MCA vs MOCP vs FLA vs LRA 4

[julesm]

It's been a while so I'd appreciate some help. What is the difference between and the significance of MCA (minimum current ampacity) MOCP (maximum overcurrent protection) FLA (full load amps) and LRA (locked rotor amps)?

I've got a chiller with MCA, MOCP, and RLA
and a ac unit eith FLA and MOCP.

I need to size an MCC with about 10 loads, and of course, provide conductors and breakers.

help! Thanks -
JM

 

[peebee]

MCA (minimum circuit ampacity) = WSA (wire sizing amps) = your wire ampacity must be this big or larger. You may need a larger size (for voltage drop or energy savings or whatever) but you cannot use a smaller size.

FLA (full load amps) = RLA (running load amps) = use this for load calcs. If you don't have MCA/WSA data, then this will get you in the ballpark for feeder size (but may ultimately be wrong).

MOCP (maximum overcurrent protetive device) = maximum breaker or fuse rating. You may see some equipment with a "maximum fuse rating" -- in that case, you MUST use a fuse (it can ALSO have a breaker in the circuit, but a breaker alone won't cut the mustard). The MOCP rating will often be larger than the MCA -- that might seem strange to you, that the fuse/breaker is bigger than the wire, but that is OK and 100% per code. You can size the wire bigger if you want to or that makes you feel better, but you are not required to per code or UL.

LRA (locked rotor amps) is what a motor will draw with a locked rotor, which will very nearly equal what it will draw during starting. This will usually be about 700% of full load amps and will last for about 6 seconds. You use this to calculate motor starting voltage drop. Code only requires you to calculate motor starting voltage drop for fire pumps and very few other specific motor types -- but practially, if your voltage drops below 85% or 80% of nominal voltage, your motor may refuse to start. For short runs (say, less than 300 feet at 480v), motor starting probably won't be an issue for you.

Your MCC would generally be sized based on FLA. 125% of largest motor FLA plus 100% of all other smaller motor FLA would be a good minimum MCC size (and would be in accordance with NEC 430 motor feeder sizing requirments).

See NEC 430 for motor & motor feeder requirements general to all motors.

See NEC 440 for motor requirements specific to HVAC motors. The MCA/MOCP questions above are specific to HVAC motors and are covered in NEC 440.

 

[itsmoked]

peebee... An excellent synopsis! Thank you.

A star for the most useful post of the day.(For me)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[peebee]

PS -- Your chiller vendor has given you complete data, but your AC unit vendor has not given you complete information, he still owes you MCA data. You could take a good guess at conductor size based on the FLA info, but you could not be sure until you got MCA from him too.

Itsmoked -- thanks!

 

[davidbeach]

Sizing wire at 125% of FLA will always get you a number at least as large as MCA, often two or more sizes bigger. You won't get in trouble code wise using FLA for wire sizing but you might get in trouble budget wise for doing so.

 

[julesm]

Thanks peebee! After reading your post 430 and 440 were a breeze.
While you are on a roll...
There is also a condenser made up of three 1 hp fans, 3.3 amps each (480V) 4.53kW MCA=10.74 MOCP=15

This makes sense except isn't 1 hp 1.8 amps??
I am happy to use my manufacturers recommendations but I want to understand it, too. I did go to engineering school after all..
thanks again

 

[itsmoked]

I am curious too as yesterday the 2HP 460V motor I was looking at wasn't even 3.3A I don't believe.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

ROT for estimating motor FLA from HP if not shown (i.e. not arrived yet), it's a non-linear guesstimate. For 460V, make a curved graph extrapolated from 3 points with 1HP = 2A, 10HP = 14A and 100HP = 124A. Above 500HP just use 1.2A/HP. For 230V, double everything, for 208V, add 12% to the 208V numbers. Close enough for estimating until you get to see the nameplates.

Or just look it up in the NEC! Those charts are pretty accurate in my experience.

 

[LionelHutz]

15A is the smallest power breaker size available in North America so using 15A on any small motor is allowed by code.

The RLA is the maximum operating current based on the load the chiller puts on the motor which is why you are given RLA and MCA. The manufacturer will use the same chiller frame size for multiple chiller capacities so the same frame size will have multiple RLA ratings. The motor FLA is not given but it will be higher and since the motor starter circuit needs to be sized to the motor they give you MCA to tell you how to size it.

I suspect the manufacturer holds back FLA so you don't know the potential maximum capacity of that chiller frame size.

 

[peebee]

A 15A breaker is the smallest breaker recognized by NEC 240.6 as noted by LionelHutz. However, there are smaller fuses, so if you are using fuses (and fuses are required for some equipment), then a 15A fuse may be too large in some applications. See 240.6 for additional smaller fuse sizes down to 1 amp.

As mentioned by LionelHutz, FLA is the rating of the motors in the equipment, where MCA is based on the mechanical load applied to those motors, so FLA will be larger than MCA (assuming they didn't design their motors to run in an overload condition -- not that I'd put that past them....). So equipment containing a 1.8A motor will likely have a somewhat smaller MCA. The overall equipment ratings (circuiting requirements) are determined by UL rules, not NEC, so they can size the feeders based on actual equipment load rather than general motor rating.

 

[davidbeach]

MCA is not (directly) load based. MCA is 125% of the RLA (why they can't use FLA like the rest of the industry is beyond me) of the largest motor plus 100% of all other motors. If you just take the system FLA you will size your conductors at 125% of the whole FLA.

Fan motor RLA and FLA are the same values for a given HP of fan motor. Compressors are a different beast, not built to HP sizes, so the RLA can be application dependent.

 

[peebee]

davidbeach -- are you sure that different fan designs using different fan blades (CFM) on the exact same motor would result in the exact same FLA & MCA? I was under the strong impression that MCA depended on mechanical load, whereas FLA was based on motor size. In contrast, you are stating that mechanical load has zero impact on MCA.

 

[davidbeach]

When I was dealing with equipment with such ratings, it certainly appeared that fans had current ratings based on the HP of the fan motor. In a line of air cooled chillers, for example, the fan RLA would be constant across a range of chiller sizes (number of fans wouldn't be) but the compressor RLA would step up with capacity of the chiller. Since hermetic compressors aren't given HP ratings there is no way of comparing FLA and RLA.

But, it has been a while since I had to circuit up such equipment, and hopefully never again, and things may have changed in the mean time.

 

[dpc]

My memory is the same as David's. I could generally compute the MCA if I knew the FLA data of the motors.

FLA is generally a term used with a single motor. The MCA and RLA terminology is used mainly for package HVAC equipment.