NEC 2008 Articles 220.51 & 424.3(B) Electric space heating load calculations

[DW98]

A coworker and I were discussing how to calculate feeder size for fixed electric space heating. There is some confusion regarding the two code articles mentioned in the post title above. 424.3(B) says to consider fixed electric space heating continuous load. 220.51 says fixed space heating shall be calculated at 100% of connected load. A similar situation exists for "all others" in the lighting load demand factors. I've always included a 1.25 factor in my feeder calculations for lighting and since NEC 2005 electric heating. Is there a conflict between the two articles or am I misunderstanding 220.51. Should I just size the branch circuits for continuous load or the feeder also?

For example: a branch circuit for 10 kw 208V-3PH wall heater would be 10kw * 1.25 /360 = 35A. for ten of these heaters, should feeder be 100kw * 1.25 / 360 = 350A or just 100kw / 360 = 278A. I've been using the 1.25 factor.

Also, article 424 says it doesn't apply to room air conditioning. I would think only PTAC would fit this description.

I tend to overthink things and get myself confused. Thanks in advance for your replies.

 

[jwelectric]

220.51 is addressing feeders and 424.3 is addressing branch circuits.

If we look at 220.82(C) we see a 65% if the heat is part of a heat pump system and 40% if it is baseboard heat for dwelling service calculations.

It all depends on what is being calculated and the amount the unit is being used.

 

[DW98]

I think I understand this more clearly now. I googled feeder calcs for fixed heating and found earlier threads on this forum (I should start there first next time). My understanding is that you have to include 100% of the connected heat load by 220.51 and article 215 then tells you to apply 125% to this continuous load for feeder sizing.

 

[suemarkp]

As Mike said, it depends on some factors:

Is it dwelling unit heat or not?
Is it one central furnace, or multiple "central" units, or separate wall/baseboard heaters?
If a central type unit, are there multiple stages of heat (and if so how many)?

There are specific factors you can apply if it is residential (which allow the feeder to be smaller). If you have multiple units, I think you could make a good case that the feeder won't be loaded continuously. Same with the multiple stages, especially if the furnace is a bit oversized or at least properly sized.