Derating

[MistaObvious]

Hey, gang! First I have to say that this site is an awesome resource and want to thank everyone involved for keeping it up.

I'm a 3rd week student at Porter and Chester in Western MA. I'm lucky enough to have started in their third term, which means I get to start with load calc's, raceway and box fill and finally (next week or week after) motor controls.

A question came up yesterday in regards to derating for continuous loads and I had a slight issue with the conclusion. We were trying to size a conductor for an AC unit and then size the breaker. The calculation the text book used was called into question when it sized the conductor at 125% of the load and then sized the breaker based on the conductor size. The conclusion in class was that because the max load to a breaker for continuous loads is %80, then the product of the load for the conductor must then be multiplied again by 125% for the breaker.

So for a 15A load, we came up with #10AWG. But then multiplied the result again by 125% (15x125%=18.75 (conductor), 18.75x125%=23.4375 (breaker)). The problem I have is if you've already done the work for coninuous load for the conductor, isn't it safe to assume the work's been done for the breaker? In my example, it still works out to be the same (25A breaker), but for larger loads it would work out that a larger breaker would be required than is necessary.

It also should be noted that in another example calculating a similar situation, the original load was used to calculate the breaker, seperate from the conductor's calculation (conductor was 125% for continuous load, whereas the breaker was 175% of the load).

I'm actually on my way out to class after I submit this, so I'm going to discuss this with my teacher. But I figured I'd ask a bunch of experts their opinion for the sake of knowing this come time for my exam. It also seems that a lot of workbook questions are based on similar questions and I'd hate to get this wrong each time because I'm overdoing my math. I do, however, understand that not much of this will be applied in my career, but since I need to know this for my Journeyman's test....

TIA,

Liam

 

[don_resqcapt19]

Actually for most AC units you don't do any calculations. You size the wire and OCPD based on the nameplate information. The nameplate should have a number for the minimum circuit ampacity and the maximum size of the OCPD. You use a conductor that has an ampacity equal to or greater than the minimum circuit ampacity and you use an OCPD that is equal to or smaller than the maximum permitted size.
Don

 

[infinity]

I agree with Don. Forget about a calculation. Article 440 requires you to use the nameplate data supplied by the manufacturer to size the OCPD and the conductors.

 

[Jim W in Tampa]

Name plate tells it all.Min.circuit amps,max breaker/fuse size and min.breaker/fuse size.Best to use the max on the breaker and unless voltage drop is an issue just size the wire to atleast be large enough for the min supply amps.The real trick is knowing what the unit will say when it arrives.Usually its too late to up size.With the new 13 seer the wire size should be getting smaller.

 

[hardworkingstiff]

The problem I have is if you've already done the work for coninuous load for the conductor, isn't it safe to assume the work's been done for the breaker?

Yes.

So a 15-amp load multiplied by 125% = 18.75-amp rating for the conductor feeding the 15-amp continuous load. #12 wire with a 20-amp breaker.

 

[hmspe]

So a 15-amp load multiplied by 125% = 18.75-amp rating for the conductor feeding the 15-amp continuous load. #12 wire with a 20-amp breaker.

Not necessarily. 125% of the largest motor plus the sum of the other motors handles wire size. We don't have the break-out of the motors so we can't do a full calc. Let's assume a split system with a 14A compressor and a 1A outdoor fan. That would be (14A X 125%) + (1A X 100%) = 18.5A. #14 copper would meet the requirement. For the breaker or fuse it's 175% of the largest motor + the sum of the remaining motors [unless the unit won't start, in which case you can go to 225% of the largest motor + the sum of the remaining motors]. (14A X 175%) + (1A X 100%) = 25.5A. Use the next smaller breaker size, 25A.

That said, I fully agree with the other posters who said to follow the nameplate. I've seen several A/C brands which require the 225% of largest motor in the breaker calc, and some that have one ampacity for fuses and a different ampacity for breakers.

FWIW, my office standard is to size the breaker, then match the wire size to the breaker. This is partly because it's hot in Arizona, so we should be doing a temperature derate on the wiring, and partly because some jurisdictions in the area only allow reduced conductor size from the last fuse or breaker to the unit (we always call out fused disconnects at the unit). It also provides some cushion in case the unit installed has higher loads than what the mechanical engineer specified.

Martin

 

[LarryFine]

It also provides some cushion in case the unit installed has higher loads than what the mechanical engineer specified.

. . . or in case the unit is someday replaced with one that has a higher current usage.

 

[MistaObvious]

Thanks a lot, guys. I should have thought it through a bit more what we were talking about, as the example given probably wasn't AC afterall. I honestly can't remember that detail (school = crammed cranium). Hard enough to remember the important stuff (fortunately it'll be over tomorrow). Anyway, the point was the calculation for a continuous load for both the wire and the breaker. Thanks for answering that for me. I figured logically speaking it didn't make sense to do two calculations for the breaker. We never did get around to discussing it yesterday, but the module review we did sorta confirmed my suspisions (reenforced by your answers).

So it could be safe the assume that it's best to calculate for the breaker, the size the wire for that (we're talking MA, so temp. derating isn't a common practice like Arizona ). One calculation, 2 birds... right? I appreciate the help.