I made earlier still stands true; it is not necessary to multiply a conductors ampacity by 125% when dealing with continuous loads. All you need to do is multiply the continuous rated load by 125% to accomodate the breaker loading of 80% and then select the appropriate conductor based on the breaker size.

So Infinity, at the beginning of this post, you stated "it doesn't" when I stated that T310.16 does indeed take into account continuous loading. You made this statement without backing up your claim which resulted in an unsatisfactory response in my initial observation.

When I post a thought or question on this board, I'm expecting educated and enlightening responses that will enhance the knowledge base of not just me, but anyone who reads the posting. I'm making the assumption I'm dealing with fellow professionals in the electrical field.

Anyone can tell me I'm wrong about a hypothesis but you're going to have to illustrate your point creditably or it's just background noise.

 

[iwire]

Re: Sizing Conductors for Continuous Loading

So what is your issue rlee?

Do you not believe 210.19 requires the conductor to increased for continuous loads?

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

Apparently when the conditions of use include supplying continuous loads the conductor must be rated 125% of the load.

 

[iwire]

Re: Sizing Conductors for Continuous Loading

By the way we are generally a friendly bunch here so why not drop the attitude.

 

[roger]

Re: Sizing Conductors for Continuous Loading

rlee, you definitely need to loose the attitude.

If you want to answer exam questions using your hypothesis go for it.

Your question has been answered from top of the line electricians as well as an engineer, if you don't want to listen so be it.

BTW, I hope you get better grades on your electrical test than you do on courtesy and manners.

Roger

 

[bphgravity]

Re: Sizing Conductors for Continuous Loading

To understand the requirements of the NEC it is sometimes required of you to look a little deeper than a few lines of code and a definition. The history and logic of continuous loading rules are tried and tested.

The first requirements showed up in the 1965 NEC. The requirement then looked similar to the requirement today. This changed in 1993 when the requirement was dropped to provide the additonal capacity of 25% to the circuit conductors leaving only the requirement to increase the OCD size.

The problem with this was that UL/NEMA standards didn't change to reflect this change and testing of OCD with continuous loading was done so with conductors sized at 125% as well. This resulted in smaller conductors being installed in compliance with the NEC but not in coordination with how the OCD was tested. This was corrected in 1996.

See, the problem really isn't with the conductors, its with the OCD. If only the OCD is increased in capacity for continuous loading and not the conductors, the terminals of the OCD become a heat sink for the hotter operating conductors. It is possible that the insulation rating of the conductors will never be exceeded, but it is likely that OCD rating will.

This is because the greater area and mass the conductors represent will dissipate heat more efficently than will a breaker. In the end, conductors and overcurrent devices need to be sized and used in the manner in which they were tested by UL/NEMA standards to protect the entire system.

By the way. There does not appear to be any breakers listed for 100% under 250A and 250V. So follow the rules of the NEC and submit a proposal for the 2011 NEC if you think you can provide significant substantiation for you conclusions.

[ November 19, 2005, 07:41 PM: Message edited by: bphgravity ]

 

[bob]

Re: Sizing Conductors for Continuous Loading

rlee

The reason you multiply the continous load, in this case 16A, by 125%, which would equal 20, is so you don't load the non-continuous rated breaker to more than 80% of its rating.

Absolutely correct

So the point I made earlier still stands true; it is not necessary to multiply a conductors ampacity by 125% when dealing with continuous loads.

Not correct.
You don't multiply the conductor ampacity by 125%. You multiply the load by 125% and then chose the conductor to match.

All you need to do is multiply the continuous rated load by 125% to accommodate the breaker loading of 80% and then select the appropriate conductor based on the breaker size.

Not correct.
You select the conductor based on the load and then select the breaker.