Load Calculations

[6506gary]

I have always calculated loads using 80% of the equipment rating providing the wire is sized properly and recently I have been questioned about where that is in the code book. Well, I have read Article 220 until I am red in the face and can't find it, so I am either in the wrong section or I am off my rocker. Help!!!!!!!!!!!!!!

 

[Dennis Alwon]

It depends on the type of load that you are calculating. Most circuits can be loaded to 100% if they are not continuous loads. What specifically are you trying to calculate?

 

[6506gary]

Load Calculations

This is a basic panel load calculation.

 

[Dennis Alwon]

This is a basic panel load calculation.

Articles in 220 tells you how to calculate so if the calculated load is 198 amps then a 200 amp breaker will suffice. Now if this was 198 amps of continuous load then the breaker and equipment must be listed for 200 amps continuous or you would have to size the feeders and breaker to 250 amps.

 

[6506gary]

Load Calculations

Thanks for the help to everyone. It appears that Art 210.20(A) and Table 210.21(B)(2) has the answer I was looking for.

 

[facemre]

I don't know if I am mixing load calculations and box fill but doesnt ambient temp come into play sometimes? I know one chart allows you to multiply by a number greater that 1.0 when doing a calculation. sorry in advance if I have drifted to the wrong subject, I don't have my codebook in the house to check before writing this.

 

[skeshesh]

Elaborate a bit more please. Ambient temp would affect conductor ampacity. I guess temperature may affect the actual load in many ways, but that affect is not considered in load calculations, save some sort of special industrial application dealing with big numbers or special conditions. I dont know of any consideration the NEC or even good engineering practice would give a typical residential or commercial load save conductor derating as mentioned above.

 

[romex jockey101]

i thought generally even if its not a continous load the crk should be at 80 % ,,ex if your feeding a waterheater and the unit draws 18 amps instead of feeding it with 12 awg you would use 10 awg ..am i on the rit track

 

[jumper]

i thught generally even if its not a continous load the crk should be at 80 % ,,ex if your feeding a waterheater and the unit draws 18 amps instead of feeding it with 12 awg you would use 10 awg ..am i on the rit track

WH is a continuous load.

422.13 Storage-Type Water Heaters. A fixed storage type
water heater that has a capacity of 450 L (120 gal) or
less shall be considered a continuous load for the purposes
of sizing branch circuits.

 

[Dennis Alwon]

i thought generally even if its not a continous load the crk should be at 80 % ,,ex if your feeding a waterheater and the unit draws 18 amps instead of feeding it with 12 awg you would use 10 awg ..am i on the rit track

4500watts/240=18.75
18.75*1.25=23.43

So yes a #10 is needed however you could use a 25 amp breaker if you wanted.

 

[romex jockey101]

so when does the 80 % load come into play and were is it in the neci checked branchcircuits and calcs ill look again ...thank u for ure time

 

[jumper]

so when does the 80 % load come into play and were is it in the neci checked branchcircuits and calcs ill look again ...thank u for ure time

210.19(A)(1)

(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Where
a branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-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.