Simple Question

[tbro723]

Hope I am not wasting anyone's time but why is the minimum ampacity of a conductor 1.25 times the continous load. Why the 1.25 factor again? From what I understand you do not need to apply this factor if your circuit breaker is rated for 100% operation.

I always check to make sure the max load on a wire is not greater than .8 times the circuit breaker rating, in this case am I double counting this factor? Once for minimum cable sizing and one for 80% CB operation? Hope my qustion makes sense

Thanks

 

[480sparky]

Simple answer: Heat build-up.

 

[Bobhook149]

I thought 125% was only if the load was continuos (more than 3 hours) ?

 

[tbro723]

Good call, pretty obvious too, except for me. So I guess that I am not double counting then because one factor is for heat build up and the other for 80% operation. Thanks

 

[tbro723]

I thought 125% was only if the load was continuos (more than 3 hours) ?

Yes that is what I meant, only for continous load

 

[Bobhook149]

Yes that is what I meant, only for continous load

I know, i should read better. Sometimes i just give it a glance sorry:roll:

 

[480sparky]

125% and 80% are reciprocals. 1/0.8 = 1.25 and 1/1.25 = 0.8. Both refer to continuous loads.

 

[e57]

It's a neat math coincidence that 80% circuit size, and all of 125% of the load of the same - are the same there-abouts...

i.e.
A 20A circuit load max is 16A. (80% of 20A) A 16A load would need a circuit rated at 125% Which is 20A... :roll:

However sizing should always go from the continuous load to find the circuit size. Not seeking a circuit size that fits load in under 80% of the rating... And the key words there are "continuous load".

 

[infinity]

It's a neat math coincidence that 80% circuit size, and all of 125% of the load of the same - are the same there-abouts...

i.e.
A 20A circuit load max is 16A. (80% of 20A) A 16A load would need a circuit rated at 125% Which is 20A... :roll:

However sizing should always go from the continuous load to find the circuit size. Not seeking a circuit size that fits load in under 80% of the rating... And the key words there are "continuous load".

Just clarify you're saying that this statement only applies to a continuous load? A 20 amp circuit can be loaded to 20 amps if the loads are non-continuous.

 

[Doug S.]

It's a neat math coincidence that 80% circuit size, and all of 125% of the load of the same - are the same there-abouts...

It's easier to see when you work w/ fractions 4/5 (80%) and 5/4 (125%).
I do often wonder if the simplicity of the math was a factor in choosing THOSE numbers?

Doug S.

 

[iMuse97]

Just clarify you're saying that this statement only applies to a continuous load? A 20 amp circuit can be loaded to 20 amps if the loads are non-continuous.

If the loads are not continous then there would be a type of "load factor" and the 20 amp circuit would not be loaded to 20 amperes. For instance, say I have two receptacles in a bath, and my wife plugs hair rollers and her blow dryer into one of them, and my daughter attaches the curling iron in another. These appliances alone add up to more than 20 amperes, and yet the CB (10 yr old SquareD's.) will hold (I know; they actually do this). You cannot really "load" a circuit unless you know exactly what equipment is supplied by that circuit.

Code requires load calculations for sizing services and continuous load branch circuits not for general use circuits.

 

[iMuse97]

It's easier to see when you work w/ fractions 4/5 (80%) and 5/4 (125%).
I do often wonder if the simplicity of the math was a factor in choosing THOSE numbers?

Doug S.

Are you suggesting that if actual engineering were involved in choosing those factors, they probably would have been figured to several points to the right of the decimal?

 

[iMuse97]

a type of "load factor"

I mean: "demand factor"

 

[Doug S.]

Are you suggesting that if actual engineering were involved in choosing those factors, they probably would have been figured to several points to the right of the decimal?

I said what I said, letting others read into it what they please when the fun begins.

 

[infinity]

If the loads are not continous then there would be a type of "load factor" and the 20 amp circuit would not be loaded to 20 amperes. For instance, say I have two receptacles in a bath, and my wife plugs hair rollers and her blow dryer into one of them, and my daughter attaches the curling iron in another. These appliances alone add up to more than 20 amperes, and yet the CB (10 yr old SquareD's.) will hold (I know; they actually do this). You cannot really "load" a circuit unless you know exactly what equipment is supplied by that circuit.

Code requires load calculations for sizing services and continuous load branch circuits not for general use circuits.

I'm not sure what you're getting at but I was just trying to clarify that a circuit can be loaded to it's ampacity if the load is non-continuous. At times there's a general misconception that a circuit can only be loaded to 80% of it's value. For non-continuous loads this is untrue. 210.19(A)(1).

 

[e57]

Just clarify you're saying that this statement only applies to a continuous load? A 20 amp circuit can be loaded to 20 amps if the loads are non-continuous.

Yeah - I guess I should.... In most cases (As they can differ on what the application is) the circuit is sized by the sum of the non-continuous, and 125% of the continuous load. Only the continuous get rated at 125% - so yes - in most cases you could load up to 100% if only non-continuous are present

Quote of convienience....

210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.~~~~~~~

The list goes on and on, including the table in 210.21 which does not have this specific language, but has the maximum load as 80% of the circuit size - but doesn't say that....

Anyway - a note: How does one know "really" if something is continuous or non-continuous? Yeah sure the defintition is 3 hours or more, but when in doubt.... figure it continuous. IMO

 

[suemarkp]

Also one little nit to the original question: the ampacity of the conductor is what 310.16 says it is, whether the load is continuous or not. A grounded conductor which doesn't connect to an overcurrent device, does not need a 125% factor applied for continuous operations (Exception 2 to 210.19(A)(1)). However, most circuit breakers require that this 125% factor be added unless you're up into the giant sized ones.

So if you can find a 100% rated breaker for your circuit, all wires can be sized at 100% for continuous use. If you can't find one, then the ones that attach to an overcurrent device must have a 125% factor applied to the load.

 

[steve66]

Are you suggesting that if actual engineering were involved in choosing those factors, they probably would have been figured to several points to the right of the decimal?

Something like "The wire ampacity shall not be less than (pi/e^1) times the continuous load."

I like it.

Steve

 

[infinity]

Also one little nit to the original question: the ampacity of the conductor is what 310.16 says it is, whether the load is continuous or not. A grounded conductor which doesn't connect to an overcurrent device, does not need a 125% factor applied for continuous operations (Exception 2 to 210.19(A)(1)). However, most circuit breakers require that this 125% factor be added unless you're up into the giant sized ones.

So if you can find a 100% rated breaker for your circuit, all wires can be sized at 100% for continuous use. If you can't find one, then the ones that attach to an overcurrent device must have a 125% factor applied to the load.

Where can I find this information? Doesn't the 125% factor have to do with the continuous load and nothing else?

 

[kevin]

"Hope I am not wasting anyone's time but why is the minimum ampacity of a conductor 1.25 times the continous load. Why the 1.25 factor again? From what I understand you do not need to apply this factor if your circuit breaker is rated for 100% operation".


The answer has to do with how overcurrent devices (OD's) are tested. UL requires that an OD be able to carry its rated ampacity in free air continuously (3 hours or more) without tripping. The OD depends in part on the wiring conductor(s) to act as a heat sink to minimize temperature rises that would result in a failed test. Terminals of OD's are rated 60/75 deg C, so the attached conductor for the test is selected from the 75 deg C column in Table 310.16. When the OD's are installed in panelboards or other enclosures it has been found that, unless they are listed for 100% of their rating, they often nuisance trip unless the current is limited to no more than 80% of the rating of the OD. That is the basis for the requirement to add 25% ampacity to branch circuit [210.19(A)(1)] and feeder[215.2(A)(1)] conductors that supply continuous loads. In the 2008 Code cycle, Ex. No. 2 was added to both those Code Sections to exclude from the 25% additional burden those grounded (neutral) conductors that do not terminate on an OD, since the rationale for the requirement (to prevent nuisance tripping of OD's) does not exist in such instances.