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#175141 02/23/08 11:46 AM
Joined: Mar 2003
Posts: 849
Code says take Highest over current device ( say on 50 hp. 65 amp Times Largest allowable fuse size 300% plus full load amps of other mts. ) to get feeder Overcurrent Device size. Example
50 Hp = 65 a,30 hp= 40a, 10 hp=14a 10hp.= 14a
250 A overcureent Device.Max (there are Exceptions I know)
But I'm wondering how many use the 300% max and how many use less say 150% to 175% x largest OC device???
65x150%=97.5+40+14+14=165.5 amps then round up to 200A.
H?????and if so have you had any problems??

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Joined: Apr 2003
Posts: 139
As you are indicating, this is a design consideration which will usually be very case specific.

In your example, it appears you are refering to nontime delay fuses. In this case, it is likely higher ratings will be need due to the short time/trip curves provide by this type of overcurrent devices. Perhaps another type is more suitable such as your standard inverse time circuit breaker.

The bottom line is since the code gives maximum allowances, it gives designers plenty of flexibility when considering proper protection.

In your example, a 200A device and 250A device will have nearly identical trip times for short-circuits and ground-faults. However, the possibility of nusiance tripping at 200A is greater.

Bryan P. Holland, ECO.
Secretary - IAEI Florida Chapter
Joined: Mar 2003
Posts: 849
But with the Mtr overloads set at 125% Per per 430.32 Max.
seems a bit over kill.
Also if you go to inverse time breakers then its 250%x65=165.5+40+14+14=233.5 then 200 Right.
So Yer saying Fuses the Feeder Should be Larger Breakers can be smaller.

Joined: Jul 2007
Posts: 1,335
the 125% comes in play in the future. It is a padding if you swap ouy the motor in the future there is a posssibility that the draw can be higher. It is more economical to install 1 circuit not the one now and replace it later. Motors die when you need them the most.

I can see if the cost for a larger breaker is very expensive. The breaker or fuses provide only short and fault protection. In theory, a 50 will trip just as quick as a 30 on a fault if wired properly. Granted you want to use as small as a breaker as practical for several reasons, larger the breaker less likey the nescience tripping.

I typically use the maximum percentage from 430.52 even with larger loads. I have used less pending on the type of loads and situation. I would not drive all the way across town for a 250 breaker if I had a 200 or 225 on hand just because the calcs says I should. The situation should tell you what will work and what will not. If I had to buy a breaker, I would but the largest allowed and be done with it. It is much more economicsl then going back to put a larger breaker in and explaining to the customer why their assembly line is shut down. He or she will not want to here you sam him $50 by using a smaller breaker. Like I said the situation and experience will help you determine if a smaller breaker will work or not. When in doubt, presume (never assume) worst case scenario and wire it to code.

Last edited by sparkyinak; 02/23/08 01:40 PM.

"Live Awesome!" - Kevin Carosa
Joined: Mar 2003
Posts: 849
125% Furture?? That Max Overloads can be set at. As Mtr wires are 125%.
I know most engineers do not use the 300% on fuses and 250 on breakers for the ist Mtr, when doing a MMC. They usually use 150 to 200% .
What about Mtrs with VFD's and soft starts???
hardly no inrush there .

Joined: Jul 2007
Posts: 1,335
I mis-read about the 125% my bad. Not having the best of a day. I was thinking about sizing the feeder conductors not the OCPD. The rest of my post is correct. As for table 430.52, you are not to exceed it. You can use what you thick the is sufficent. Yu want it big enough to prevent neuscience tripping but small enough not to cause sags and surges in the system. What you use is dependent of the loads you are powering.

I wired a system that has a I think it was a 2hp, and 2, 1/2 hp motors. It is fed with only a 30 amp breaker and runs fine. Max was 40 at 250% for using a breaker. One of the 1/2 hp was primer pump that would never run while the other two were running so I did not use it to size the breaker. Also the other two pumps start under no load so the inrush is minimal for motors of their size. I do not use a set percentage, I figure the max than let the conditions dictate reducing it from there if it was worth the time.

"Live Awesome!" - Kevin Carosa
Joined: Mar 2003
Posts: 849
Great to be able to bounce things off of other people here Thank You

Joined: Jan 2004
Posts: 1,507
Ernie— I don't see people using one-time fuses much even tho there is a considerable cost savings over duel element fuses. In most cases the cost of the increased size of the disconnect is not justified.

George Little

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