This is the third article of the series reviewing the most popular questions that have appeared in NECA’s online Code Question of the Day and have generated the most comments from subscribers. All answers have been updated to comply with the 2008 National Electrical Code (NEC). If you are not a subscriber and would like to receive the Code Question of the Day online, visit www.neca-neis.org, and follow the links.
The NEC requirements relating to the installation of electric motors and controllers have by far produced more comments and questions than any other subject.
The first question seeks to determine the reasoning behind the overload relay table shown in a motor starter having precedence over Table 430.52.
My question stems from NEC 430.52(C)(2) and the interpretation by a Nationally Recognized Testing Laboratory (NRTL). A circuit supplying power to a motor load begins at a 20-ampere 10,000 AIC breaker using 12 AWG THHN copper conductors. The conductors terminate on the supply side of a size 0 starter with a start-stop station in the cover of the enclosure. The motor is 208-volt, three-phase, which also is supplied by 12 AWG THHN copper conductors and protected by overload relays that correspond directly to the chart provided on the inside of the starter’s cover in relationship with the full load amps listed on the motor nameplate.
The equipment in question had no prior NRTL listing, and the state of Alaska specifically requires this in order for approval during inspections. A testing laboratory was brought in for consultation, and their inspection report stated, “The motor must have branch circuit protection rated per the overload relay chart.” The basis for this report involved the maximum fuse size column on the starter’s label based upon the size of the overload relay used. According to this label, the maximum fuse size listed in the column for the particular overload relay is 15 amps with the motor’s load shown appropriately. NEC 430.52(C)(2) states that the manufacturer’s overload relay table shall not be exceeded even if higher values are allowed in Table 430.52. The NTRL report states I must use a 15-ampere breaker to provide overload protection for the supply conductors, or, in the case of a motor that has less than 15 amps, a fuseholder would have to be installed. Along the same line—and this is extreme—if a motor is connected to a size 3 starter supplied with 2 AWG copper conductors and proper overloads are installed to protect a motor with only 3.2 FLA, by this NEC article, UL/NEC requires that the branch circuit supplying the starter have an overcurrent device (e.g., circuit breaker or fuse holder) not to exceed the listed column on the inside of the starter’s cover. If the motor starter is adequately sized for the motor it serves, the branch breaker is in bounds for the size of the conductor it is protecting, and the motor itself is protected by listed overloads, what is the purpose of the additional requirement? It appears redundant and expensive.
The answer is based on the last paragraph of the question, which says in part, “If the motor starter is adequately sized for the motor it serves.” The motor starter has an overload relay chart, and on this chart is the “service limit current rating,” which specifies the maximum size branch-circuit short-circuit and ground-fault protective device that may be used with that motor starter. NEC 430.52(C)(2) requires, where the maximum branch-circuit short-circuit and ground-fault protective device ratings are shown on the manufacturer’s overload relay table, they shall not be exceeded even if Table 430.52 permits higher values.
A proper installation would be to go to a larger-size motor starter that complies with the proper sizing of the branch-circuit short-circuit ground-fault overcurrent device. The reasoning behind the “service limit current rating” is that the power circuit contacts of the motor starter must be sized on the basis of motor current. The ability of the contacts on the starter to carry the full load current without exceeding a rated temperature rise and the spacing from adjacent contacts corresponds to NEMA standards set to indicate the NEMA size of the starter.
Here’s a question looking for help related to matching available building system voltage to equipment voltage.
I’m installing an electrical system that operates on 230 volts, three-phase. My building power is 208 volts, three-phase. I will have about four or five electrical motors that operate on 230 volts AC. Can I use my present building system to operate my machinery or do I have to install 230 volts?
You can probably use your existing 208Y/120V, three-phase system for the 230-volt motors operating some of your machinery by using three-phase buck-boost transformers. You didn’t mention the kilovolt-amperes (kVA) loads presented by these motors, but I suggest you contact a manufacturer and talk to a representative who can help you work out the details. Or you can check out three-phase buck-boost transformers yourself. You can also check out NEC requirements in 450.4.
Voltage available for a 10 kVA output:
208V AC, 3 PH (X1, X2, X3)
Voltage desired for a 10 kVA output:
230V AC, 3 PH (H1, H2, H3)
The size of the auto-transformer would be 230-208/230 ×
10 kVA = .9 kVA approx.
Overcurrent protection for remote motor control circuits can become very confusing.
Are motor control circuits feeding remote-control devices required to have overcurrent protection?
Overcurrent protection for motor control circuits is covered in 430.72(B). The requirements for conductors, which extend beyond the enclosure (remote) can be found in Table 430.72(B) in Column C.
For example, if your motor branch circuit protective device is rated at 60 amperes and you are using copper control circuit conductors, then you find 60 in the copper column and move to the left to control circuit conductor size, where you will find 12. This means that you need to install control circuit conductors not smaller than 12 AWG copper. Smaller conductors may also be used, but they would require supplemental overcurrent protection.
Motor control wiring schematic
I have a question concerning NEC Table 430.7(B). -I understand how to compute the locked-rotor current of a motor using the table, but am not clear on how you determine kilovolt-amperes per horsepower. Each motor design code letter has a sliding scale. For example, when using code letter “G” you have a choice of kilovolt-amperes per horsepower that ranges from 5.6 to 6.9. In all the examples I’ve seen, the highest kilovolt-amperes per horsepower is used (second column—6.9), but I’ve never turned up an explanation. Your help on this issue would be greatly appreciated.
NEC 430.7(A)(8) requires that code letters or locked-rotor amperes be marked on motor nameplates to show the motor current contribution under locked rotor conditions. Motor design code letters are required to be in accordance with Table 430.7(B). The kilovolt-amperes per horsepower input code letter, prior to the 1996 edition of the NEC, was used for determining branch-circuit overcurrent protection. It referred to what was then Table 430-152 and is now Table 430.52.
Motor code letters indicate a range that the motor designer could use to assist in determining necessary characteristics for overcurrent protection and associated motor equipment. Code letters were deleted from Table 430-152 in the 1996 NEC, since motors are already identified by design letters that incorporate the locked-rotor characteristics of the motor. The reason your research always comes up with the highest kilovolt-amperes is because that’s a worst-case scenario that reflects the best protection (if not the most economical protection).
Check out National Electrical Installation Standard (NEIS), which covers this subject in more detail.
NECA 230-2003, “Standards for Selection and Installing of Electric Motors and Motor Controllers”
Section 440.22(A) on rating or setting for individual motor compressors states, “The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor.” Please tell me when the Code started requiring ground-fault protection on compressors?
The term “branch-circuit short-circuit and ground-fault protective device” means the overcurrent device, usually a circuit breaker or fuses, protecting the branch-circuit conductors. This overcurrent protective device (OCPD) also protects against overload on the branch-circuit conductors.
I believe you may be confusing the ground-fault protection provided by the OCPD with ground-fault protection of equipment (GFPE) or ground-fault circuit interrupter protection for personnel. Branch-circuit ground-fault protection (fuses or circuit-breakers) generally follow closely the ampacity of the branch-circuit conductors, except in motor installations where Table 430.52 can be used to allow for the starting current of a motor as required by 440.22(A).
TROUT answers the Code Question of the Day on the Web site, www.neca-neis.org. He can be reached at 352.527.7035.