wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
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Taps
- Thread starter Djelite
- Start date
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
430.31 Overload Protection & 430.51 Ground Fault Protection - the sizing of ground fault device can sometimes be based on 430.52(C)(3), and up to 1700% for motor full load current for design b energy efficient motors (based on the exception). You wouldn't need to install 1700% of FLA size wire. You would just need to size the wire based on Part 2 of 430.Where does it say that?
P.S. I dislike the definitions in art.100 on overload vs overcurrent. The NEC tends to lump ground fault and overload into the same device and call it overcurrent. The idea being, any device suitable to protect the conductors from overload (excessive current leading to overheating) should also be acceptable for available ground fault at the same device.
Option 3 exists so that you can install the motor feeder tap. If you couldn't meet options 2 or 1 then you wouldn't be able to install a feeder tap on a motor feeder circuit. It also means that whatever size wire was installed on the motor feeder and it's appropriate ground fault protection, will stay suitable for that wire size until you terminate in your respective tap's overload device which will be sized based on the motor it feeds.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Consider this example:
We have three motors of 20A FLC. One feeder supplies them all and requires an ampacity of 65A (125% * 20 + 2 * 20), in which case we may have OCPD up to 90A (250% * 20 + 3*20). [If we exceed 90A OCPD (other than due to 430.52(C) Exception 2, which might get us to 120A), we need to provide an ampacity that complies with the OCPD size as normal for non-motor circuits.] So let's say we use 65A conductors protected at 90A.
Further, say one motor is physically distant from the others, and it would be convenient to split its supply from the feeder and run it 30 ft to the branch circuit OCPD for that motor. 430.62(A) tells us that after the split, our single motor supply must have an ampacity of at least 25A, in which case the OCPD can be up to 50A. But as the upstream 90A OCPD is larger than 50A, we are by default back to needing to use a conductor of size 90A, unless we can use one of the tap rules in 240.21 or 430.28. As we are over 25', and let's say indoors, none of the normal tap rules apply.
Thus absent 430.28(3), we would need 90A conductors, which makes little sense coming off a 65A feeder. 430.28(3) fixes this for us.
Cheers, Wayne
We have three motors of 20A FLC. One feeder supplies them all and requires an ampacity of 65A (125% * 20 + 2 * 20), in which case we may have OCPD up to 90A (250% * 20 + 3*20). [If we exceed 90A OCPD (other than due to 430.52(C) Exception 2, which might get us to 120A), we need to provide an ampacity that complies with the OCPD size as normal for non-motor circuits.] So let's say we use 65A conductors protected at 90A.
Further, say one motor is physically distant from the others, and it would be convenient to split its supply from the feeder and run it 30 ft to the branch circuit OCPD for that motor. 430.62(A) tells us that after the split, our single motor supply must have an ampacity of at least 25A, in which case the OCPD can be up to 50A. But as the upstream 90A OCPD is larger than 50A, we are by default back to needing to use a conductor of size 90A, unless we can use one of the tap rules in 240.21 or 430.28. As we are over 25', and let's say indoors, none of the normal tap rules apply.
Thus absent 430.28(3), we would need 90A conductors, which makes little sense coming off a 65A feeder. 430.28(3) fixes this for us.
Cheers, Wayne
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
It doesn't. Both 240.4 and 240.21 have subsections that reference Article 430 as alternatives to the main rule.
Did you mean 240.3?
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
No, 240.4(G) and 240.21(F) (2020 NEC).Did you mean 240.3?
Cheers, Wayne
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I guess my larger beef with the discrepancy between 240.21(B) and 430 is that I don't see what the feeder ampacity or or downstream overcurrent device have to do with opening an overcurrent device for a faulted tap conductor.
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
Overcurrent lumps two concepts into one. Overload protection and Ground fault.
Ground faults, in order to operate fast, need a high amount of current. If you were to install #10 on a 1000A feeder circuit, you will destroy the #10 (or its connections) before that 1000A breaker opens. You would rather that device operate prior to the destruction of #10. So we limit the size of the wire based on how the upstream device is sized.
Motors are an anomaly as the motor in-rush current can appear like a fault. But in rush is based on the motor's size, so a conductor sized properly for the motor can handle it's inrush. So for motors, we size things differently. We separate overcurrent protection into overload and ground fault. Now to ensure that the upstream device trips, we looks at (1),(2),(3). As long as the conductor is still properly sized for the motor it feeds, and that motor has it's proper overload protection. Then, the fault current should not be able to destroy the conductor prior to operation of the upstream device.
I believe that is the theory.
Reading 430.55 might help as well. It essentially says, you can combine the overload and ground fault into overcurrent protection as long as it is sized based on the overcurrent protection. Which is what is normally done in 240.
Ground faults, in order to operate fast, need a high amount of current. If you were to install #10 on a 1000A feeder circuit, you will destroy the #10 (or its connections) before that 1000A breaker opens. You would rather that device operate prior to the destruction of #10. So we limit the size of the wire based on how the upstream device is sized.
Motors are an anomaly as the motor in-rush current can appear like a fault. But in rush is based on the motor's size, so a conductor sized properly for the motor can handle it's inrush. So for motors, we size things differently. We separate overcurrent protection into overload and ground fault. Now to ensure that the upstream device trips, we looks at (1),(2),(3). As long as the conductor is still properly sized for the motor it feeds, and that motor has it's proper overload protection. Then, the fault current should not be able to destroy the conductor prior to operation of the upstream device.
I believe that is the theory.
Reading 430.55 might help as well. It essentially says, you can combine the overload and ground fault into overcurrent protection as long as it is sized based on the overcurrent protection. Which is what is normally done in 240.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
OK, how about this:I guess my larger beef with the discrepancy between 240.21(B) and 430 is that I don't see what the feeder ampacity or or downstream overcurrent device have to do with opening an overcurrent device for a faulted tap conductor.
With a tap conductor, the potential danger increases with (a) length of the tap conductor and (b) ratio of upstream OCPD size to conductor ampacity. The tap rules represent acceptable limits on the potential danger. You would think that there's some "isodanger" curve for which you could, say, plug in the tap conductor length and it tells you the OCPD ratio that achieves the fixed maximum allowable danger. But 240.21 only gives us 2 main points on this curve, namely 10', 10x and 25', 3x.
The motor SC/GF OCPD sizing rules are similar to the tap rules in that they allow the upstream OCPD to be rated higher than the conductor ampacity, relying on a downstream overload device to protect against overloads. They are also different, as they have no length limit, just an OCPD/ampacity ratio limit. For inverse time circuit breakers, the typical ratio limit for one motor is 250/125 = 2x (but round up the OCPD size), with an allowance for a ratio of up to 300/125 = 2.4x or 400/125 = 3.2x if 2x is insufficient for the motor to start (and no rounding up on these limits).
Then 430.28 defines how these two allowances interact. For the 10' tap rule, 430.28(1) specifies an OCPD ratio of maximum 10x; it could more or less just say "see 240.21(B)(1)" instead. You don't get to take the 2x motor allowance on top of the 10x 10' tap rule allowance. Where as for the 25' tap rule, 430.28(2) effectively says you can combine these two allowances, as it references the feeder ampacity, rather than OCPD size.
Cheers, Wayne