Derating and conductor sizes

[Plano12345]

I'm having a discussion with an engineer. It's as follows:

We have wires for 10 current carrying conductors for 5 circuits in a 2" conduit and a bare for grounding
The size of all the wires are #8 copper.

The breaker sizes are the following: 4 at 20 amps and 1 at 30 amps. The current using the 30 amp breaker is 19 amps.

At 75 C for XHHW, you have 50 amps available for the current. Since there are 10 CCC, you really have 25 amps from derating.

My opinion is one set of wires needs to increase in size to completely cover the 30 amp breaker, so we'd make the wires for the 30 amp breaker #6. He is telling me the conductors are ok in that at 25 amps, we cover the necessary current in the design and would cover the breaker in the sense that 25 x 1.25 is greater than 30.
I always saw this as the derating current you create must cover the breaker completely.

I went to the NEC 2011 and couldn't find anything specific to what he is saying. Am I simply being too conservative? I'd rather be safe but it'd be nice to know if you confirm my colleague is 100% correct.

Thanks!

 

[augie47]

Are you using the 75? rating of the XHHW due to a wet location ?
If so, I would agree with you. 10 ccc's would result is a 25 amp rating and since 240.6 lists 25 amps as a "standard" size, you would need to use that breaker. If you could use the 90? ampacity the story changes.

 

[Plano12345]

Yes, 75C XHHW.

In Texas, the standard illumination breaker sizes are as follows:

15,20,30,40,50,60,70,100,125,150,175,225.

does this change the answer?

 

[Dennis Alwon]

If I understand you correctly the max current on the #10 is 19 amps then there is no reason not to use #8 IMO. 25 amps breaker or a 30 amp breaker will work as long as the load is not greater than 25 amps.

 

[Dennis Alwon]

I don't know about illuminating breaker and if there is no 25 amp breaker then that is how I answered above.

 

[Plano12345]

If I understand you correctly the max current on the #10 is 19 amps then there is no reason not to use #8 IMO. 25 amps breaker or a 30 amp breaker will work as long as the load is not greater than 25 amps.

Maybe I need someone to give me a 101 type lesson. I have always made sure that the conductor size selected covers both the breaker size (say 30 amps) and design current (19 amps). Should I only be covering the design current when selecting conductor sizes and not the breaker? It sounds from Dennis's response that only the design current is necessary.

thanks. Sometimes you think too much and can get yourself confused on something basic.

 

[Dennis Alwon]

Maybe I need someone to give me a 101 type lesson. I have always made sure that the conductor size selected covers both the breaker size (say 30 amps) and design current (19 amps). Should I only be covering the design current when selecting conductor sizes and not the breaker? It sounds from Dennis's response that only the design current is necessary.

thanks. Sometimes you think too much and can get yourself confused on something basic.

It depends but in general you are correct however look at 240.4(B)

(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above
the ampacity of the conductors being protected) shall be
permitted to be used, provided all of the following conditions
are met:
(1) The conductors being protected are not part of a branch
circuit supplying more than one receptacle for cordand-
plug-connected portable loads.
(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).
(3) The next higher standard rating selected does not exceed
800 amperes

 

[JDBrown]

If I'm understanding you correctly, your conductors for the 30A breaker do indeed need to be increased to #6 AWG.

The issue here is NEC 240.4(B):

240.4 Protection of Conductors.
(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes.

What (2) above means is that you can only use the next larger breaker (above the conductors' ampacity) IF the conductors' ampacity DOES NOT correspond with one of the standard ampere ratings.

Since the derated conductor ampacity of 25 amps IS one of the standard ampere ratings listed in NEC 240.6, you can't go up to the next size breaker without also increasing the size of the conductor.

If the derated conductor ampacity had been, say, 26 amps, then your colleague would be right and he could connect it to a 30 amp breaker (as long as the actual load was less than 26 amps).

I'm not sure what you mean by "standard illumination breaker sizes," but since NEC 240.6 lays out the ratings that "shall be considered" standard, I would think you have to go by what the NEC says -- unless, of course, there is a Texas amendment that deletes 25 amps from that section.

 

[Dennis Alwon]

The op said that there is no 25 amp illuminating breaker. If that was a standard size then I would agree. The NEC states 25 amp OVERCURRENT PROTECTIVE DEVICE is standard but I understood that is not the case in Texas

 

[Dennis Alwon]

I think JDBrown and I are on the same page

 

[Plano12345]

In texas roadway illumination, only certain breaker sizes are used.

I know that 25 amps is NOT available in Texas when it comes to roadway illumination (20 amps is and 30 amps is). However, it does exist in the NEC.

 

[GoldDigger]

In texas roadway illumination, only certain breaker sizes are used.

I know that 25 amps is NOT available in Texas when it comes to roadway illumination (20 amps is and 30 amps is). However, it does exist in the NEC.

Which would mean that unless Texas has also amended the NEC for their use, you can (will have to) go to a 30A breaker, but to permit that under the NEC that the derated conductor ampacity will have be more than 25. You can only go up to the next standard size. Rock and hard place....

 

[JDBrown]

Which would mean that unless Texas has also amended the NEC for their use, you can (will have to) go to a 30A breaker, but to permit that under the NEC that the derated conductor ampacity will have be more than 25. You can only go up to the next standard size. Rock and hard place....

... so the solution would be to increase to #6 AWG for the 30 amp circuit.

 

[Plano12345]

^^^^ Exactly. Even if it's a rock and hard place, go for the guarantee that it's ok.

thanks for the conversation.

 

[suemarkp]

Are these conductors old or new? Aren't most recent XHHW conductors really XHHW-2 now, so you could derate from the 90C column?

 

[augie47]

I'm trying to see whole picture and if I do.

You have roadway illumination project using XHHW conductors.
A actual 19 amp load being roadway illumination would have to be considered continuous thus
23.75 amps. A #8 XHHW in a wet location and 10 cccs would have an ampacity of 25 amps.
Under NEC rules you would need a 25 amp breaker.
IF TX has a Code that does not recognize a 25 amp breaker as "standard", and still recognized 240.4(B) I guess you cold go to a 30 amp breaker.

 

[charlie b]

Maybe I need someone to give me a 101 type lesson. I have always made sure that the conductor size selected covers both the breaker size (say 30 amps) and design current (19 amps).

As you wish. Allow me to present circuit design 101. The design process starts with calculating the load. You stated it was 19 amps. I will presume that that includes the 125% factor for the lights being a continuous load. At any event, let's take 19 amps as being the design basis load. Step two is to select a conductor size that has an ampacity of at least the amount of the load. You have already accomplished that, in that you have a wire that is good for 25 amps under the conditions of use. The next step (and here is where you have the process in the wrong order) is to select a breaker that is capable of protecting the wire that you selected in step 2. The wire has an ampacity of 25 amps, so you need a breaker that is either exactly the same as the conductor's ampacity, or that is the next higher standard rating, if the conductor's ampacity does not exactly match a standard rating. In this case, 25 amps is a standard rating, as far as the NEC is concerned. So you cannot go to the next higher rating. To be specific, a 30 breaker is not allowed to be used to protect a 25 amp rated conductor.

Now you say that a 25 amp breaker is not used for this application, and that you couldn't buy one if you tried. That does not change the law, and the NEC is the law, unless Texas has enacted a specific exception to the list of standard breakers in 240.6.

If the wire you are using really does have a 90C rating, then you can derate the 50% from the 55 amp value. That gives the wire an ampacity of 27.5 amps. Now you can use the next higher standard breaker rating of 30 amps, since 27.5 amps is not a standard rating.

 

[Julius Right]

Since NEC Art.240.4 Protection of Conductors (B)(2) states that in case there is a circuit breaker rated exactly as conductor ampacity[25 A] you have to take this one and not the next higher standard overcurrent device[30 A].
If, nevertheless, you intend to use the 30 A circuit breaker you have to change the conductor to #6.
I am sorry this is the law!:rant: Calculated, the ampacity of 10 conductor of #8 in 2? pvc buried conduit in 90 RHO 30 dgr.C earth at 4 ft. depth will be 31 A.
According to Table 310.15(B)(3)(a) the adjustment factor of 0.5 is stated for 10-20 conductors. Calculated for 10 conductor is only 0.6 and for 20 is 0.45.
[for 9 conductors is 0.7]