No, your missing #2, and lack understanding on HOW to read the NEC. For the last time, 424.3 deals with BRANCH circuits for the heaters, not the FEEDERS!!Its written in BOLD the lists (A) and (B) under that section, and since they under that section, it for the BRANCH circuits.
Again, I am NOT missing your issue #2. You are. You say for #2 "What is considered continuous and non-continuous on a FEEDER." How in the world can 20A on a branch circuit be CONTINUOUS, but the same 20A be NON-CONTINUOUS on the feeder that feeder that branch circuit? It
CANNOT be. If the 20A on the branch circuit continues for 3 hours or more, then that 20A on the feeder continues for 3 hours or more. The load on the feeder is CONTINUOUS if the load on the branch circuit is CONTINUOUS.
This is not my logic and definitely not what I have posted or said, you've missed it all together! This shows you lack comprehension and reading of section 220.51 and what I have posted. The latter part of 220.51 clearly says, "....However in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied." So your question is moot and already answered if you read 220.51 correctly. Your feeder in that example you gave would have to be at least 50amp. Because it cannot be smaller than the largest branch circuit (notice this is SINGULAR NOT PLURAL,)and since this is the only heater you mention it has to be at least that size since its the 'largest branch circuit'. This would probably change if you had more heaters the feeder was feeding.
You're right - bad example, but it doesn't change the point above that continuous load on the branch circuit equals continuous load on the feeder. You seem to be confusing NON-CONTINUOUS load with DEMAND FACTOR. If I understand your argument, the load is NON-CONTINUOUS on the feeder, because all of the heaters on the feeder may not be operating "at the same time." Whether the loads on the feeder are operating at the same time has nothing to do with whether the load is CONTINUOUS or not.
Again, you are wrong the NEC clearly shows you are!! 424.3 is for branch circuits for heaters (125%,) 220.51 is for feeders for heaters 100%. I noticed you never responded to the example I gave about water heaters. According to YOUR logic, since the water heaters are considered a continuous load on the branch circuit, they have to be continuous loads on feeders. This would not be case, you would be installing an overpriced and and unecessary large wire for a job. This practice of 'demand factors' is done all the time on multifamily housing jobs.
220.51 is for determining the
LOAD on the feeder, not for determining the
SIZE of the feeder (the feeder size is determined by 215.2.) Again, a load
CANNOT be continuous on a branch circuit, but non-continuous on the feeder feeding that branch circuit. I didn't see your example regarding water heaters, but yes, the load is continuous on the feeder if it is continuous on the branch circuit. If the water heater is 30A (for more than 3 hrs continuously) on the branch circuit, then the load for the water heater will be 30A (for more than 3hrs continuously) on the feeder. How will the current for the water heater magically stop flowing on the feeder when it is still flowing on the branch circuit? The practice of "demand factors" is done when/where the code says they can be done, not just any place you feel like providing a demand factor. And again, this is clearly where you are going wrong, you are confusing "demand factor" with "non-continuous load."
The Demand factors would have ALOT to do with calculating the load on feeders and services. Following your logic, all these multi family apartment complexes and other buildings are not code compliant because they used the DEMAND factors for sizing the load to get the ampacity of what wires to install. This includes the 100% load of the feeders for heaters.
Where did I suggest that demand factors don't have anything to do with calculating loads on feeders and services? Now it is really clear to me that you are confusing "demand factor" with "non-continuous load" because I said that demand factors have nothing to do with whether a load is
CONTINUOUS OR NOT. These are completely different things.
Let me give you an example that hopefully will help you comprehend that you are confusing "demand factor" and "Non-continuous load."
You are suggesting that 220.51 says that the feeder should be size for 100% of the heater load, that is to say, that the load is "non-continuous" on the feeder, even though it is continuous on the branch circuit. 220.51 is under section III. Feeder & Service Load Calculation, so lets look at another load in calc in that section. How about 220.42 for lighting. Imagine a 3000sf store. The branch circuit loads will be 3va/sf, so the total lighting load on the feeder for the store will be 9000VA. Section 220.42 tells us that the lighting load for the store should be calculated at 100% (just as 220.51 says that heaters should be calculated at 100%.)
By your logic, the feeder load for the lighting (on a 240V feeder) will be 37.5A. If a feeder was feeding only this lighting, a #8awg (60deg term) would be allowed? However, take a look at example D3 in the Annex D. You will see that the 9000VA for the lighting is calculated as continuous load @ 125% for the feeder. Why? Section 220.42 says to figure it at 100%? Because the load, calculated at 100% in Article 220 is
continuous on the feeder for sizing the feeder per Article 215.
Now lets look at the OP again. If there are four 5kW, 240V heaters connected to the same feeder, each heater has a load current of 21A. 424.3 says that the load for the branch circuit shall be considered continuous load, so by 210.19(A)(1), each branch circuit must have an ampacity of at least 26.25A. Let's for the sake of argument say that the heaters will run for 4 hours continuously, that would make them "continuous load." Section 220.51 then tells us that the heating load shall be calculated at 100% of connected load, that is to say that all of the heaters are operating at the same time. 100% of 4x21A=84A. Now imagine that these four heaters turn on at 8AM and off at noon. Each branch circuit sees 21A continuous load (as per 424.3) and the feeder sees 84A of continuous load (per 220.51.) The size of the feeder per 215.2 shall have an ampacity of at least 125% of the continuous load, or 105A.
If we say, well one heater will come on at 8AM and run for 4 hours, and one will come on at 9AM, one at 10AM and one at 11AM, so that the feeder never sees the full load continuously, then we would be violating section 220.51. We are calculating the load on the feeder at LESS THAN 100%.
Now imagine, in the OP, that the four heaters, each in its separate conex, are running overnight when the outside temperature drops to below zero. Since it is so cold, the heaters run all night. But by using the 100% size for the feeder, the heater are connected by a #3 feeder to a 90A c/b (using 60 deg terms.) How is that feeder and circuit breaker going to respond when it is trying to carry the full 84Amps for 12 straight hours?