Feeder for Heat Loads

[augie47]

In another thread, Dennis(Alwon) touched on the situation where the Code requires the Branch Circuit to be rated at 125% of the load.
Got me to thinking. Art 424 .3 tells us the Branch Circuit for Space heating must be considered a continuous load thus figured at 125%.
If one had two 240v space heaters 10kw each, our current would be 41.6 amps each and the branch circuit would be calculated at 52 amps. We wire each with a #6 NM and a 60 amp breaker.
424.3 says nothing about feeders, only that the branch circuit needs to be figured as continuous.
Would it be Code compliant to install a #3NM feeder to these two units with an ampacity of 85 amps ?

 

[texie]

In another thread, Dennis(Alwon) touched on the situation where the Code requires the Branch Circuit to be rated at 125% of the load.
Got me to thinking. Art 424 .3 tells us the Branch Circuit for Space heating must be considered a continuous load thus figured at 125%.
If one had two 240v space heaters 10kw each, our current would be 41.6 amps each and the branch circuit would be calculated at 52 amps. We wire each with a #6 NM and a 60 amp breaker.
424.3 says nothing about feeders, only that the branch circuit needs to be figured as continuous.
Would it be Code compliant to install a #3NM feeder to these two units with an ampacity of 85 amps ?

I'm inclined to say that the feeder would be calculated at 100% per 220.51. So yes, the #3 NM would be compliant.

 

[Dennis Alwon]

What bothers me about 215.2(A)(1) is the use of continuous load as singular. With motors we only do 125% of the largest load.

 

[Smart $]

I'm inclined to say that the feeder would be calculated at 100% per 220.51. So yes, the #3 NM would be compliant.

I agree. The 125% rating for branch circuit is not carried through to the calculation for a feeder. This is even true if one heater load is all the feeder serves... but would seem strange having a feeder of a smaller size than the branch circuit wiring... :blink:

 

[Dennis Alwon]

I agree. The 125% rating for branch circuit is not carried through to the calculation for a feeder. This is even true if one heater load is all the feeder serves... but would seem strange having a feeder of a smaller size than the branch circuit wiring... :blink:

Can you explain 215.2 (A) (1). I thought it was as you are saying but...

 

[Smart $]

What bothers me about 215.2(A)(1) is the use of continuous load as singular. With motors we only do 125% of the largest load.

It's sort of the opposite of a Victorian "we" meaning "I". In this case, the analogy is flipped and "I" means "we".

By using the singular form continuous load, it means the sum of all individual continuous loads.

 

[texie]

What bothers me about 215.2(A)(1) is the use of continuous load as singular. With motors we only do 125% of the largest load.

After further thought, I think that 215.1(A) would apply here and the feeder ampacity would have to be 104 amps. And the feeder OCP would have to be per 215.3. In other words, 220.51 would only apply to the overall load calculation not the individual feeder. To do otherwise the feeder would be operating a continuous load at more than 80%.

 

[Dennis Alwon]

Well I still don't feel like I am closer to an answer. If we add the loads and multiply by 125% we get the same as if we multiply the loads by 125% and add them. Either way it seems strange that the feeder would have to be 125% of all the loads.

 

[Smart $]

Can you explain 215.2 (A) (1). I thought it was as you are saying but...

After further thought, I think that 215.1(A) would apply here and the feeder ampacity would have to be 104 amps. And the feeder OCP would have to be per 215.3. In other words, 220.51 would only apply to the overall load calculation not the individual feeder. To do otherwise the feeder would be operating a continuous load at more than 80%.

Now y'all are making me rethink the premise of my belief... and as it currently stands, my belief may have been in error. While 424.3 says nothing about feeder rating and sizing, 220.40 says...

The calculated load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, as determined by Part II of this article, after any applicable demand factors permitted by Part III or IV or required by Part V have been applied.

As we know, there is no specific section in Part II for fixed space heating equipment, so it falls under 220.14(A). No problem to this point with my existing belief. Now read 220.3 and note 424.3 is listed in Table 220.3.

Comments?

 

[Smart $]

Well I still don't feel like I am closer to an answer. If we add the loads and multiply by 125% we get the same as if we multiply the loads by 125% and add them. Either way it seems strange that the feeder would have to be 125% of all the loads.

I do not understand where you are confused. makes it difficult to help you get un-confused.

Your middle sentence is correct.

Its not 125% of all the loads... only 125% of the continuous load (or loads, if you prefer).

 

[Dennis Alwon]

It seems to me the 125% rule is enforced here.

 

[augie47]

If you look at the load calculations in Appendix D they don't show water heaters, back-up heat, etc as continuous loads in their load calculations. In both cases the branch circuit is continuous but they don't appear to take that into consideration on the feeders.

 

[texie]

I think I'm sticking with 215.2(A) and 104 amps. As I said previously, otherwise you would have a feeder and OC device supplying a continuous load at more than 80% of its rating.

 

[texie]

If you look at the load calculations in Appendix D they don't show water heaters, back-up heat, etc as continuous loads in their load calculations. In both cases the branch circuit is continuous but they don't appear to take that into consideration on the feeders.

Yes, but that is for overall calculating purposes (220.51). I think an individual feeder would fall under 215.2(A).

 

[Dennis Alwon]

I do not understand where you are confused. makes it difficult to help you get un-confused.

Your middle sentence is correct.

Its not 125% of all the loads... only 125% of the continuous load (or loads, if you prefer).

Yes in this case all the loads are continuous

 

[Dennis Alwon]

If you look at the load calculations in Appendix D they don't show water heaters, back-up heat, etc as continuous loads in their load calculations. In both cases the branch circuit is continuous but they don't appear to take that into consideration on the feeders.

It is odd because it appears in some examples they do and others they don't. Example D3 they do take the continuous load times 125% for overcurrent protective device and it follows thru for the feeder directly above the next example in the 2011

Example D(3)(a) where they mention ocp they mention 125% when when they add the continuous and non continuous loads they somehow forgot the 125% of the continuous load. They do the same for the feeder load.

 

[Dennis Alwon]

So Texie--- if the overall calculation is for the feeder then why would we need 125%. This is the confusion. I have nothing but heaters and I have 2 areas in the code saying different things. It makes no sense to say calculate it at 100% then say they are considered continuous loads so they must be calculated at 125%. Still confused here.

 

[Smart $]

So Texie--- if the overall calculation is for the feeder then why would we need 125%. This is the confusion. I have nothing but heaters and I have 2 areas in the code saying different things. It makes no sense to say calculate it at 100% then say they are considered continuous loads so they must be calculated at 125%. Still confused here.

There's nothing that says heater loads are to be calculated at 125% under Article 220.

For 424.3(B), it would be 210.19(A)(1) and 210.20(A) which says branch circuit ampacity and overcurrent must be sized at 125% for continuous loads plus 100% of noncontinuous loads.

 

[Dennis Alwon]

There's nothing that says heater loads are to be calculated at 125% under Article 220.

For 424.3(B), it would be 210.19(A)(1) and 210.20(A) which says branch circuit ampacity and overcurrent must be sized at 125% for continuous loads plus 100% of noncontinuous loads.

What about 215.2

 

[texie]

What about 215.2

Gee, such a simple question from augie and look how complicated it gets. We'll have to think this through a bit. Like you, I'm a little confused.