Conductor Ampacity Continuous Load with other derating factors

[jonbritt]

I am very confused due to recently reading the 2011 handbook. There is an example on page 329 of the 2011 handbook that states. It applies the 60 % adjustment factor due to the fact that over 20 conductors are bundled over a length of 24" of MC cable. That is understandable. Now it states also that a 16 ampere nonlinear load is present which would come to 20 Ampere load after multiplying by 125%. A 12 AWG THHN = 30 Amps and after 60% adjustment due to more than 20 conductors on bundled MC cable..., this comes to 18 amperes. How does the NEC say this is applicable? This is a 20 amp load with a 18 amp rated conductor? Any ideas?

 

[david luchini]

I am very confused due to recently reading the 2011 handbook. There is an example on page 329 of the 2011 handbook that states. It applies the 60 % adjustment factor due to the fact that over 20 conductors are bundled over a length of 24" of MC cable. That is understandable. Now it states also that a 16 ampere nonlinear load is present which would come to 20 Ampere load after multiplying by 125%. A 12 AWG THHN = 30 Amps and after 60% adjustment due to more than 20 conductors on bundled MC cable..., this comes to 18 amperes. How does the NEC say this is applicable? This is a 20 amp load with a 18 amp rated conductor? Any ideas?

I think you mean 16A "continuous" load where you wrote "nonlinear"...

The load on the circuit is 16Amps. The adjusted ampacity of the conductor is 18A. The conductor has an ampacity (18) which is larger than the load to be served (16), so it is acceptable (see 210.19(A)(1)).

The 125% for continuous loads is only used to determine minimum conductor SIZE, not conductor ampacity.

 

[jonbritt]

Thanks for your help!
So are you saying that if I have a 12 AWG THHN that has a bunch of derating factors dropping it all the way down to lets say 5 amp rating(original ampacity is 30 amps), and I had a continuous load of 5 amps. The continuous load means nothing in this problem because (5 * 1.25 ) = 6.25 which is less than the rated 30 amp wire? So the wire was technically calculated to carry a max of 5 amps. The continuous part of it doesn't pertain to the calculation because the 6.25 is smaller than the 30 amp wire before correction factors? So in a nutshell are you saying continuous loads should be considered on a wires ampacity before any of the derating factors are calculated?

 

[MasterTheNEC]

(1) General.

Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Conductors
shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b).


(a) Where a branch circuit supplies continuous loads or
any combination of continuous and noncontinuous loads, the
minimum branch-circuit conductor size shall have an allowable
ampacity not less than the noncontinuous load plus 125 percent
of the continuous load.


(b) The minimum branch-circuit conductor size shall have
an allowable ampacity not less than the maximum load to be
served after the application of any adjustment or correction
factors.

Along with the previous post David should assist you.

 

[jonbritt]

So after looking at this... After the correction factors, the wires from example on page 329 can carry 18 amps. And also a 16 amp continuous load requires a 20 amp rated wire and circuit breaker. The only thing I don't understand about David's response is that the load to be served is 20 amps, not 16(due to the fact it's is a continuous load)? So as I questioned before... Is the 125% of continuous load only considered on the wires actual ampacity before any correction factors are to be considered?

Also MasterTheNEC said...
Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Conductors
shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b).


(a) Where a branch circuit supplies continuous loads or
any combination of continuous and noncontinuous loads, the
minimum branch-circuit conductor size shall have an allowable
ampacity not less than the noncontinuous load plus 125 percent
of the continuous load. This would be 16 * 1.25 which is = 20 amps


(b) The minimum branch-circuit conductor size shall have
an allowable ampacity not less than the maximum load to be
served after the application of any adjustment or correction
factor. This would be the = 18 amps

The larger of these two is what
Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Conductors
shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b)

would be the 20 amps?


?Please help me understand what I am not seeing here?

 

[Smart $]

The noncontinuous plus 125% continuous load current is only used in conjunction with sizing for terminal [and/or equipment] temperature limitations as covered in 110.14(C).

The conductor ampacity after the application of adjustment and correction factors cannot be less than the load current, where the load is as calculated under Article 220 (i.e. no factoring of continuous loads at 125%).

Both preceding requirements must be met, so the minimum conductor size is the larger of the two preceding minimum size determinations.

 

[Smart $]

...?Please help me understand what I am not seeing here?

Also, unless you include a temperature limitation and wire type to your evaluation, you will not be able to see the complete picture.

 

[david luchini]

So after looking at this... After the correction factors, the wires from example on page 329 can carry 18 amps. And also a 16 amp continuous load requires a 20 amp rated wire and circuit breaker. The only thing I don't understand about David's response is that the load to be served is 20 amps, not 16(due to the fact it's is a continuous load)? So as I questioned before... Is the 125% of continuous load only considered on the wires actual ampacity before any correction factors are to be considered?

You are making an error here. The LOAD is 16A, not 20A. Lets say you had two loads, both 1920VA @ 120V, one which would be continuous, one which would be non-continuous. BOTH circuits would have a load current of 16A. (The load that is continuous doesn't have a load current of 20A.)

Also MasterTheNEC said...
Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served.

Read this sentence again..."shall have an ampacity not less than the maximum load to be served." Again, the conductors need to have an ampacity of at least 16 to serve the load (whether the load is continuous or non-continuous.) Note that here they are talking about conductor ampacity, where later they talk about conductor size.

Conductors
shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b).


(a) Where a branch circuit supplies continuous loads or
any combination of continuous and noncontinuous loads, the
minimum branch-circuit conductor size shall have an allowable
ampacity not less than the noncontinuous load plus 125 percent
of the continuous load. This would be 16 * 1.25 which is = 20 amps

Note that here, they are talking about conductor size. So what size conductor has an allowable ampacity of 20? Assuming the terminations are rated for 75 deg, #14 AWG has an allowable ampacity of 20. So for part (a) the minimum conductor size is #14.

(b) The minimum branch-circuit conductor size shall have
an allowable ampacity not less than the maximum load to be
served after the application of any adjustment or correction
factor. This would be the = 18 amps

So what size conductor would have an allowable ampacity of not less than 16 after the adjustment or correction factors have been applies? For THHN conductors, #14 AWG with the 0.6 adjustment factor would have an ampacity of 25*0.6=15...15 is less than the load current of 16, so it is too small. #12 AWG with the 0.6 adjustment factor would have an ampacity of 30*0.6=18. So for part (b) the minimum conductor size is #12.

The larger of these two is what
Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Conductors
shall be sized to carry not less than the larger of 210.19(A)(1)(a) or (b)

would be the 20 amps?


?Please help me understand what I am not seeing here?

The larger conductor of the requirements of 210.19(A)(1)(a) or (A)(1)(b) is #12 Awg. That would be the minimum conductor size allowed.

Hope this makes sense.