Water heater test question

[tortuga]

Greetings all I am interested how you all answer this question:

A standard residential 50 gallon water heater has two 4500 watt elements that operate non-simultaneously.
The markings on the water heater indicate 240 Volt, 4500W, copper conductors only.
The manufacturers literature that comes with the water heater has a table for recommended sizing the breaker and wire.
The table lists 10 Awg copper and a 25 amp breaker for this unit.
The breakers are marked for 60/75 C terminations.

If the wiring method chosen will be type MC cable what is the minimum branch circuit conductor size permitted by the NEC?

A) 10 AWG

B) 12 AWG

C) 8 AWG

Spoiler: My answer

(A) is correct 10 AWG , the water heater is a continuous load per 422.13 and since 240.4(D)(5) limits 12 AWG MC to 20 amps and table 240.4(G) only references 'motor operated' appliances not all appliances.

 

[infinity]

The correct answer is A. #12 cannot be used because of 240.4(D)(5). I didn't look at your answer.

 

[Dennis Alwon]

I agree with infinity

 

[Tulsa Electrician]

That is a well written question.

Greetings all I am interested how you all answer this question:

I would start with understanding what it is asking. In this case the keywords for me are.

what is the minimum branch circuit conductor size permitted by the NEC?

I would start in 422.

In this case 422.13 storage type INF note go to 422.10

part 2 installation

422.10 (A)
marked rating

The markings on the water heater indicate 240 Volt, 4500W, copper conductors only.
The manufacturers literature that comes with the water heater has a table for recommended sizing the breaker and wire.
The table lists 10 Awg copper and a 25 amp breaker for this unit.

On to 422.11, (A-G) over current protection.
422.11 (A)

If the wiring method chosen will be type MC cable what is the minimum branch circuit conductor size permitted by the NEC?

Says to go to 240.4 plus marked rating.
Off to 240.4
240.4 (D)
Use (D-1 thru D-7).
Unless E or G apply.
Neither do apply.

So back to (D)
1 nope
2 nope
3 nope
4 nope
5

Plus I cheated; cliff note

water heater breaker size

My customer just installed a 36 gal water heater. The label says upper and lower elements are both 4500 w. But it says "total watts connected : 4500 watts" I've always wondered how two 4500 w elements can be only 4500 w connected, I obviously don't understand how they work. ( I always...

forums.mikeholt.com

 

[infinity]

I agree it is a good question and as it contains some irrelevant information to possibly confuse you. When I look at a question like this I try to start with what's relevant and what's irrelevant to finding the correct answer. I outlined my process below. Bold/Black relevant, Bold/Red irrelevant.

A standard residential 50 gallon water heater has two 4500 watt elements that operate non-simultaneously. The markings on the water heater indicate 240 Volt, 4500W, copper conductors only. The manufacturers literature that comes with the water heater has a table for recommended sizing the breaker and wire. The table lists 10 Awg copper and a 25 amp breaker for this unit. The breakers are marked for 60/75 C terminations. If the wiring method chosen will be type MC cable what is the minimum branch circuit conductor size permitted by the NEC?