|
| Ek's Home | Forum | Chat | Electrical Links | |
| Subject - 3 PHASE WIRE SIZING | |
| CJH | I AM ESTIMATING A JOB TO WIRE A3PH 480V IRRIGATION PUMP WITH A 75AMP LOAD. THE DISTANCE IS TO BE 500 FEET. I WANT TO FEED THE FUSED DISCONNECT WITH A 100 AMP BREAKER. ACCORDING TO MY CALCULATIONS FOR VD I CAME UP WITH 3/0 COPPER. IS THAT TOO LARGE ? |
| David Hyatt | CJH, Thanks for your post. My voltage drop calculation gives me #1cooper. CM=1.732x(k)xDxI/vd permitted 75ampFLC?x125%= 93.75amp for VD calculation. 1.732x12.9x500x93.75/14.4= 72,730.465 TBL(8)CM= #1 cooper. 75amp x 250%CB = 187.5 Section 240.6 next higher size CB 200amp breaker. Hope this helps and feel free to comment if you see an error in my calculation. Thanks Also someone help me here, a pump motor, it does require the 125% increase, right? 
|
| Scott Vickrey | David's calculations look correct to me. The #2 copper wire will have less than 3% voltage drop of 480 volts at 500'. 3% is the maximum voltage drop allowed for a branch circuit by code. Remember #2 is the minimum, you can increase efficiency by using a larger wire. That's up to you though. To answer your question David, This depends on duty cycle. Check out section 430.22(A)for a single continuous duty motor which tells you to use 125 of FLA. Also see section 430.22(E) for a single other than continuous duty motor. Clayton what is the horse power of the motor? What would you think the longest period of time this motor will run without resting? Remember if this is a continuous duty motor you can't use the name plate rating of the motor for sizing the circuit conductors. you have to use the HP and Tables 430.147-150. If it's other than continuous duty you use the FLA in conjunction with Table 430.22(E) |
| CJH | Thank you for the great advice .Where are you getting your formulas from. Also iwas not so clear on what you were telling me about having to go to a200A breaker. |
| David Hyatt | I have a six step motor calculation. 1) FLC= TBL430.148- TBL430.150 2)overloads= 430.32 minimum size & 430.32c maximum size. 3)Branch circuit conductors= 430.22 FLC x 125% 4)Branch circuit OCP= TBL430.52 FLC x listed percentage for type of OCD. Note: Branch circuit OCP go to the next higher standard size. 5)Feeder condutor size (if more than one motor on one feeder)= 430.24 6)Feeder OCP= 430.62 Note: on a feeder you may not go to the next higher standard size breaker, you must go down. Hope this helps. The only other thing I am not possitive about is: do you take the 75amp at 125% for the VD calculation? 
|
| tony thrower | NEC 430.22 Conductors supplying power to a single motor shall have a minimum ampacity not less than 125% of the motor full load current. 75 x 1.25 = 93.75 amperes minimum conductor size NEC 315.16 #3 75 c NEC 210.19(A)(1) fpn #4 Limits the voltage drop on branch circuits to 3% of the system voltage. 120 x .03 = 3.6v 208 x .03 = 6.27v 480 x .03 = 14.4v I selected a #3 thw conductor which has an a circular mil area of 52,620 cm. 3-phase voltage drop= 1.732 x 12.9 x 75 x 500 / 52,620 cm =15.922v This voltage drop exceeds my allowable voltage drop. To determine the minimum circular mil area that will limit the voltage drop to the desired value use the formula: 3-phase circular mils = 1.732 x 12.9 x 75 x 500 / 14.4v minimum cir mils = 58,184 cm Now take this (58,184) to NEC Table 8 Chapter 9. Under the cir mil column select a number equal to or greater than 58,184 cm. This number is 66,360 which is the circular mil area for a #2 copper conductor. Using this cir mil you can now determine the voltage drop at full load using the formula 1.732 x 12.9 x 75 x 500 / 66,360 = 12.625v Voltage drop using #1 copper is 1.732 x 12.9 x 75 x 500 / 83,690 = 10.01v. 1.25 should not be included in voltage drop formula. Electical Formulas are taken fron the 2002 edition of Ugly's Electical References which is carried by many electrical supply companies. A book store can order it using the ISBN # 0-9623229-6-2. |
| David Hyatt | Wasn't sure if the 125% was included or not for motors in a VD calculation. |
| eleccon | My electricalc pro calculator says that #1 copper is the wire size.......... I love this thing!
|
| karlwayne | I would almost have to agree with Tony, although we took different roads,I'll try to explain. According to 'American Electricians Handbook' (Croft,Car, and Watt) states as a general rule, voltage drop of a circuit depends upon many variables, the value of which cannot be determined accurately. The constant of copper changes with temperature and in their examples they use 12. There had to be many ways to do this but this is the only way I learned. If you used the Vd formula for a 2-wire dc circuit with the paramaters you specified, and multiplied the result by .8666 you end up with your 3 phase drop. 2*k*L*I/cm(.8666) 2*12*500*75/52630*(.8666)=14.8volts for #3 thhn If you use a lower constant and dropped down to 10.6 you will be within the limits, however,what is the voltage proper that you're starting with? Is it always practical to use the bare minimum? Will it work,of course, but for a five hundred foot run I would put in #2. |
| iwire | As a side note to these fine calculations the NEC does not require anything be done about voltage drop. As was mentioned voltage drop is referenced in a fine print note, that is nothing more than a suggestion. I do agree 3% is a great figure to use just not NEC required. Now if we run a 1 AWG copper to this 75 amp load what is the correct size equipment grounding conductor. Remember 250.122(B) requires the EGC to be increased in size in proportion with the increase of size of the ungrounded conductors. |
| JimmyDee | Iwire, I sure learned something from you here. I would have been willing to bet a lot of money that you were wrong on the fpn statement but: Here is proof I was wrong and you were correct.  90.5 Mandatory Rules, Permissive Rules, and Explanatory Material. (A) Mandatory Rules. Mandatory rules of this Code are those that identify actions that are specifically required or prohibited and are characterized by the use of the terms shall or shall not. (B) Permissive Rules. Permissive rules of this Code are those that identify actions that are allowed but not required, are normally used to describe options or alternative methods, and are characterized by the use of the terms shall be permitted or shall not be required. C) Explanatory Material. Explanatory material, such as references to other standards, references to related sections of this Code, or information related to a Code rule, is included in this Code in the form of fine print notes (FPNs). Fine print notes are informational only and are not enforceable as requirements of this Code. Jim |
| iwire | Jim thanks, I am sure I will learn things here too.:) FPNs aside if it was up to me I would not allow more than 3% drop on the FLA of this motor, after all we still have to get the thing to start up decently at the end of 500'.:) |
| JimmyDee | quote: #8 copper as per: Table 250.122 Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding Amps---Copper 15------14 20------12 30------10 40------10 60------10 100-----8 200-----6 300-----4 400-----3 Jim |
| iwire | If it was only that easy.quote: We increased the size of the supply conductors so the code requires us to up size the equipment grounding conductor. As we are going to use a 200 amp breaker it is likely (I did not do the math) that the EGC will end up being a 1 AWG also. |
| David Hyatt | This is something I didn't know either, just checked my study notes and it did say code suggest 3%VD onbranch circuits and 5% total. Note: there was a question on the electrical exam on VD. |
| JimmyDee | I was going with the original question's breaker size. quote: And I agree that the 100 amp breaker won't be big enough for the 75 amp motor load. Can we find out what the HP of the motor is? Jim |
| iwire | I would say it would be a 60 HP motor to need 75 amps @ 480 3 phase. CJH can you provide more details, the 100 amp breaker is the code minimum for a 75 amp motor load but we are not supposed to be sizing the breaker off of the motor nameplate amps, we should be sizing everything except the motor overloads from the tables in 430 based on name plate HP. |
| tony thrower | The 75 ampere motor load would require a branch circuit conductor sized based on NEC 430.22. 75 x 1.25 = 93.7 amps If the O.C.P.D. temperature rating is 75 c this would require a #3 copper conductor that has a circular mil area of 52,620. NEC table 250.122 requires a #8 copper EGC that has an area of 16,510 cir. mils. I selected a #2 thw copper conductor to limit the voltage drop to a desired (not required) value of 3% or less. #2 has an area of 66,360 cir. mils. #2- 66,360 / 52,620 = 1.26. This is an increase in size of my branch circuit conductors of 126%. NEC 250.122(B) tells me that I must increase the size of my EGC proportionately according to the area in cir. mils. Initially my EGC was a #8-16,510 cir mils. #8-16,510 x 1.26 = 20,802.6 cir. mils. This would require a #6 EGC-26,240 cir. mils. |
| iwire | Hi Tony thanks for trying to solve this one. This one is tough as the EGC is based on the breaker size not on the wire size. If this motor draws 75 amps, the breaker we choose could be 100 to 200 amps (125% to 250% of FLA). In order to determine the EGC size we will need to know the breaker size. The ungrounded conductors will stay at 3 AWG no matter what breaker we choose and the EGC will never be required to be larger than the ungrounded conductors. So it is not as easy as say a long lighting circuit. |
| tony thrower | You are correct in saying the OCPD rating is used to select the MINIMUM EGC size. I know also that the 100 amp breaker would probably never allow this motor to start. However, seeing that I did increase my ungrounded conductor size I must also increase the EGC size proportionatly(NEC 250.122(B). Excessive voltage drop on conductors decreases the efficiency of our electrical system which is not a code violation (NEC 90.1(B). Low voltage has the same effect on an electric motor as digging a ditch ,by hand, in Alabama, on a hot day in August has on me! It will decrease my life expectancy! A technician told me that a representative for a motor manufacturer told him if a motor is ever subjected to a temperature that exceeds it's rating by 10 degrees the motor's life expectancy will be decreased to 50%. |
| iwire | Tony I think we are on the same page. What I was trying to get at is if we decide to use a 150 amp breaker for this 75 amp motor we will need to apply 250.122(B) to the EGC required for a 150 amp breaker. We may find that once we do the math for 250.122(B) it will show the need for an EGC larger than the wires feeding the motor. No matter what the result of 250.122(B) is the EGC will not have to be larger than the ungrounded conductors.  |
| tony thrower | Bob I agree. I am curious. Is this electrical knowledge repository habit forming? If so I think I am addicted! |
| JimmyDee | quote: Sure is. It a hole lot of teaching and learning, one on one, except the whole world is watching. Jim |
| Scott Vickrey | This site is averaging 150 new visits a day for the past 10 weeks. We are just starting to get queried search traffic. This forum is now answering peoples specific questions everyday. Remember when you answer someones question you actually answer it for a great number of people in the future. |
| iwire | quote: Tony You are asking an addict if it is addicting, my answer would have to be yes it is. Now don't be digging any ditches in the Alabama sun, that does not sound like fun.
|
| shocky | NEVER REFER TO THE NAMEPLATE AMPS,ALWAYS USE THE HP RATING AND THE CODE BOOK WILL GIVE YOU THE AMP RATING |
| Ryan_J | quote: I would disagree with that. The table values are to be used for ground fault and short circuit protection, but the nameplate is to be used for overload protection. See 430.32 By the way, please turn off your caps lock button. |
| shocky | RYAN J,CHECK OUT TABLE 430.150.THINK ABOUT START UP.NEVER USE NAMEPLATE AMPS UNLESS UNDER 1200 RPM.AND BY THE WAY...DO BIG LETTERS SCARE YOU?1.25%? |
| JimmyDee | quote: No I don't think so. In forum talk, the all capital letters are equivalent to shouting or yelling and it is hard for some people to read. I am a speed reader and can not do it with all caps so several of us would appreciate it if you would turn the cap lock off. Thank-you, Jim |
| shocky | Sorry guys,not screaming.Caplock off for now on.Didn't mean to confuse anyone. |
| Ryan_J | Once again, you need to read 430.32 and differentiate between ground fault and short circuit protection vs. overload protection. I am well aware of table 430.150....but what code section tells you to go to that table? What code section tells you how to size overload protection? You can't go to a table until code text tells you to go to a table. |
| shocky | ryan j, read 430.6(a)(1) "table 430.150, including note, shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit, short-cicuit, and ground-fault protection, instead of the actual current rating marked on the motor nameplate." referenced 2002 handbook.if i'm reading this wrong, someone please tell me because i've been using this rule for years. |
| shocky | amp rating on nameplate is for overload protection devices only,if the horsepower is not shown on nameplate, only then will you use amps for conductor sizing. |
| Ryan_J | I think we are starting to argue the same point. Take a look at my thread called "Education: overload vs. overcurrent" and let me know if you agree. Go here: http://www.electricalknowledge.com/forum/topic.asp?TOPIC_ID=609&SearchTerms=education |