Feeder for Heat Loads

[Smart $]

If the feeder supplies only a single branch circuit and that circuit has only heating loads, then the effect is to apply the 125% to the feeder also. But if there is more than one branch circuit, it is very easy to have the feeder smaller than the calculation of the heater loads as continuous for the feeder ampacity calculation would require.

In the latter case, the stipulation of 220.51 would prevent the feeder from having a lesser size than the heater's branch circuit.

Say for example we had a feeder with one 3kW noncontinuous heater and two 120V receptacles, non-dwelling... perhaps a guard shack. The heater branch circuit minimum size is 3.75kW while the feeder minimum by calculation would be 3kW + .36kVA = 3.36kVA. The feeder minimum must be upsized to 3.75kVA.

 

[Dennis Alwon]

Smart a heater cannot be noncontinuous, IMO. Where do you get that a heater is noncontinuous?

 

[Smart $]

Smart a heater cannot be noncontinuous, IMO. Where do you get that a heater is noncontinuous?

Definitions...

Continuous Load. A load where the maximum current is
expected to continue for 3 hours or more.

Noncontinuous load is any that is not a continuous load by definition or through requirement (such as motors, other than the largest, in general).

I don't know how much truth there is to this, but I read a long time ago that a properly sized and installed space heater should cycle about 5-6 times per hour.

 

[Dennis Alwon]

Definitions...

Noncontinuous load is any that is not a continuous load by definition or through requirement (such as motors, other than the largest, in general).

I don't know how much truth there is to this, but I read a long time ago that a properly sized and installed space heater should cycle about 5-6 times per hour.

Yes but the NEC says to consider it continuous. I have never seen an electric furnace stay on for 3 hours or more but we have to size our conductors based on that. Same is true for water heaters. I cannot run 12/2 to a 4500 watt 240 water heater even though it calculates at 18.75 amps

 

[Smart $]

Yes but the NEC says to consider it continuous. I have never seen an electric furnace stay on for 3 hours or more but we have to size our conductors based on that. Same is true for water heaters. I cannot run 12/2 to a 4500 watt 240 water heater even though it calculates at 18.75 amps

The NEC says to consider it/them continuous to size the branch circuit... and that's all, nothing more. Says nothing about considering continuous for feeder and service sizing.

You agree these loads are typically noncontinuous by observation. So why do you feel these loads must be considered continuous for feeder and service sizing if the NEC doesn't specifically say so?

 

[Dennis Alwon]

The NEC says to consider it/them continuous to size the branch circuit... and that's all, nothing more. Says nothing about considering continuous for feeder and service sizing.

You agree these loads are typically noncontinuous by observation. So why do you feel these loads must be considered continuous for feeder and service sizing if the NEC doesn't specifically say so?

Okay I see where you are coming from finally. As long as the total load is at least as large as the heater load *125% then we are good as in the case cited above. Makes sense but not the way I was reading it. And as Golddigger stated one heater branch circuit fed from a feeder would certainly have to be sized at least the same as the branch circuit. :thumbsup:

Thanks- this is making sense

 

[Smart $]

...Thanks- this is making sense

Glad to hear it. I hope you still bring it up at your April meeting. I'd like to know the CMP members' opinion on the matter.

 

[augie47]

my work here is done

 

[Dennis Alwon]

Glad to hear it. I hope you still bring it up at your April meeting. I'd like to know the CMP members' opinion on the matter.

Already sent in -- questions need to be in by March 3 so I sent 10 in today.

 

[Dennis Alwon]

my work here is done

Great job gus...you didn't even come and try and help me....:lol:

 

[Dennis Alwon]

Here is what I can make of the response-- maybe Roger can help when he gets back to Asheville tonite or tomorrow- he has a long ride ahead of him.

Let me post my question as the members saw it.

Calculate the feeder size in the following example. I have 2 electric space heaters rated 10kw-- 41.7 ( lets use 42 amps). For a branch circuit it would calculate at 52.5 amps (53 amps) because it is considered a continuous load in Art. 424.3(B). 220.51 states that fixed electric heat shall be calculated at 100% while 215.2(A)(1) says both use art 220 sections III, IV, and V - which would include 220.51 and then goes on to state to calculate a feeder at 100% for noncontinuous loads plus 125% for continuous loads. This seems to be a contradiction. Am I missing something. I would say that the feeder must be sized 52.5 *2= 105 amps or (42 + 42) * 1.25 = 105 amps rather than 42*2=84 amps. Which is correct and why?

Jeff Sargent's response that both were correct. He stated art. 220 was for the load calculation but art 215 was for sizing the conductor and the OCPD

 

[Smart $]

...
Jeff Sargent's response that both were correct. He stated art. 220 was for the load calculation but art 215 was for sizing the conductor and the OCPD

Hmm... both are correct regarding an either one or the other question???

 

[Dennis Alwon]

Hmm... both are correct regarding an either one or the other question???

I was confused a bit also but it seems the feeder must be sized using art. 215 but he didn't elaborate on the fact that 215 mentions 220. I took that to be that 220 is not used when you calculate the conductor size.

 

[roger]

I agree with Dennis's interpretation of what was implied as far as the way Jeff worded his answer. It might have been that he didn't have a definate opinion and just answered with a CYA.

Roger