Fwd: Large Motors

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George Stolz

George Stolz

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DPC said:
I am confused by the use of the standard slide rule values for three phase 480volt motors i.e. A 200 HP 3 phase 480Volt motor calls for a 350MCM conductor , ampacity 310 amps at 75 deg C.......the specified breaker is a 350 ..sometimes bumped to a 400Amp.......Many times these feeders are not sized to the breaker. The Breaker rating is higher than the Ampacity of the feeder cable...Can you help me in this?
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George Stolz said:
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Originally Posted by DPC
I am confused by the use of the standard slide rule values for three phase 480volt motors i.e. A 200 HP 3 phase 480Volt motor calls for a 350MCM conductor , ampacity 310 amps at 75 deg C.......the specified breaker is a 350 ..sometimes bumped to a 400Amp.......Many times these feeders are not sized to the breaker. The Breaker rating is higher than the Ampacity of the feeder cable...Can you help me in this?
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In a motor circuit, the breaker is there to provide short circuit protection, it is the OVERLOAD RELAY that is technically providing the long term over current protection for the conductors, and the OL relay rules are much tighter than the breaker sizing rules. So the allowed increase in breaker sizing is based on the fact that the instantaneous trip capability of the breaker will have to allow the motor starting current to accelerate the motor without nuisance tripping, but trip fast if there is an actual short circuit / grounded fault. This is specific to motor controller circuits, because it is assumed that there are not going to be any other (significant) loads down stream of that breaker.
 
Sahib said:
If the fuses are designed both for overload and short circuit protection (called dual element, time delay fuse) of a motor, then the size of motor protection device corresponds to the size of the the feeder cable to the motor. For more information see
http://www.cooperindustries.com/con...library/BUS_Ele_Tech_Lib_Fuses_Time_Delay.pdf
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The standard time delay fuse is not considered overcurrent protection for a motor - as it can be sized up to 175% motor full load current in most cases. If you can apply one at 125% of full load current and it doesn't blow during starting you could use it for overload protection though. The thing I don't like about that is many times it will not be replaced with the same fuse and then it compromises the original design selection.
 
kwired said:
The standard time delay fuse is not considered overcurrent protection for a motor - as it can be sized up to 175% motor full load current in most cases. If you can apply one at 125% of full load current and it doesn't blow during starting you could use it for overload protection though. The thing I don't like about that is many times it will not be replaced with the same fuse and then it compromises the original design selection.
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Yeah, the only people I ever see promoting that concept are, no surprise, the fuse manufacturers. I have seen it once or twice in cheap OEM equipment with small motors, but it rarely works out when someone tries to do that in the field. I doubt the numbers would ever work on something that big however.

By the way Sahib, who mentioned fuses? This was about breakers. Do you happen to work for Cooper?
 
Jraef said:
Yeah, the only people I ever see promoting that concept are, no surprise, the fuse manufacturers. I have seen it once or twice in cheap OEM equipment with small motors, but it rarely works out when someone tries to do that in the field. I doubt the numbers would ever work on something that big however.

By the way Sahib, who mentioned fuses? This was about breakers. Do you happen to work for Cooper?
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It is done around here all the time on irrigation fertilizer injector pumps. Usually it is a 1/2 hp 480 3 phase motor, with FLA of 1 amp. Technically needs to be a 1.25 amp fuse. Often though is done with 2 amp fuse - I kind of accept that to some degree though it is not my preference of how to do it, 2 amp is easier to find then 1.25 and cost less. But what I usually find is after some time has gone by the farmers have replaced the original fuses with a 6 amp fuse - which really means all you have is short circuit and ground fault protection. Then they ask why that motor burned up:slaphead:
 
kwired said:
... the farmers have replaced the original fuses with a 6 amp fuse - ...
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Or a piece of pipe, bolt, wire, whatever was on the truck that day...
But at least when they do that, they don't bother to ask why it burned up, they know. There was an AHJ in eastern Washington who for decades would reject any pump panel that used fuses instead of CBs. Several of the low-end pump panel people who had no CB version took him on, but lost. Others just saw it as an opportunity to displace those guys.
 
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Jraef said:
Or a piece of pipe, bolt, wire, whatever was on the truck that day...
But at least when they do that, they don't bother to ask why it burned up, they know.
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When I was still pretty green in this trade, I remember a story of one farmer getting tired of replacing a fuse in a disconnect for an irrigation service. It happened to be a corner grounded system and common practice then was to put a slug of some sort in fuseholder of the grounded phase. He decided to swap that slug into the position that kept blowing fuses:huh:

Said it wasn't too long and there was a fire going on up top of the pole (transformers were on this pole). He said he then switched it back and called POCO and told them there was a problem with his service:roll:
 
jraef: You replied to the OP's query in your first post. My post tried to supplement it. Iam not sure whether our code or other code such as British prohibits use of time delay fuse for both overload and short circuit protection of motor above a minimum HP.
 
I had a project where I got a separate RFI every week for each motor feeder - why didn't the breaker match the wire size? I felt like sending the contractor a bill for continuiing education. I am always surprised at how many people haven't absorbed this info.

There are 3 kinds of over-current protection; short-circuit, ground-fault and overload. Breakers that feed motor starters are usually for short-circuit and ground-fault protection only (see 430.52 and 53.) The wire is sized according to motor FLA (430.22 and 24) and is protected from overload by the heaters in the starter (see 430.31.)
 
JoeStillman said:
I had a project where I got a separate RFI every week for each motor feeder - why didn't the breaker match the wire size? I felt like sending the contractor a bill for continuiing education. I am always surprised at how many people haven't absorbed this info.

There are 3 kinds of over-current protection; short-circuit, ground-fault and overload. Breakers that feed motor starters are usually for short-circuit and ground-fault protection only (see 430.52 and 53.) The wire is sized according to motor FLA (430.22 and 24) and is protected from overload by the heaters in the starter (see 430.31.)
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I had that EXACT conversation with a consultant today. He undoubtedly made a lot more money on this project than I will, with far less work, yet I had to 'splen it to heem.* Then I got a call from the end user, had to do it all over again. Then I got a call from MY site manager, saying they wanted HIM to 'splen it to dem too, so he wanted me to 'splen it to heem first. Sheesh...

(obscure reference to I Love Lucy there).
 
This is going to really upset the OP, but his example of a 200 HP @480 volt motor can have a 600 amp inverse time breaker with 350 MCM conductors. The 350 amp breaker he mentioned won't likely start that motor.
 
JoeStillman said:
I had a project where I got a separate RFI every week for each motor feeder - why didn't the breaker match the wire size? I felt like sending the contractor a bill for continuiing education. I am always surprised at how many people haven't absorbed this info. )
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If you send them a bill early in the project they will learn faster:happyyes:
 
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