Feeder for Multiple Motor Circuit

[tlona1]

I am running a single feeder that will power a air handler (AHU) and and Return Air Fan. (RAF) The AHU nameplate states a FLA of 42A but limits the MOCP device to 50A. The RAF has a FLA of 3.4A.

NEC 430.62 states the protection device for the multiple motor feeder shall not be greater than largest protection device and the sum of all other motors so in this case, the feeder protection device must be 50A (50+3.4).

If it is sized at 50A and my total FLA is 45.4 (42+3.4), won't I overheat my 50A breaker and get false tripping? It seems to me the AHU MOCP device should be greater that 50A since the 42 FLA is comprised of 4 compressors and 2 fans, but I cannot exceed the manufactures specs.

 

[poeboy]

I believe if your going to run one circuit for multiple motors, then you are also required to have a FUSED disconnect in front of each. Fuse the d/c with appropriate OCP and your fine. Local AHJ has the final say in it.

for only 2 motors, I can't see an advantage for the additional costs of the time, JB's, fused d/c's and fuses vs an extra set of wires in the conduit or another cable. Everything is always circumstantial though.....

 

[tlona1]

yea, I am fusing the AHU at 50A and fusing the RAF at 6A. The main with have a 50A CB. I am concerned that I will get tripping because my FLA of the two is 45.4 and my main is only 50A.

 

[poeboy]

Which code cycle are you using in your area??

It would take me forever to cite codes, but I can tell you that YES, 50A is way too small for BOTH of the loads being served. 80% of the OCP is the rule of thumb.

I was taught that in multiple motors using one feeder, you take the largest motor X 125%, then add in the rest. In your case this would mean (42*1.25 = 52.5. Then add in the 3.4 = 55.9. Since 55.9 is not a standard size then 60 is the next size up and is OK. There are even exceptions for start up that will allow a bigger OCP than 60 (70 might be a good place to start. ).

After your feeder is sized, then use appropriate fusing at the d/c for each load served as well as wiring sized for the fusing. That has been acceptable from all local authorities wherever I have worked.

 

[tlona1]

yes agree with your calculation and that is exactly how I would do it. The predicament I am running in to is that NEC2005, section 430.62 states that the feeder OCP can not be greater than the largest motor OCP plus the sum of the remaining motor currents (3.4A in this case). The AHU manufacturer limits the AHU OCP to 50A max. So based on this and the code, my feeder OCP cannot be greater 53.4A or 50A.

 

[Dennis Alwon]

The AHU nameplate states a FLA of 42A but limits the MOCP device to 50A. The RAF has a FLA of 3.4A.

NEC 430.62 states the protection device for the multiple motor feeder shall not be greater than largest protection device and the sum of all other motors so in this case, the feeder protection device must be 50A (50+3.4).

If it is sized at 50A and my total FLA is 45.4 (42+3.4), won't I overheat my 50A breaker and get false tripping? It seems to me the AHU MOCP device should be greater that 50A since the 42 FLA is comprised of 4 compressors and 2 fans, but I cannot exceed the manufactures specs.

Would it not be easier to run a feeder based of 60 amps and then set a 4 circuit panel. Feed the AHU with a 50 amp circuit and the RAF with a 15 amp circuit. I would bet, but I would check, that the RAF has a built in oveerload protection so 15 amps should be fine.

 

[poeboy]

The AHU manufacturer limits the AHU OCP to 50A max. So based on this and the code, my feeder OCP cannot be greater 53.4A or 50A.

The 50 A fuses in your d/c suffice for the manufacturer's spec. I can't see in that part of the code where it says the MANUFACTURER limits the size of your feeder, it says the CALCULATION limits the size of your feeder (IMHO), based on table 430.52

 

[tlona1]

thank-you for your help. Yes I guess I could run a feeder to a panel. Not a bad idea thank-you. Poeboy, I see your point, if I just do the straight calcs ignoring the manufactures limit of 50A, it works fin as you had previously suggested. In fact the manuf. 50A is in itself a multiple motor feeder. thank-you all for your help.

 

[kwired]

..In fact the manuf. 50A is in itself a multiple motor feeder. thank-you all for your help.

That may be what has had you mislead into thinking that the 50 amp OCPD was too small in the first place. Take the load of the largest compressor x 1.25 and add 100% of the remaining motors to it and it will be more clear where the minimum circuit ampacity comes from