Continuous Loads and Load Calculations

[steve66]

215.2(A)(1) is confusing the heck out of me:

"The minimum feeder conductor size, before application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125% of the continuous load."

This only applies to the feeder conductors, not the circuit breaker. Is there a similar requirement that the circuit breaker has to be sized for 125% of the continuous load? I thought there was, but I cant find it in 240.

Steve

 

[raider1]

Steve, take a look at 215.3.

Chris

 

[steve66]

Steve, take a look at 215.3.

Chris

Thanks Raider. I knew that was in there somewhere.

Now, on to the confusion part. I'm doing a load calculation on a small substation to see if some load can be added. There are some lighting circuits, some receptacle circuits, some window shakers, and a large AC unit with a condenser, supply fan and return fan.

I would appreciate opinions on the following:

1. The large condenser has a 200 A MCA rating. Do I add another 25% to this number???

My understanding is that the MCA has the total load with an additional 25% of the largest motor or compressor. But that wouldn't be as much as adding 25% to the entire condenser full load current.

2. Do I add 25% to the full load amps of the supply and return fans?

2. If I have 20 window shakers, do I add 25% to every one?

Thanks in advance.

 

[raider1]

1. The large condenser has a 200 A MCA rating. Do I add another 25% to this number???

If the manufacture has marked a minimum circuit ampacity then you don't need to add 25% to this number.

2. Do I add 25% to the full load amps of the supply and return fans?

I would say that they most likely would be continuous and would need to be figured at 125%.

2. If I have 20 window shakers, do I add 25% to every one?

Pardon my ignorants but what is a window shaker?

Chris

 

[Dennis Alwon]

Pardon my ignorants but what is a window shaker?

Chris

I assume they are window a/c units.:grin:

 

[Dennis Alwon]

Art. 440 Part Vii talks of room a/c units but heck I think that the nameplate plus 125% of the largest motor would work for the feeder but I am having a heck of a time finding that info.

 

[raider1]

I assume they are window a/c units.:grin:

OK cool, never heard of a window A/C unit refered to as a "window shaker".

Chris

 

[bob]

Now, on to the confusion part. I'm doing a load calculation on a small substation to see if some load can be added. There are some lighting circuits, some receptacle circuits, some window shakers, and a large AC unit with a condenser, supply fan and return fan.

What is the definition of "a small substation"?
You are not required to add the 25% for a transformer. What is the size of the transformer? May I assume this is a customer owned transformer?

 

[LarryFine]

Do I add 25% to the full load amps of the supply and return fans?

Two? In one air handler?

 

[steve66]

I think Iwire was the first one I saw use the term "window shaker", so I though most of the forum members would be familar with the term. Anyhow, I kind of like the term - it seems very descriptive

Bob:

It's a 208V, 225 KVA customer owned substation, but unfortunately, it feeds a 600 amp main breaker (which I would consider to be somewhat undersized.) If it had a 750 or 800A main, I wouldn't have to be quite so concerned about the 125% of continuous loads.

Steve

 

[steve66]

Two? In one air handler?

Yes, 1 supply & 1 return in a single AHU with an outdoor condenser.

Steve

 

[steve66]

Don't the following two paragraphs confict with each other? (From the 2005 NEC).

215.2(A)(1):

"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 conductor size, before application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125% of the continuous load."

220.40:

"The calculated load of a feeder or service shall not be less than the sum of the loads on the branch circuit supplied, as determined by Part II of this article, after any applicable demand factors permitted by Parts III or IV or required by part V have been applied."

One says add 25% of the continuous loads, the other does not??

Steve





Steve