Motor and Transformer Calculations for the C-10 Exam
Motor and transformer problems are where a lot of otherwise-solid electricians lose points on the C-10. Not because the math is hard — it's arithmetic — but because motors break the pattern every other circuit follows. For a receptacle circuit you read the load, size the wire, size the breaker, done. Motors make you pull two different current values from two different sources, and they split protection into two separate jobs. Get clear on that structure and these become easy points.
The rule that catches everyone: use the table, not the nameplate
Here's the single most important idea. When you size motor branch-circuit conductors and short-circuit protection, you do not use the current stamped on the motor's nameplate. You use the full-load current (FLC) from the NEC tables — Table 430.248 for single-phase and Table 430.250 for three-phase motors. Section 430.6(A) says so directly.
Why? Consistency. Two 10-horsepower motors from different manufacturers might have slightly different nameplate amps, but the Code wants the branch circuit sized the same way every time. So the tables give one standardized FLC per horsepower and voltage.
The nameplate current (FLA) isn't useless — it's exactly what you use for overload protection, because overload protects the specific motor sitting in front of you. So the split is:
- Branch-circuit conductors and short-circuit/ground-fault protection → table FLC (430.250)
- Overload protection → nameplate FLA
Mix those up and you'll land on a plausible-looking wrong answer. The exam loves to hand you both numbers precisely to see which one you grab.
Two layers of protection, two different jobs
Every motor circuit has two independent protective schemes, and the exam expects you to know which protects what.
| Protection type | Protects | Sized from | Typical setting |
|---|---|---|---|
| Overload (430.32) | The motor windings | Nameplate FLA | 115%–125% of FLA |
| Branch-circuit short-circuit/ground-fault (430.52) | The conductors & equipment | Table FLC | Up to 250%+ depending on device |
Overload guards against the slow burn — a motor pulling too many amps for too long because it's mechanically loaded down or single-phasing. For a continuous-duty motor with a service factor of 1.15 or higher, overload is generally set at 125% of nameplate FLA; for others, 115%. That's a different world from the fast trip you need for a dead short.
Branch-circuit short-circuit/ground-fault protection is the breaker or fuse at the panel. It has to ignore inrush — a motor draws six to eight times its running current for a fraction of a second on startup — while still clearing a genuine fault. That's why the percentages in Table 430.52 run so high: a non-time-delay fuse can go up to 300% of FLC, an inverse-time breaker up to 250%, with provisions to bump those higher when the motor won't start otherwise. That number would be absurd for overload, but it's right for this job because the overload relay handles the sustained-current protection separately.
If you've worked through overcurrent protection on the C-10 exam, notice how motors flip the usual logic: the branch-circuit device is deliberately oversized relative to the conductor, and a separate overload device makes up the difference. That's legal only because of the motor rules — don't carry it over to general circuits.
Sizing the conductors: the 125% bump
Motor branch-circuit conductors for a single continuous-duty motor are sized at 125% of the table FLC per 430.22. The workflow:
- Look up FLC in Table 430.250 (three-phase) for the motor's HP and voltage.
- Multiply by 1.25.
- Pick a conductor with an ampacity at least that value from Table 310.16.
Quick example. A 10 HP, 230-volt, three-phase motor has a table FLC of 28 amps. Conductor sizing: 28 × 1.25 = 35 amps, so you size the wire for at least 35 amps of ampacity. Easy once you know which current to start from — and starting from the table value is the whole game.
For the short-circuit device on that same motor, take 28 amps × 250% (inverse-time breaker) = 70 amps, then round to a standard size per the rules in 430.52. Two different multipliers, one FLC, two answers. Drilling this pattern until the sequence is automatic is how people pass the calculation-heavy items — spacing out C-10 practice questions on this exact structure pays off more than re-reading the code text.
Feeders and multiple motors
When several motors share a feeder, 430.24 gives the rule: size the feeder conductor at 125% of the largest motor's FLC plus the sum of the FLCs of all the other motors. Only the biggest motor gets the 125% bump; everything else is added at 100%. The feeder protective device (430.62) is the largest branch-circuit device rating plus the sum of the other motors' FLCs. These two rules look alike and get confused constantly, so slow down and confirm whether the problem asks about the conductor or the protective device.
Transformers: Article 450
Transformer questions on the C-10 usually come down to overcurrent protection, and that all lives in Article 450. The core table is Table 450.3(B) for transformers 1000 volts and less — the everyday case for most C-10 work.
The key distinction is whether you're protecting the primary only or the primary and secondary. With primary-only protection and a primary current of 9 amps or more, the primary device can be sized up to 125% of rated primary current (with a rounding-up allowance when 125% doesn't land on a standard size). When you protect both sides, the primary can go up to 250% because the secondary device is handling the closer protection — that secondary device being limited to 125% in this arrangement.
To find rated current, use the transformer's kVA and voltage. For a single-phase unit, I = VA ÷ V. For three-phase, I = VA ÷ (V × 1.732). So a 45 kVA, 480-volt three-phase primary draws 45,000 ÷ (480 × 1.732) ≈ 54 amps. Primary-only protection at 125% works out to about 68 amps, rounded up to the next standard device size per 450.3(B) and 240.6.
Don't forget the transformer secondary conductors still have to be protected and sized as conductors — the transformer rules cover the transformer, but the wire on either side follows the conductor rules you already know. And any transformer tied to a grounded system pulls in the bonding and grounding-electrode work covered in grounding versus bonding, which shows up as its own line of questions.
How to actually study this
The exam isn't testing whether you can multiply 28 by 1.25. It's testing whether you reach for the right table and the right percentage. So when you drill these, say out loud which table each number comes from — 430.250 for FLC, 430.52 for short-circuit protection, 430.32 for overload, 450.3(B) for transformers. Once the table lookup is reflexive, the arithmetic is the easy part.
These concepts are laid out here to help you study; always confirm exact values against the current adopted California Electrical Code and defer to the official code language on exam day. Get comfortable with the two-layer structure for motors and the primary/secondary split for transformers, and this whole topic turns from a trap into a reliable batch of points.