Feeder calculations for fixed electric space heating.

[djake11]

I?m confused about fixed electric space heating load calculations for feeders and services. The requirements of NEC Articles 424.3(B), 215.2, and 215.3 indicate that the load should be calculated at 125% for branch circuits, feeders, and services calculations.

Article 424.3(B) requires that fixed electric space heating be considered as a continuous load.

Article 215.2 states that feeders shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

Article 215.3 states that the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

However, Article 220.51 states that fixed electric space-heating loads shall be calculated at 100 percent of the total connected load. I understand that ?total connected load? refers to the 100% calculation; e.g., total connected load for a 10 Kw heater at 240 v would be 10,000/240=41.7 amps.

Should feeders be calculated at 100% or 125%? I?ve seen arguments both ways; where am I confused?

 

[augie47]

Welcome to the Forum.
I stand to be corrected but I believe you will find the "secret" is in the wording. 424.3 addresses the branch circuit for fixed electric heating and states that for sizing the BRANCH CIRCUIT the load shall be considered continuous..
In making service calculation per Art 220.51, the requirement is at 100 %.
(note the feeder can't be smaller than the branch circuit)

 

[Hv&Lv]

I see 220.51 as stating that fixed electric space heating cannot be derated for any reason for calculations, whereas a few stoves or dryers can.
The feeders will have to be 125%

 

[Dennis Alwon]

I see 220.51 as stating that fixed electric space heating cannot be derated for any reason for calculations, whereas a few stoves or dryers can.
The feeders will have to be 125%

What about the exception? Also 220.82(C)(5)

 

[Hv&Lv]

What about the exception?

I have a hard time believing an AHJ will give permission, but stranger things have happened....
I have never counted on that exception, but you are right, it is there.
For 220.28 (C)(5), was he using the optional method?

 

[djake11]

I know this is not a code requirement, but an explanatory note is in the NEC 2008 Handbook with 424.3; it states "Note that this revision to consider fixed electric space heating as a continuous load also impacts feeders and services...". This sure seems (to me) to conflict with 220.51. I think I'm still confused.

 

[Dennis Alwon]

It would probably be better to say that it can affect the feeder or service load. I don't agree that it always does. If the feeder serves one branch circuit of heaters then that feeder cannot be smaller than the branch circuit and thus is figured at 125%. However, if your feeder is feeder many heater branch circuits then I don't agree. I also think 220.82 comes in also.

 

[augie47]

It would probably be better to say that it can affect the feeder or service load. I don't agree that it always does. If the feeder serves one branch circuit of heaters then that feeder cannot be smaller than the branch circuit and thus is figured at 125%. However, if your feeder is feeder many heater branch circuits then I don't agree. I also think 220.82 comes in also.

That's the way I see it. 424.3 addresses branch circuits, 220 Part III and 220.51 address feeders
and 220.51 and 220.82 services. The 125% is only addressed in the 424 branch circuit section.

 

[djake11]

OK, I think everyone is saying that defining a heating load to be continuous only applies to branch circuits, regardless of what the note in the handbook says. I'll go with that. Thanks very much for your comments.

 

[brother]

OK, I think everyone is saying that defining a heating load to be continuous only applies to branch circuits, regardless of what the note in the handbook says. I'll go with that. Thanks very much for your comments.

I remember having an interesting 'debate' with someone here about this calculation issue. Check out this article it should explain it more for you. And yes you are right, 125% is for branch circuits only when calculating heaters. Feeders are only required to be calculated at a 100% for heaters.


http://www.ecmag.com/?fa=article&articleID=7781

 

[kwired]

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

125% of a continuous load is supposed to be included in sizing a feeder, or any conductor as a general rule.

 

[augie47]

125% of a continuous load is supposed to be included in sizing a feeder, or any conductor as a general rule.

I don't read 220.51 that way and, when you look at the Appendix D Examples, I only see them using 100%

 

[Dennis Alwon]

I see a feeder for a motor as needing 125% of the largest plus the sum of the others but I don't see that for heaters.

 

[augie47]

I see a feeder for a motor as needing 125% of the largest plus the sum of the others but I don't see that for heaters.

Probably due to motors having an "inrush" and heaters not having one.

 

[Smart $]

Probably due to motors having an "inrush" and heaters not having one.

What about forced air heating? The blowers are motor driven.

 

[djake11]

Very clear-cut and interesting article; thanks!

 

[kwired]

Probably due to motors having an "inrush" and heaters not having one.

I always thought it was because of continuous load purposes. We are sizing conductors here not overcurrent devices.

What about forced air heating? The blowers are motor driven.

125% of the largest motor full load current is typically included in nameplate ratings on HVAC equipment. Nameplate has done all calculations for you and you install what it says.