Load Calc

[Flapjack]

Main question:

I have a 3-phase 480/277V panel with 6 welding receptacles, 3 heaters, lights and a roll up door, as well as a breaker feeding an existing lube shop and a breaker feeding a single phase 120/240 panel for vent fans, emergency lighting and duplex receptacles.

I can apply the rating of the welders from NEC 630.11 and 125% to the lighting for the non-motor continuous loads.

NEC 424.3 says 125% for the heaters. Say the roll up door was not my largest motor and the largest motor was in the 50kW heaters. How would you apply the 125% of the largest motor in NEC 430.24(1)? Do you add 125% of just the heater motor and ignore the other heater motors because they are already accounted for in the heater load? In other words, 50,000*1.25*3 heaters = 187500. 187500+(1.25*heater motor ampacity per NEC tables).


Side question (probably a similar answer as my main question):

I have read in other threads that the largest motor is sometimes in the HVAC. The manufacturer calculates the MCA. But how would you apply the HVAC load to your feeder calculation if it involves your largest motor? Do you just take the MCA value and say that the 125% of the largest motor has been applied in the MCA calculation, or do you add the 125% of the largest motor to the MCA value, or something else?

 

[Dennis Alwon]

I don't believe you need to add 125% for the heaters when doing a load calculation that isn't for the branch circuit.

 

[Flapjack]

I don't believe you need to add 125% for the heaters when doing a load calculation that isn't for the branch circuit.

At the bottom of 424.3(B) it says, "The requirement in 424.3(B) eliminates any question about whether the heating equipment is a continuous load, as defined in Article 100. This requirement also impacts feeders and services that supply fixed electric space-heating equipment branch circuits."

 

[Dennis Alwon]

I see nothing in 424.3(B) that states it applies to feeders and services calc. What do you mean the bottom of 424.3(b)..I agree they are continuous loads and when you run a branch circuit then the conductor must be sized 125%

 

[Flapjack]

I am looking in the blue box underneath 424.3(B) in NEC 2011.

 

[Dennis Alwon]

I am looking in the blue box underneath 424.3(B) in NEC 2011.

Are you looking in the handbook?

 

[Dennis Alwon]

I am not sure I agree with the authors of the handbook on this and that is just their opinion. I do not see substantiation for that but I could be wrong

 

[Flapjack]

Yeah, I am looking in the NEC 2011 Handbook.

Just say you were to apply the "continuous" 125% factor to the feeder. How would you treat the largest motor factor?

 

[Dennis Alwon]

Yeah, I am looking in the NEC 2011 Handbook.

Just say you were to apply the "continuous" 125% factor to the feeder. How would you treat the largest motor factor?

If the heater is larger than I see no reason to dd 125% to the motor also