NEC Article 430
Master, RW, Contractor and Journeyman Electrician tests all have motor questions. You will have 10 questions at least on Motor Feeder short-circuit and ground fault protection, Motor disconnecting means, Motor branch-circuit short-circuit and ground-fault protection, Motor circuit conductors, Motor controllers, and Motor overload protection. So lets take a look at some.
Figure 430.1 Article 430 Contents
Please go to article 430 in your code book and look at this figure. This is the start of all motor questions on your test. What are they asking you to find? What piece of this one-line diagram "figure 430.1" is the question looking for?
Table 430.247 through 430.251(B)
Do you see in figure 430.1 the very last of the "Part I - Part XVI"?
Part XVI is the very next thing you do. Go and find your Motor in these Tables. You cannot answer any load calculation questions until you find your motor here.
Real World Check.
Don't put #12 THHN on a 40 amp breaker. You will never be able to explain it in court if you have to defend yourself.
Open your code book. Now put it about five feet away. This is how your peers will see it.
Here is a typical motor and pump.
Scope of work: We need you to provide power for a continuous duty, 5 H.P., 208 Volt, 3 phase motor, induction type wound rotor.
What size conductors will we need?
What size disconnect will we need?
NEC Article 430.22 (single motor, continuous duty) says 125% of the motor full load rating.
The only answer is to go to Table 430.250 and get the full-load current.
The nameplate for this particular motor says 5 HP, 200 Volts, 3 phase, 16.1 Amp. Only use what you need from the nameplate to find your motor in the tables.
The question on the exam is going to tell you the motor they are looking for. Example 5 HP, 208 volt, 3 phase, wound rotor.
Table 430.250 : Full-load current = 16.7 amps
Conductor size shall be 125% FLC
16.7 x 1.25 = 20.875 amps
NEC Table 310.15(B)(16) @ 60 degrees C = 10 AWG
Note: NEC 2008 310.16 Answer will be 12 AWG
Well a 20 amp disconnect won't work. The next standard size disconnect is 30 Amp.
I want you to understand that your conductors and equipment must be rated at or above the Full-load.
What size instantaneous Trip Breaker will we need for Branch Short-Circuit and GFCI protection?
Standard breaker is 20 Amps. Note: If the motor doesn't start on a 20 Amp breaker you can go up to 800% of full load current. Example 17.5 x 8 = 140 Amps Table 430.52
I know your saying that is crazy. I can have a 10 AWG wire on a 100 amp breaker? Well short-circuit and ground-fault inrush current is over 10,000 amps and the breaker trips so quickly protection is provided. That is why overload protection calculations are completely separate from short-circuit and ground-fault protection..
What is the overload protection needed?
What is overload protection? Overload protection protects the conductors and the motor from staying overloaded. It is a time delay so the motor has the current it needs to start.
If overloaded for a long enough period of time the fuse or heater protection will burn up instead of the motor and conductors.
If you have motor that is constantly burning up check the overload protection or add if necessary.
It's one of those "Ghost" things.
Here you will use the nameplate Amps
16.1 x 1.15 = 18.515
Your fuse size will not round up or go over this value.
Standard Fuses 15 amps. In this case it is a heater in the motor controller. H29
So here it is you will see that;
The motor is on a 3 pole 20 Amp breaker.
The overload protection is 15 Amps
The Disconnect is rated for 30 Amps.
The conductors are # 12 AWG (2008 code)
I have some great motor calculations on the way but some words of encouragement would speed things up. I am concentrating on the study guide but would be happy to chat about motors with you.