help me understand

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Good morning all,
I have a burning question that I seem to encounter a lot lately. My responcibilities in this facilty is to manage and coordinate installation of new equipment as well as the maintenance electrician. One thing I encounter often is machine builders will specify, for example they need a 100 amp service to their MCC. As it is with our next new piece of equipment. I have a EE get P.O.ed at me because I had requested information on how many motors would be on the feeder circuit I provided so I can size the Conductors correctly and protect them with a breaker. He has stated just provide me with a 100amp service and protect it with 125amp breaker and it will be fine. If this is the case I'm multipling the 100amp by 1.25 percent and call it good. What is the correct way to do this. Am I sizing this correctly form the MCC to my Buss? I have list a few parameters and will provide more to those that chose to reply. Thank you in advance for your time.

480V 3 phase
100amp service to MCC
GE series molded case circuit breaker 125amp plug
50ft. to MCC from buss
120 ambiant temp.
2 motor on circuit 50hp and a 5hp
considering #3 copper thhn with 8ga. ground

As always thank you for your time and patients


LHarrington
 

dinkelja

Member
This is what I got:
50 hp 3-ph @460V = 65A
5 hp 3-ph @460V = 7.6A
(65A * 1.25) + 7.6 A = 88.9 (90A Breaker)
#4 THHN = 95 A * .82 (derate to 120 deg ambient) = 78A XXXXX BAD
#3 Ampacity is 110A * .82 = 90A, is okay, although #3 is a bastard size
I would use #2 = 130 * .82 = 107A, and a #8G, in 1 1/4" EMT (although that is another bastard size) That is "100A service"
 

RRelec

Member
Location
Florida Keys
LHarrington said:
Could you please tell me how you determined this? Or where can I read this information?

Figure 430.1 in the NEC will guide you. Take a look. Motor Feeder(S) 430.24, Motor-Feeder Short-Circuit and Ground-Fault Protction, 430-62 (A), and so on. Don't overlook Table 250.122 for the ground. It's worth the time to check it out, then you can see for yourself how to put it together and be confident in your installation. We don't always get the answers we want, remember where we are in the food chain.

R
 

e57

Senior Member
LHarrington said:
He has stated just provide me with a 100amp service and protect it with 125amp breaker and it will be fine.

LHarrington

Huh? :-? Should that not be reversed? Say, 125 sevice protected @ 100... :rolleyes:
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Motors

Motors

I agree with the EE. 100 amp feeder, 125 amp breaker.
To me the (65 * 1.25 + 7.6 = 88.9) provides you with the feeder conductor size, not the short-circuit protection. #3 THHN @ 120 ambeient (110 amps * .82 = 90.2 meets the requirement).
The short circuit (breaker) could be 65 x 2.5 + 7.6 or as large as 170 amps.
If you size it at 125 or 150, the equipment ground would be a #6
 

S'mise

Senior Member
Location
Michigan
motors control ctr?

motors control ctr?

E57, Motors are a funny animal, you can have a ocpd's larger than the ampacity of the conductors feeding the motor. I don't know what the heck a MCC is (forgot how to find abreviations on here) but I assume both motors are separately fused in this equiotment. Heres the formula I used; 20hp fla= 65 amps * 175% (fuses)=113.75 next size up = 125A fuses just for that 20hp motor bc in your mcc. Feeder; would be the LARGEST OCPD plus the fla of the rest of the motors. 125A + 7.6=132.6, Next size LOWER is 125A so your power drop ocpd is 125A. Like Augie said the 3awg is fine and the egc needs to be 6awg per 250.122
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I disagree with everyone---LOL--- I want the nameplate rating and nothing else. To often have I come back after a wire was run to find out I either oversized or undersized my wire. No thanks--- give me the motor or the nameplate info
 

Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
LHarrington,


For future fast answers at the turn of a page, I'd recommend checking this out...Click here. What you get is something better than a code handbook for $120, for field related activity, at half the price. I have both books, but get a lot more bang for my dollar from Mike's Book. When it comes time for fast answers in dealing with engineers, etc., this book is the best desk reference in town.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
undersized ?

undersized ?

Dennis Alwon said:
I disagree with everyone---LOL--- I want the nameplate rating and nothing else. To often have I come back after a wire was run to find out I either oversized or undersized my wire. No thanks--- give me the motor or the nameplate info


When the NEC states you determine the wire size based on Tables 430.247-.250 how do you determine the wire is undersized? If it meets the requirements of National Electrical Code, I considered it acceptable, even if doesn't meet Dennis' code :)
 

e57

Senior Member
S'Mise, While I agree you could use #3 protected @ 125A via 240.4B, or 215.3.
Your example confuses me... "125A + 7.6=132.6, Next size LOWER is 125A " I don't understand that part.

And I'll tend to agree with Dennis on the Nameplate info. So far we know about 2 motors on a machine, but nothing else about it. There could be much more involved...
 

e57

Senior Member
Back to the OP's "120 ambiant temp" and not knowing the isulation and termination ratings he could need to be using #2 or larger for tempature correction....
 

S'mise

Senior Member
Location
Michigan
Feeder to several motors

Feeder to several motors

I agree. In most cases your term's are rated at 75C but yes, ampacity could vary depending on the type of conductors. I usually see THHN wire used.

430.62(A) (rating or setting-motor load); "A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch circuit short-circuit and ground fault protective device for any motor supplied by the feeder[based on the maximun permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A)..], plus the sum of the full load currents of the other motors of the group." So this means the feeder to several motors can be based on the largest OCPD plus the sum of the rest of the motors flc. Since it is the maximum amount alowed you pick the next lower OCPD if it is not a standard size.
 

dahualin

Senior Member
There are two currents that must be found before designing. The first one is FLA from Table 430.248 and Table 430.250. This FLA is used to size circuit conductor and OCPD. The second one is FLA on the nameplate. This FLA is used to size the overloads (OL's) to protect the motor windings and conductors.

1.The FLA on Table 430.250 : 50hp@460v=65A; 5hp@460v=7.6A
2.The FLA on nameplate: ?

Use first FLA to size circuit conductors and OCPD
Minimum conductor amp = 65x1.25+7.6=88.85A. We assume the temperature rating of terminal is 75 degree so that the minimum conductor will be #3AWG.

OCPD amp = 65x2.5+7.6=170.1A this is maximum over current protection device rating so that the maximum OCPD will be 150A. This OCPD only provide short circuit and ground fault protection. It will not provide over load protection.

Use second FLA to size overload protection device with factor 115% for motors other than those with a marked service factor 1.15 or greater or termperature rise 40 degree or less.
 

S'mise

Senior Member
Location
Michigan
Feeder?

Feeder?

Yes, David I follow your numbers but you are using a single CB to protect both motors? First off, no one has answered my question as to what a MCC is, but I am looking at this like a power drop to a machine. On this machine, each motor has its own fuse, motor starter and overload set. (I still see Duel element fuses used most of the time). So to figure the feeder OCPD, I calculated the branch fuses for the bigest motor at 125A and then added the remaining fla. Largest OCPD= 125+7.6=132.6 Next lower= 125A feeder OCPD. Or, the feeder OCPD would be 175A if the 20HP was using an IT breaker. Please correct me if I'm looking at this wrong. I see three groups of OCPD's here, one for the power drop, and one for each motor.
 
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dahualin

Senior Member
John,

MCC is Motor Control Center that is used for control of a lot of motors such as pumps, exhaust fans etc. In your situation, each machine has its own fuse (OCPD), starter and overload protection, you don't need any MCC. There are two installations you can do. The first one is what I mentioned in my previouse reply that is one feeder from a electrical panel with 150A (or 125A) breaker (OCPD) in the panel. Tap two feeders to feed the two motors. The second method is to install two feeders and two breakers in a electrical panel, size conductor and breaker using FLA from Table.
 

S'mise

Senior Member
Location
Michigan
430.62

430.62

David thank you for the explanation, but I am still a little confused. I was looking in partV:Motor Feeder sc and gf protection."430.62(A)Specific motor loads" when I should have been in partVIII:Motor control centers. "430.94 OCP" It just says OCP has to be in accordance with article 240. But I was unable to find anything on motor feeders there. (other than taps)
Yes, I see 430.24 for conductors but where is OCP on feeders?

I guess I'm confusing when I should apply which article;Several motors, Multi-motor,specific fixed load, MCC....? I been in the code book for over an hour now and I still come back to 430.62
 

S'mise

Senior Member
Location
Michigan
430.62=Feeder

430.62=Feeder

dahualin said:
Article 430.52, Table 430.52, Article 430.53(c), Article 430.62.

Well NO, 430.52 and 430.53 is for BRANCH circuits, NOT Feeders. Yes,430.62 is for Feeders, and I believe 430.62(A) would apply.
Can you elaborate?
 
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