Journeyman Test Question

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Max Hudson

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A recent Journeyman Test given in my state asked the following:
Given a 230Volt, single phase 3 HP motor, find the ampacity rating for the disconnect.
I'm wondering if I should use 746 Watts per HP and ohms law to calculate the ampacity or to use table 430.248 to determine the ampacity. According to section 430.110, I multiply the FLC by 115% to determine the disconnect size.
I'm thinking that since it asks for FLC, I should use the table, which would mean 17Amps X 1.15 = 19.55 amps. If either calculation were correct, a standard size disconnect would be at least 20 amps. Most electrical supply houses start with 30Amp disconnects however. What do you think should a student should put as an answer to the question?

A second question on the same test asked the following:
A 10KVA range is on a 120/240Volt circuit. Find the amps for the circuit.
Should I use table 220.55 and take 8KW as the power to use in ohms law to calculate amps or use 10KW instead? If I use 8KW, I end up with 33.33 Amps and a 40 Amp Circuit Breaker. If I use 10KW, I end up with 41.7 Amps and end up with a 45 Amp circuit Breaker. How would you answer the question?
 
mdshunk said:
Disconnects only come in certain size steps. :wink:
Click to expand...

I agree Marc but I don't know the answer to the question.

The NEC does not require that we use a "disconnect' as the disconnecting means.

What if I wanted to use a 20 amp circuit breaker in a single breaker enclosure?
 
(2005)
430.101 ? disconnecting means required
430.109 ? types of disconnecting means allowed (circuit breaker, etc)
430.109 D 2
- switch with 125% of flc (I think this is the minimum required ?)

3hp 230V ? table 430.248 ? FLC = 17
Flc x 1.25 = 21.25

I?m guessing smallest switch amperage is 25A ?


since they asked for the disconnecting means, and not the size of the ocpd, I'm guessing that this is the answer they are looking for (even though, as you said the cb could be the disconnecting means if lockable in open position ?)
 
Max Hudson said:
A second question on the same test asked the following:
A 10KVA range is on a 120/240Volt circuit. Find the amps for the circuit.
Should I use table 220.55 and take 8KW as the power to use in ohms law to calculate amps or use 10KW instead? If I use 8KW, I end up with 33.33 Amps and a 40 Amp Circuit Breaker. If I use 10KW, I end up with 41.7 Amps and end up with a 45 Amp circuit Breaker. How would you answer the question?
Click to expand...
First question I ask is the range in a dwelling? Table 220.55 is for dwellings only. Otherwise use table 220.56
 
They usually don't won't you to read too much into these test questions. With that said:

3 HP 230v motor=17 FLC x 115%=19.55-->20 amp rated disco.

10kva range=10000/240=41.7 amps. I went with this answer because they didn't specify it was for a load calculation.

That's how I'd answer the questions. Are they multiple choice?
 
Note 4 to table 220.55 states that the table should also be used for branch circuit calculations. 210.21(4) restates the same thing as does 422.10(A).
 
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