Impact of Breaker Derate on Required Conductor Size

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  #1  
Old 11-09-06, 07:47 PM
mho
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Impact of Breaker Derate on Required Conductor Size

I am planning to add 4 kW of hard wired baseboard electric heat to a hobby room. It will consist of 2-1.5kW and 1-1.0 kW 240v units all controlled by a single wall-mount thermostat on a single dedicated breaker position. I understand that in this "continuous duty" application, the code requires me to derate the supply breaker by 25% which drives me to a 30A breaker. The 16.7A actual max load current could normally be supplied by #12 wire. However, the derated 30A breaker does not protect #12 wire against "overload". As a result, am I required to install #10 wire to all units or can I use #12 wire since the actual max load will not exceed 20A?
Thanks for any help you can offer on this question.
 
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  #2  
Old 11-09-06, 08:01 PM
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You have to derate if the breaker isn't rated for continuous duty. If the breaker IS rated for continuous duty, you can size it at 100%. For cheap house breakers... it's probably 30A

You should use #10 Cu wire if you use a 30A breaker. Article 424 gives a few technicalities where you might be able to get away with as little as #14 for parts of your circuit, but really, #10 is the best & safest to go.
 
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Old 11-10-06, 06:25 AM
mho
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Tnx

Tnx for the advice Grover. I was trying to avoid working with #10 wire and since the maximum conductor loading is so well defined it seemed like a great opportunity to do so. Your suggested methodology is the cleanest and safest however.
Mho.
 
  #4  
Old 11-10-06, 08:28 AM
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Consider using two separate runs of #14 on two separate 15A double pole breakers. This would require two separate thermostats (possibly heating different areas of the room), or some sort of contactor arrangement where one thermostat controls two circuits, or some other arrangement that provides control for the two separate circuits.

Article 424 requires that the conductors have an ampacity of 125% of the load, with no exceptions for '100% rated' OCPD, and there are no exceptions to the requirements of 240.4(D) protection for small conductors. So there is no way around the 125% rule for your heaters.

-Jon
 
  #5  
Old 11-10-06, 10:04 AM
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Space heaters are excepted from the small conductor requirement, per 240.3. Article 424 requires conductors be sized to 125 percent of the LOAD, not the OCP. Article 424 makes no explicit mention to OCP, merely stating that space heaters are considered a continuous load- which usually requires OCPs be oversized 125%, but not if they're rated for continuous duty.

Edit: my bad, Table 240.3 doesn't except the 240.4(D) small requirement, Table 240.4(G) does, and heaters aren't listed, just A/C equipment. So, not even the sqeakiest interpretation would let you use #14 here. If it was a heat pump, yes, but not a strip heater Either way, I'd never size 30A as anything less than #10 under any circunstance, nor would I ever recommend anyone else do so.
 

Last edited by grover; 11-10-06 at 10:27 AM.
  #6  
Old 11-10-06, 10:27 AM
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Note: this is not relevant to the recommendations being made to the original poster. Grover and I both agree that 30A and 10ga is correct, and I presume that he would agree that dual 15A and 14ga is a reasonable alternative. However to continue the discussion of the code issues:

I don't see how 240.3 _over rides_ 240.4, especially given that the small conductor requirement 240.4(D) explicitly spells out the other articles that are exempted from the requirement. (table 240.4(G) ).

424.3(B) explicitly states: The ampacity of the branch-circuit conductors and the rating or setting of the overcurrent protective devices...shall not be less than 125 percent of the totla load of the motors and the heaters.....

Seems like a pretty explicit mention of the OCPD to me.

I do agree that in the general situations where the 125% rules apply, using '100% rated' breakers provides an exception. However I don't believe that any residential branch circuit breakers are 100% rated, and I don't see an exception for 100% rated breakers in article 424.

-Jon
 
  #7  
Old 11-10-06, 10:34 AM
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Are you using NEC 2002? 424.3(B) was changed in NEC 2005; the way it reads now, all the general rules apply. It now reads only:

424.3(B) Branch-Circuit Sizing. Fixed electric space heating equipment shall be considered continuous load.


424.2, 424.6, 424.12(B), 424.13, 424.36, 424.44, 424.94 and 424.95(B) changed, too, but I don't have a copy of 2002 to tell you what changed.
 
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Old 11-10-06, 11:58 AM
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Grover,

Can you post some tech info on the continous duty vs non continous duty breakers.

I have never, in all my years heard of this. I would love to learn about it.

Jeff
 
  #9  
Old 11-10-06, 12:27 PM
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I stand corrected on 424.3(B). As you note, in the 2005 code we are simply instructed to treat heaters as continuous loads, and then use the general rules for continuous loads.

I still contend that 240.3 does not override 240.4, and that the small conductor limits apply.

-Jon
 
  #10  
Old 11-10-06, 01:39 PM
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chapters 1-4 are the basic code. chapters 5 - 8 can modify or override 1-4.

since table 240.3 mentions sections from articles 4 to 8 each must be taken seperately to decid if one overrides the other or just adds information.
 
  #11  
Old 11-10-06, 03:30 PM
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Jw I personally have never had experience with 100% rated breakers. My understanding is that you must use 90C rated conductors but you may not use ampacities from the 90C column they must come from the 75C column. Also the breakers have special compression lugs. I'm just not sure if you would find a frame that would fit a residential load center. I believe that they are commonly used in hospital applications and also with hospital emergency power generators.

I remember seeing something from siemens as for as tech info. You might query that web-site for some literature.;

Roger
 
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Old 11-10-06, 03:50 PM
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I cant find the exact literature I was thinking of but I looked around the Siemens site and found this......

http://ecatalog.squared.com/pubs/Circuit%20Protection/Insulated%20Case%20Circuit%20Breakers/0600DB0101.pdf

Roger
 
  #13  
Old 11-10-06, 04:05 PM
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Ok, so show me where this has anything to do with the calculations for a circuit.

Since all breakers meet this requrement, (now that zinsco and fpe are out of business)

Trust me, NO consideration needs to be given to the brand, rating of the circuit breake, when calculating load, except for if the load is HVAC and needs a HACR type breaker to handle the inrush from the motor starter.
 
  #14  
Old 11-10-06, 04:38 PM
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Well if I understand this correctly and considering the next size up rule and 30 amp max. for more than one outlet if you are using a standard inverse time breaker then 125% of the continuous load is the breaker size needed.

If you are using a 100% rated breaker then the continuous load is the breaker size.

If you have continuous plus non-continuous load its 125% of the continuous load plus the non continuous load for a standard breaker.

If 100% rated it is just the sum of the different loads.

But I dont believe there are any residential load centers that are going to be able to be used with 100% rated breakers in them at 100% load. Even if you put one in them they would still have to be operated at 80% on a continuous load.

There seems to be several restrictions on the use of these breakers so I would be hesitant to use them without more information.

I also notice this is a 1999 article. I believe you are saying the breaker sizing for fixed space heat is omitted in 2005?


Is that what your asking?

Roger
 

Last edited by Roger; 11-10-06 at 05:00 PM.
  #15  
Old 11-10-06, 05:09 PM
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In residential applications, the question of '100% rated' breakers is a non-issue. You won't find them available. 100% rated breakers assume importance in larger settings, eg. industrial feeders. Unless they are specifically marked otherwise, breakers are rated for _continuous_ loading at 80% of their trip rating.

There are many parts of the code that essentially require you to limit your continuous load to 80% of the breaker trip rating, to match with the capabilities of the breakers. The code also requires that the wire size be increased so that the now larger breaker will properly protect the wire. Thus if you had an 80A continuous lighting load, you would be required to use a 100A breaker and conductors that would be protected by a 100A breaker. The net result is that to properly and safely load the _breaker_, you end up oversizing the wires. By using '100% rated' breakers, you need not oversize the wires, and for large feeders this can make a very significant difference in wire size and cost.

-Jon
 
  #16  
Old 11-10-06, 05:46 PM
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jwhite, re-read 210.19(A)(1), the paragraph that requires branch circuit OCP to be sized at 125%. Notice exception 1? See also, 210.20(A), 215.2(A)(1), 215.3 and pretty much everywhere else NEC stipulates circuits be sized at 125% of continuous load.

I think winnie nailed it- continuous duty rating is rare in small (residential) molded case breakers with 60C wires, but common in more sophisticated adjustable trip breakers. I work mostly industrial where you'd be hard pressed to find a 1600A or 3000A breaker that wasn't continuous duty rated, and this is usually a non-issue the other way! The way mho asked the question, I figured this and the "fine print" exceptions to normal hard and fast rules (like #10=30A), was what he was referring to. And make no mistake, there are some cases where you can run #14 on a 30A circuit and have it be perfectly legal. (Again, not that I'd ever do it!)

FYI, OCP is normally sized at 125% because thermal conduction through the wire can cause simple thermal trip designs to malfunction and nuisance trip if held at near 100% load for too long. More sophisticated breakers are designed to trip accurately regardless.
 
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