NEC vs IEC ?

[chandra]

Gentlemen,

I am new to this forum and also not a NEC Code user in day-to-day work. I am working outside North America. We use IEC 60364/IEE 16 th edition etc. for conductor sizing and other IEC stds/recommendations. But I am interested in clarifying the following issues with reference to NEC.

As an example we need to size a branch circuit (3 core cable with 90 deg insulation material, directly buried), which is about 1500 feet long, and having a continuous current of say 250 Amps at an ambient temperature say 30 deg C at 480 Volts (we have got 400 Volts). If I am not mistaken, we can get the size from Table 310-16. It will be AWG 4/0. Now,
1) Since there is no table in the NEC which gives the voltage drops for different sizes, how do we make sure that 1500 feet long cable will not give more than 3% drop from 480 V input supply? ( FPN #1 of section 310-15 of NEC 2002 says that the ampacities given are without voltage drop considerations. North American manufacturers are also not publishing d.c resistance, a.c. resistance, reactance & voltage drop details of cables in their product catalogues. But the same is freely available with leading European manufactures.)
2) Since there is no table in the NEC how do we make sure that the conductor is properly sized for the short circuit current? (the 3-phase bolted fault current at the end of the cable is say 25 kA )
3) Why it is a practice in North America to use bare copper conductors as grounding conductors? (It is against our national codes to use them and should be with insulated conductors having green/yellow stripes. Bare copper can be used only in overhead transmission lines)

These may be very simple issues for NEC code experts in this forum. I honestly welcome them to explain these issues to a non NEC user.

Thanks in advance!

Chandra

 

[bob]

Re: NEC vs IEC ?

Thanks for your post.
The NEC does not require any specified voltage drop. 210.19A FPN suggests 3% VD on branch circuits and a total of 5% VD including the feeder
If you need the R and X value for the conductor,
they are in Chapter 9 of the NEC.

The 90 degree rating of conductors can not be used because the connectors are only rated for 75 degrees max. Therefore your must use the 75 degree Col. If the 250 amps is continuous, then the NEC required that the load be increased by 25%.
250 amps x 1.25 = 313 amps is the minimum conductor capacity. This requires 400 kcm cu.
For voltage drop you need the use the computer program on the Mike Holt site under "FREE STUFF".
Using 1500 ft and 250 amps
the caculations revealed the following:
kcm cu
400 31v 6.4%
500 27v 5.6%
2-250 21v 4.4%
2-350 17v 3.5%
I'm not sure wwhat you mean by "we got 400 volts".
If that true then the voltage level will be much lower that shown above.

In order to caculate the fault current you need to know the primary voltage and transformer size.
I used a 2500 kva 12.46 kv 480 volt 5% and
caculated 60 ka at the transformer and 6 ka at the end of the 2 250 circuit. There is also a program on this site to do this.

[ May 24, 2003, 04:19 PM: Message edited by: bob ]

 

[chandra]

Re: NEC vs IEC ?

Hello Bob,

Thanks a lot for your informative reply. Now we can do some calculations to cross check the IEC sizing. Your answer will really be helpful in that context for all of us at this end.

Factor 1.25 - Does it signify a load growth on the feeder or just a safety multiplying factor on the continuous load?

Note: I am sorry, by 400 V I meant our supply voltage as per IEC.

Could you also comment on the use of bare copper conductors as equipment grounding conductors?

I would post some more IEC related situations as the time permits,so that we can get the NEC version from this valuable site.

Thanks again!

 

[don_resqcapt19]

Re: NEC vs IEC ?

Bare copper is permitted for the EGC (equipment grounding conductor) by the NEC. It is commonly used in underground runs with a nonmetallic raceway. Some feel that this provides better protection if there is an insulation fault in the circuit conductors. It is not commonly used, at least in my area, for interior circuits unless it is part of a cable assembly. As far as sizing this conductor when the size of the circuit conductors has been increased, 250.122 requires that the EGC be increased in the same proportion as the circuit conductors were increased. This is based on looking at the conductor size required for the load in Table 310.16. If you use a conductor that is larger the that required, then the size of the EGC given in Table 250.122 must be increased in the same proportion.
Why would the EGC be required to be an insulated conductor?
Don

 

[bob]

Re: NEC vs IEC ?

Chandra
The 1.25 multipler is used to increase the size of the copper wire so that it acts as a heat sink
for the breaker. By removing some of the heat it prevents the breaker from tripping prematurely.
The grounding equipment conductors called for in 250.122 are green or green striped most of the time just at you have. The grounding electrode conductors are where you see most of the bare copper.
I did not metion the problem of flicker. When you have conductors that long, motor starting can cause the voltage to dip and can cause problems
depending on the type of load.
Where are you located?

[ May 25, 2003, 06:31 PM: Message edited by: bob ]

 

[ron]

Re: NEC vs IEC ?

chandra,
Your calculated fault current was 25kA? If so, a properly sized circuit breaker or fuse will almost always provide sufficient protection for the conductors withstand ratings. The withstand ratings are found in many IEEE documents as well as manufacturers literature.

 

[bphgravity]

Re: NEC vs IEC ?

Not to generalize the topic too much, but it appears to me that a possible difference is overall scope of the two codes.

The NEC is the barest minimal requirements for a practically safe and hazard free electrical installation. Anything less than the NEC is essentialy dangerous, even only slightly less. The NEC also separates these minimum requirements with good design and better procedures.

Knowing nothing about the IEC and based on the two examples given of voltage drop considerations and EGC requirments, it appears to me that maybe the IEC is part code and part instruction manual. This may be similar to the American Electricians Handbook that not only gives the NEC requirments, but also tries to teach you something in the process.

This is not to say that either or is a better code. Like I said, I know nothing about the IEC, so cannot make a fair judgement of it. Simply put, when using the NEC, your only going to get what you must do and not necessarily what you should do. Just my opinion.

 

[chandra]

Re: NEC vs IEC ?

Hello Gentlemen,

Thanks to all who contributed to my issue. I have got a somewhat clear picture now. See you soon with some other issue.

Thanks

Chandra

 

[arneykaner]

Re: NEC vs IEC ?

Originally posted by chandra:
Hello Bob,

Thanks a lot for your informative reply. Now we can do some calculations to cross check the IEC sizing. Your answer will really be helpful in that context for all of us at this end.

Factor 1.25 - Does it signify a load growth on the feeder or just a safety multiplying factor on the continuous load?

Note: I am sorry, by 400 V I meant our supply voltage as per IEC.

Could you also comment on the use of bare copper conductors as equipment grounding conductors?

I would post some more IEC related situations as the time permits,so that we can get the NEC version from this valuable site.

Thanks again!

Chandra,

You got a wrong reply about 25% additional capacity for the conductor size. In the US, all circuit breakers shall have load not exceeding 80% of their RATED (NOMINAL)size. ("Why" - is a separate topic) So, to find the size of a conductor, you don't have to multiply by 1.25. However, if you use a US-manufactured circuit breaker or fuse, and it is not 100% rated, than you have to multiply the real load by 1.25. And then you have to upsize the conductor, to match the setting of circuit breaker or fuse. However, if you use a circuit breaker made per IEC standard, then it is O.K. To summarize it: per IEC you need a 400A breaker to handle 400A load. per NEC you need a 500A breaker to handle 400A load.

 

[bob]

Re: NEC vs IEC ?

Originally posted by arneykaner:


You got a wrong reply about 25% additional capacity for the conductor size. In the US, all circuit breakers shall have load not exceeding 80% of their RATED (NOMINAL)size. ("Why" - is a separate topic) So, to find the size of a conductor, you don't have to multiply by 1.25. However, if you use a US-manufactured circuit breaker or fuse, and it is not 100% rated, than you have to multiply the real load by 1.25. And then you have to upsize the conductor, to match the setting of circuit breaker or fuse. However, if you use a circuit breaker made per IEC standard, then it is O.K. To summarize it: per IEC you need a 400A breaker to handle 400A load. per NEC you need a 500A breaker to handle 400A load. [/QB]

I hope to correct your misunderstanding of the NEC. The 125% adder for conductors and the 80%
rating applied to breakers have nothing to do with each other. It just so happens that
1/1.25 = 80%. If you had checked the UL Electrical Construction Materials Directory (Green Book) you would have seen the requirement that breakers should not be loaded more that 80% of their rating. This is because breakers can
malfunction if loaded more that 80%. This does not apply to breakers rated for 100% load. If you will check 215.2 for feeders and 210.19A1 you will find the requirements to MULTIPLY THE CONTINUOUS LOAD BY 125%. Yes Arneykaner you do multiply the load by 1.25. The breaker size depends on the type of load. Transformers and motors allow the breaker to be oversized too match the load. I hope this helps you to understand this requirement.

 

[arneykaner]

Re: NEC vs IEC ?

Originally posted by bob:

Originally posted by arneykaner:


You got a wrong reply about 25% additional capacity for the conductor size. In the US, all circuit breakers shall have load not exceeding 80% of their RATED (NOMINAL)size. ("Why" - is a separate topic) So, to find the size of a conductor, you don't have to multiply by 1.25. However, if you use a US-manufactured circuit breaker or fuse, and it is not 100% rated, than you have to multiply the real load by 1.25. And then you have to upsize the conductor, to match the setting of circuit breaker or fuse. However, if you use a circuit breaker made per IEC standard, then it is O.K. To summarize it: per IEC you need a 400A breaker to handle 400A load. per NEC you need a 500A breaker to handle 400A load.

I hope to correct your misunderstanding of the NEC. The 125% adder for conductors and the 80%
rating applied to breakers have nothing to do with each other. It just so happens that
1/1.25 = 80%. If you had checked the UL Electrical Construction Materials Directory (Green Book) you would have seen the requirement that breakers should not be loaded more that 80% of their rating. This is because breakers can
malfunction if loaded more that 80%. This does not apply to breakers rated for 100% load. If you will check 215.2 for feeders and 210.19A1 you will find the requirements to MULTIPLY THE CONTINUOUS LOAD BY 125%. Yes Arneykaner you do multiply the load by 1.25. The breaker size depends on the type of load. Transformers and motors allow the breaker to be oversized too match the load. I hope this helps you to understand this requirement. [/QB]

Sorry, Bob, but your reading of the NEC is incorrect. NEC does NOT require to multiply the continuous load by 1.25 to determine the CONDUCTOR size. It applies only to Overcurrent Protective Devices - breakers and fuses. Then, after you got the size of your OCPD, you may have to upsize the conductor in order to match OCPD rating or settings. The reason for that 1.25 multiplier is that in the US the circuit breakers
are designed and manufactured in such a way, that they would trip in about (3) hours if loaded 100% - unless, they are 100% rated. But, if a circuit breaker is, indeed, 100% rated NO 1.25 multiplier applies at all.