Service Conductor Size

[steve66]

Article 230.42(A) of the 2002 NEC covers sizing service entrance conductors:

 230-42. Minimum Size and Rating
 (a) General. The ampacity of the service-entrance conductors before the application
 of any adjustment or correction factors shall not be less than either (1) or (2). 
 Loads shall be determined in accordance with Article 220. Ampacity shall be
 determined from Section 310-15. The maximum allowable current of busways
 shall be that value for which the busway has been listed or labeled.
 1. The sum of the noncontinuous loads plus 125 percent of continuous loads
 2. The sum of noncontinuous load plus the continuous load 
 if the service-entrance conductors terminate in an overcurrent device where both
 the overcurrent device and its assembly are listed for operation
 at 100 percent of their rating

Why does the continuous or non-continuous rating of the breaker affect the size of the conductors

Assume I forget to include 125% of the continuous loads in my calculations for the service conductors (I use 100% instead). The inspector comes out and tells me the service conductors are undersized. By replacing the breaker with one that has a 100% continuous rating, the conductors are now sized correctly. This makes no sense to me. Does anyone have a clue WHY?

Steve

 

[Guest]

Re: Service Conductor Size

Originally posted by steve66:
The inspector comes out and tells me the service conductors are undersized. By replacing the breaker with one that has a 100% continuous rating, the conductors are now sized correctly.

Steve

If the inspector tells you that the service conductors are undersized, replacing the breaker with a smaller one isnt going make the service wires the correct size. The inspector will look at the load calculation you submitted and will know if you left out the 125% continuous loads. An inspector I gave a load calculation to went over it with a fine tooth comb.

[ May 10, 2004, 04:34 PM: Message edited by: iaspiretowire ]

 

[Guest]

Re: Service Conductor Size

The breaker will now be sized properly for the conductors.

But, will the conductors be sized properly for the load calculation? It does not sound like it.

You are getting off easy if this is a real life situation. Is this real, or only a test?

 

[rbalex]

Re: Service Conductor Size

The inspector has it right.

The problem is one of ancient history. It has to do with how circuit breakers and other over-current protective devices were originally tested and rated. They were tested on an open board ? not enclosed. But the early failure rate of enclosed devices showed there was a problem with the test method.

Rather than re-rate and re-label all the devices (VERY expensive) the simplest method was to just permit only 80% continuous usage of the device?s rating. The conductors were already OK since they were tested and rated at full load ? at least with respect to 65C ratings in the early days.

For a long time it was impossible to get over-current protective devices that were rated at 100% continuous load.

So the 125% over-sizing had more to do with protecting the over-current device than the conductors.

 

[steve66]

Re: Service Conductor Size

Welcome back Wayne, I think its been several months since I've seen you post. This is just a test for now (I'm still designing the service). I hope this doesn't sidetrack the thread, but I am not completely clear what loads are considered continuous and need to be calculated at 125%. So I took a closer look at 230.42, and it only adds confusion.

Let me rephrase my confusion:

By using a 100% rated breaker, 230.42(A)(2) allows the me to size the service conductors at 100% of all the loads. But using a standard 80% rated breaker, I now have to figure out which loads are continuous, and use 125% of those loads.

I just don't understand why a continuous rating of the breaker would affect the size of the conductors. It seems like a 100% rated breaker would be good for more current, and that would require larger service conductors, not smaller ones.

Steve

[ May 10, 2004, 07:15 PM: Message edited by: steve66 ]

 

[brentp]

Re: Service Conductor Size

rbalex,

Is it safe to say that todays breakers are rated at 100%, or are we still living with "ancient history"?

Brent

 

[bphgravity]

Re: Service Conductor Size

This all comes down to how OCD are tested and listed. Manufacturers since 1965 have been testing breakers and conductors at 125% of their rating. The code at the time required additional capacity in both OCD and conductors at the time, but this rule changed ove the years. By 1993, all references to conductor capacity increase had been dropped and only capacity increase for OCD remained.

The 1995 ROP showed that UL/NEMA standards were not in line with NEC requirements. With the code requiring increased OCD capacity, but allowing the "smaller" conductors sizes, the conductors had a tendency to overheat and exceed their temperature-rise limits as tested. It may seem that the problem centered with the conductors, but it was still a problem with the OCD.

The most common installed conductors today have 90 degree ratings. OCD only have 75 degree ratings. Even with the larger capacity breakers, the conductors being smaller and overloaded, were damaging OCD. By increasing the conductor capacity, the conductor will act as a heat-sink and draw heat from the OCD terminal and still have the capacity to handle continuous loading.

So, why don't we have to increase conductor size for 100% rated OCD? Essentially, a 100% rated OCD has been tested with a rating and conductors with no additional capacity. This means the 25% is already worked into the loading, and no overloading will occur. Regardless, this allowance has no real world use as it stands now. UL does not list OCD with 100% under 250-ampere. This eliminates most dwelling applications and all branch-circuits.

 

[steve66]

Re: Service Conductor Size

Thanks Bryan. Your last paragraph seems to be what I was looking for. By the way, this is for a 400A service, so a 100% rated breaker may be an option. If I really had to add another 25% to all the continuous loads, it was looking like a 600A service...for a house!!!

After starting this post, I used the optional method for dwelling units. It actually cut the calculated load down to about 300A, so now I can use a 400A service and a 80% rated breakers. But I still think it is important to know why the code is written that way.

Steve

 

[bennie]

Re: Service Conductor Size

Bob: Laguna Hills, brings back fond memories of the late 40's, spring break at Laguna Beach.

Beautiful place and art festival.

Bennie R. Palmer
Chaffee High
Ontario, Calif.

 

[rbalex]

Re: Service Conductor Size

brentp,

Not quite, see Bryans more expansive answer.

I think the strange thing is we are actually protecting the overcurrent protective device

 

[stew]

Re: Service Conductor Size

Brian could you please direct me to the code articles regarding 80% loading on breakers.? Does that mean that when you use 3/0 copper for a 200 amp service you need a 250 amp breaaker instead of the typical 200? Lets just say for drill we come up with a load calc of 195 amps.

 

[bphgravity]

Re: Service Conductor Size

Article 210.20(A) for branch-circuits and Article 215.3 for feeders covers the requirements for overcurrent protection sizing.

Using your example, the sizing of both conductors and the OCD depends on what type of load the 195-amperes is. If the load is non-continuous, then 230.42 tells us to size the service entrance to the sum of the non-continuous. Per 310.16, 3/0 minimum is permitted. The overcurrent device protects the feeder conductors or the service loads of 195-ampere. 215.3 states the OCD shall not be less than than the non-continuous loads. 240.4(B) permits 200-ampere OCD.

The feeder is sized per 215.2(A)(1) states the same as 230.42. So, 3/0 will be required for the feeder conductors as well.

Now, say the 195-amperes is continuous. The same sections require an increase of 125% of the continuous loads. This means the service conductors are required to be increased to 250-kcmils. The OCD will be increased to a 250-ampere OCD, and the feeder conductors will also be 250-kcmils.

In effect, the load will only be 80% of the conductor and OCD rating for the continuous load, being that mathematically 80% is the reciprocal of 125%. Another way to look at it is with a 250-ampere breaker, the load is only allowed to be 80% of its rating. This would allow up to 200-ampere loading which 195-ampere does not exceed.

 

[stew]

Re: Service Conductor Size

my theoretical load would include both non continous loads and continous loads at 125% for a total load of 195.I read 210,20 to say that a 200 amp breaker is fine for this application eh? where does 215.3 fit in here? And I still dont see where a breker can only be loaded yo 80% of its value anywhgre in the code. AM I missing something somewhere?

[ May 12, 2004, 02:29 AM: Message edited by: stew ]

 

[bphgravity]

Re: Service Conductor Size

Originally posted by stew:
my theoretical load would include both non continous loads and continous loads at 125% for a total load of 195.I read 210,20 to say that a 200 amp breaker is fine for this application eh?

Article 210 is for branch circuits. I doubt a 195-ampere load is a branch circuit. But yes, if your, non-continuous loads, plus the continuous times 125% is equal to 195-ampere, then 200-ampere OCD is fine,

where does 215.3 fit in here?

The main service disconnect usually incorporates an OCD. This OCD is protecting the feeder conductors. This isn't always the case but technicaly anything after the line side of the main service disconnect is the start of a the feeder. The OCD may also be protecting the bus of a panelboard without the use of a feeder.

And I still dont see where a breker can only be loaded yo 80% of its value anywhgre in the code. AM I missing something somewhere?

The actual term 80% is not specifically stated in the code, it is inferred by the requirements. Again, read my other post above concerning how OCD's are tested and listed.

 

[stew]

Re: Service Conductor Size

I meant to say that I was referring to service conductors. thanks

 

[jim dungar]

Re: Service Conductor Size

Per 240.4 OCPDs are chosen to protect the caluclated load on the conductor (whether feeder 230.79 or branch 210.20). The code requires a factors of upto 125% to be applied to the calculated loads prior to chosing the OCPD, unless a 100% rated OCPD is used. Most people mistakenly take this 125% increase to mean the standard OCPD is being derated by 80%.

Per UL, a 100% rating takes into account the OCPD, the conductors, the lugs, and the enclosure. It is unlikely to be possible to exchange any standard existing OCPD with a 100% rated one without providing additional ventilation.

Trivia:
All circuit breakers and fuses are tested by UL at 100% of their rating.

However, all circuit breakers and fuses must be installed in some type of enclosure or switch mechanism. Enclosed OCPDs require special/additional heat rise tests to be UL Listed as 100% rated devices.

 

[stew]

Re: Service Conductor Size

Thanks Jim I belive this solidifies my original thoughts in that if you size the conductor to match the load taking into consideration continous loads increased by 25% and add up all your other noncontinous loads you may then size a service condutor or feeder per the proper table and size the overcurrent device to match the resultant load.Anyway thats the way I have always understood the proper application of breakers.