I have a transformer which is supplying power to a 480V MCC bus. The MCC bus is rated for 2000A and the LV breaker coming off of the transformer feeding the cables over to the MCC is a 2000A breaker. My question is, can the feeder cables be rated to the exact bus size of 2000A or do the feeders have to be rated at 2000A plus 125%. The MCC bus has both continuious and non-continuous loads. I was looking at section 215.2(A)(1) of the NEC and it seems like you would need to have the feeders rated at 125% of the bus. There is also an exception below this which refers to an overcurrent device but I'm not sure if it applies in my case. What would the cables have to be rated for in my case? 2000A exactly or more?
Feeders to an MCC Bus
- Thread starter mull982
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- Lockport, IL
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- Semi-Retired Electrical Engineer
The feeder need not be 125% of the bus rating. But it must have an ampacity at least as high as the load, with the load calculated as 100% of the non-continuous load plus 125% of the continuous load. If that value is higher than 2000 amps, then you have undersized the bus. If that value is no higher than 2000 amps, then you can use a feeder with an ampacity of 2000 amps.
So it sounds like i would have to add up all my loads and figure out which ones were continuous and which ones were not and then calcualate my feeders bases on that. Is that the same steps that should be taken when calculating the bus size?
What if I add up all of the loads as above, and the number comes to less then 2000A? Can I use less than 2000A feeders, or do I have to size the feeders at at least 2000A because that is what the breaker is sized at?
Thanks
What if I add up all of the loads as above, and the number comes to less then 2000A? Can I use less than 2000A feeders, or do I have to size the feeders at at least 2000A because that is what the breaker is sized at?
Thanks
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
The sequence is,
(1) Calculate the load. Include 100% of non-continuous and 125% of continuous.
? For the sake of discussion, let us assume that the answer is 1900 amps.
(2) Select a bus that has a rating at least that high. The next available size is 2000 amps, so you can use a 2000 amp bus or a bus with a higher rating..
(3) Select a feeder that has an ampacity of at least 1900 amps. The feeder need not be good for the entire bus rating of 2000 amps.
(4) Select a breaker overcurrent setting that is no higher than the ampacity of the feeder. Since this is over 800 amps, we cannot use the ?next higher standard size breaker.?
So for this example, the feeder could be five sets of 500 MCM copper (i.e., 5 times 380 equals 1900). However, if you choose to use that feeder, then you must set the breaker overcurrent trip at 1900 amps or lower.
All that notwithstanding, I would make the following design choices.
(1) I would use a feeder that is good for the entire 2000 amp rating of the bus. I would likely consider using six sets of 400 MCM copper, for a total ampacity of 2010 amps.
(2) I would then set the overcurrent trip at 2000 amps.
(1) Calculate the load. Include 100% of non-continuous and 125% of continuous.
? For the sake of discussion, let us assume that the answer is 1900 amps.
(2) Select a bus that has a rating at least that high. The next available size is 2000 amps, so you can use a 2000 amp bus or a bus with a higher rating..
(3) Select a feeder that has an ampacity of at least 1900 amps. The feeder need not be good for the entire bus rating of 2000 amps.
(4) Select a breaker overcurrent setting that is no higher than the ampacity of the feeder. Since this is over 800 amps, we cannot use the ?next higher standard size breaker.?
So for this example, the feeder could be five sets of 500 MCM copper (i.e., 5 times 380 equals 1900). However, if you choose to use that feeder, then you must set the breaker overcurrent trip at 1900 amps or lower.
All that notwithstanding, I would make the following design choices.
(1) I would use a feeder that is good for the entire 2000 amp rating of the bus. I would likely consider using six sets of 400 MCM copper, for a total ampacity of 2010 amps.
(2) I would then set the overcurrent trip at 2000 amps.
Thanks for the info Charlie it really helps.
Suppose that the MCC consisted of all continuous motor loads. Would I take 125% of the total continuous load, or just take 125% of the largest motor? I thought I read somewhere that you take your total load plus 125% of the larges t motor in some applications where there are several motors. Is this a different situation that I am thinking of?
Thanks for all the help.
Suppose that the MCC consisted of all continuous motor loads. Would I take 125% of the total continuous load, or just take 125% of the largest motor? I thought I read somewhere that you take your total load plus 125% of the larges t motor in some applications where there are several motors. Is this a different situation that I am thinking of?
Thanks for all the help.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Just the largest motor. The notion of "continuous load," defined as one that runs for more than three hours at a time, does not apply to motors. They have separate rules. Check out 430.24.
I've run into exactly this question.
I need a panelboard that will feed nothing but continuously running motors.
215.2 says feeder conductor ampacity must equal 125% of the continuous load as calculated by III, IV and V in article 220.
Part III, 220.50 is the rule for motors and it just refers you to 430.
Why should motors be treated differrently from other loads in this regard? Because of the requirement for individual overcurrent protection for each motor downstream from the feeder?
I need a panelboard that will feed nothing but continuously running motors.
215.2 says feeder conductor ampacity must equal 125% of the continuous load as calculated by III, IV and V in article 220.
Part III, 220.50 is the rule for motors and it just refers you to 430.
Why should motors be treated differrently from other loads in this regard? Because of the requirement for individual overcurrent protection for each motor downstream from the feeder?
motors
motors
I am sure there are exceptions, but based on my years in industrial establishments, I'd say it would be rare where every motor was 100% loaded for 3 hours or more. Most are partially loaded a some point in a 3 hr period and pull less than FLA.
motors
I am sure there are exceptions, but based on my years in industrial establishments, I'd say it would be rare where every motor was 100% loaded for 3 hours or more. Most are partially loaded a some point in a 3 hr period and pull less than FLA.
Don't know what kind of facilities you hang in Augie, but most of the motors I deal with in industrial facilities - pumps, fans, chillers and occasionally an air compressor - easily meet the three hour rule. In fact, many of he pumps and fans run continuously for months. Some for years, especially if you toss out single shift shutdowns here and there. It is the norm, not the exception.
However, I'm still not sold on exactly what the code is trying to say.
It starts out in 215 saying feeder conductors will have a minimum ampacity of 125% of the continuous load and 100% of the non-continuous load. It refers you to parts III, IV and V of 220 for the load calculation and 220 sends you to 430 for motor load calculations.
I don't see that any of that releases you from the conductor ampacity rule in 215. In other words, even though you calculate the motor LOAD as 125% of the biggest plus 100% of the rest, the result of that calculation is "load" not conductor size. So that number becomes a part of your total load and, if its continuous, you have to add 25% to it when selecting the conductors.
However, I'm still not sold on exactly what the code is trying to say.
It starts out in 215 saying feeder conductors will have a minimum ampacity of 125% of the continuous load and 100% of the non-continuous load. It refers you to parts III, IV and V of 220 for the load calculation and 220 sends you to 430 for motor load calculations.
I don't see that any of that releases you from the conductor ampacity rule in 215. In other words, even though you calculate the motor LOAD as 125% of the biggest plus 100% of the rest, the result of that calculation is "load" not conductor size. So that number becomes a part of your total load and, if its continuous, you have to add 25% to it when selecting the conductors.
eric stromberg
Senior Member
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- Texas
no three hour rule for motors
no three hour rule for motors
As Charlie stated, there really is no three hour rule when it comes to motors.
Motors are all considered continuous, unless they must stop in performance of their duty. For example, an elevator motor must stop when it comes to a floor and so, therefore, it is an intermittant duty motor. All other motors are considered continuous. At least, as far as the calculations for the conductors going to them.
no three hour rule for motors
As Charlie stated, there really is no three hour rule when it comes to motors.
Motors are all considered continuous, unless they must stop in performance of their duty. For example, an elevator motor must stop when it comes to a floor and so, therefore, it is an intermittant duty motor. All other motors are considered continuous. At least, as far as the calculations for the conductors going to them.
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