Wire and overcurrent size

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tom baker

tom baker

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I am reviewing a book on wiring, does this sound right?
40 ampere continuous load
40x 1.25 = 50 ampere
Table 310.16 @ 75 Deg C = 8 AWG
Would the overcurrent device (circuit breaker) be 40 amperes or 50 amperes?
Pretty simple, but I want to get some comments.
 
The OCPD can be no higher than 50. You can use a 40, if you wish. You can use a 2 amp fuse, if you wish. :D For my part, I would go with 50, if for no better reason than that I can.
 
I thought 50 would be the minimum size for the breaker, unless the load is marked with a MOCP or something similar? Or unless the breaker is 100% rated.

Steve
 
210.20(A) requires that the branch circuit OCPD be rated not less than 100% of the noncontinuous load plus 125% of the continuous load. In your example, the entire load is continuous, so the minimum size of the OCPD is 0 + 1.25 x 40 = 50 amps.

You did not ask for comments on the minimum size or Type of insulation of the conductors, but that issue deserves scrutiny. 210.20(B) and 240.4 require that the branch circuit conductors be protected in accordance with their ampacities specified in 310.15, i.e.Table 310.16. That Table, however, is subject to 110.14(C)(1) which requires that the terminal temperatures not exceed their ratings. While most equipment, and ALL circuit breakers that I know of, have 75 deg C terminals allowing the use of the 75 deg C column in the Table, the book you are reviewing should explicitly state that the OCPD has 75 deg C terminals in this case. Otherwise, Table 310.16 would require that the conductors be sized not smaller than that shown in the 60 deg C column. Finally, the text should specify the type of insulation for the conductors. Mike Holt's textbooks normally, in the Introduction, have a "The Scope of this Textbook" segment that for the sake of simplification limit the variable conditions of these issues to THHN and copper.
 
OK, so I withdraw my 2 amp fuse suggestion. :roll:

I was thinking along the lines of 240.4. That is all about protecting the conductor. Since a #8 is good for 50 amps, and since 50 amps is a standard breaker rating, that imposes a maximum of 50 amps for your circuit.

But I missed the 210.20(A) requirement that imposes a minimum rating of 50. So Steve and Kevin win one Brownie Point each from me. :)

Upper limit is 50. Lower limit is 50. So I gather you are stuck with 50. :cool:

 
Aren't there situations where you can have 125% added for continuous load but the breaker if rated for usage at 100% can be the load itself. Thus a 40 amp or 45 amp breaker could be used as long as the circuit conductors where at 125%
 
The way I have looked at it is that the conductor is sized to carry the calculated load, the OCPD is sized to protect the conductor based upon the conductors rated ampacity.
Also, doesn't the code say that if as an example 50a is not a standard rating that you are allowed to go and not exceed the next higher standard rating? This would include an OCPD with ratings up to and including 800a. Above 800a the conductor rated ampacity is to be equal to or greater than thae rating of the OCPD.
 
tom baker said:
I am reviewing a book on wiring, does this sound right?
40 ampere continuous load
40x 1.25 = 50 ampere
Table 310.16 @ 75 Deg C = 8 AWG
Would the overcurrent device (circuit breaker) be 40 amperes or 50 amperes?
Pretty simple, but I want to get some comments.
Click to expand...

This sounds like one of the trick questions that you find on exams. As pointed out by others, the OCPD would be 50A per 210.20(A). However, the conductor size would only be #8AWG IF the equipment is listed for use with conductors with higher than the 60 Deg C temperature rating. Otherwise, the conductors would have to be #6AWG per 110.14(C)(1).

Dennis Alwon said:
Aren't there situations where you can have 125% added for continuous load but the breaker if rated for usage at 100% can be the load itself. Thus a 40 amp or 45 amp breaker could be used as long as the circuit conductors where at 125%
Click to expand...

If the OCPD was a listed 100% rated device, so that a 40A c/b was used, per 210.20(A)Exeption, then the conductors would also be permitted to be 100% of the load, instead of 125%, by 210.19(A)(1)Ex. 1.

So a 40A (100% rated) c/b could be used for the 40A continuous load, with #8AWG (60 Deg C) conductors.
 
Well then I guess you wouldn't need a #6 conductor in that case. Are breakers rated for 100% readily available in all sizes?
 
Dennis Alwon said:
Are breakers rated for 100% readily available in all sizes?
Click to expand...

Good question. Couldn't say for sure, but I'd guess they're not. The code says, for both feeders and branch circuits, where the "assembly," including the OCPD protecting the feeder/branch circuit, is listed for operation at 100% of its rating...

I think the UL listing for 100% rated c/b's ties into specific breaker enclosures or cubicle spaces of a specific size, presumably to ensure proper ventilation around the breaker for the 100% rating. So I don't think you'd ever see a 100% in a panelboard situation.
 
charlie b said:
OK, so I withdraw my 2 amp fuse suggestion. :roll:

I was thinking along the lines of 240.4. That is all about protecting the conductor. Since a #8 is good for 50 amps, and since 50 amps is a standard breaker rating, that imposes a maximum of 50 amps for your circuit.

But I missed the 210.20(A) requirement that imposes a minimum rating of 50. So Steve and Kevin win one Brownie Point each from me. :)

Upper limit is 50. Lower limit is 50. So I gather you are stuck with 50. :cool:
Click to expand...

But I still like your iinitial comment about using a 50A if for no other reason than you can. :)

Dennis Alwon said:
Well then I guess you wouldn't need a #6 conductor in that case. Are breakers rated for 100% readily available in all sizes?
Click to expand...

FWIW: One engineer told me that he thought all substation breakers were 100% rated. I haven't found anyone yet that has refuted that claim.

It leaves me wondering if any panelboard type breakers are 100% rated?

Steve
 
I can't imagine that panel board breakers are rated 100% for continuous duty. If they were we wouldn't have to use 125%. Remember the assembly (whatever that is) must be rated 100% also.

I don't get the exception. Why would the fact that a breaker is rated 100% have any affect on the branch circuit conductors.

I am confused now, for sure, I thought we could load a breaker to 100% on a normal circuit. If so, then aren't they rated for 100% of its rating.
 
steve66 said:
FWIW: One engineer told me that he thought all substation breakers were 100% rated. I haven't found anyone yet that has refuted that claim.
Click to expand...

Steve, what is the definition of "substation" breakers, in this example?

steve66 said:
It leaves me wondering if any panelboard type breakers are 100% rated?
Click to expand...

I've been looking at Square D specifically. It doesn't appear that the QO or E-frame c/b's meant for installation in the QO or NF panelboards have a 100% rating option. But the PowerPact H, J and D frame breakers, which can be installed in I-Line panels do indicate 80% and 100% rated. But installing a 100% rated c/b in an I-Line panel won't necessarily give you a 100% rating, as the entire assembly has to be listed for 100% rating.
 
Dennis Alwon said:
I don't get the exception. Why would the fact that a breaker is rated 100% have any affect on the branch circuit conductors.
Click to expand...

Dennis, I apologize if I don't understand your question correctly, but you seem to be asking if the 40A continuous load in the OP required a breaker to be 50A and the conductors not less than 50A ampacity (#6 at 60 Deg C) then why doesn't the use of a 100% rated 40A c/b also require the 50A ampacity conductors.

I think the issue is with overheating at the breaker during continuous load. The code asks for the 125% on the breaker to deal with overheating at the breaker, and the conductors must then be upsized to match the breaker. It is not the conductors themselves that are concerned with the continuous load.

The 100% rated breaker deals with the overheating by its being installed (according to its listing) in a way that will reduce the overheating during continuous load. The conductors, therefore, just match the breaker size in either the 100% or 125% rating.
 
Since 100% rated breakers were brought up in order to apply a 100% rated breaker it must be mounted in an enclosure that the manufacturer has listed it for use with then it is to be applied at 75 deg C.
 
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