ECN Forum Forums Code Related Discussion NEC & other Code issues Another NEC 2017 Question_240/24VAC Txfr-Neutral R

Hi,

Am I thinking correctly on this NEC2017 question?

"A 24 volt AC system fed thru a 240 volt transformer does not need to use "White" as a neutral (grounded)"
True or False

I'm thinking True based on 250.20 (p106).
But that code section doesn't exactly hit the mark. I've looked all through Article 450 and other sections but I haven't spoted this exact question terminology.

Any clues?
Thanks,
Phil

I'm not sure what kind of answer they're looking for, but I would argue true based on 200.6

The only way it CAN be white is if it is a grounded conductor. If you ground one side of the secondary that conductor needs to be white or gray. Otherwise it can't be (legally) although there is plenty of red/white doorbell and thermostat wire out there. I understand we assume the rules don't really apply to low voltage systems but Article 200 doesn't say that

I can’t tell if you’re agreeing with me or not

My reading of the question is whether the grounded wire in this situation has to be white and 200.6 says it could be gray.

True. They removed "natural" (gray) a few cycles ago. That referred to the natural rubber color of the wire we now call white. Gray PVC insulated wire has been the convention for neutral in 277 (wye) for years before the change and they finally recognized it.

The open question is whether 200.6 applies to low voltage cables. If so the white wire would have to be grounded and we know it usually isn't. In fact the green wire in thermostat cable is also non compliant. I have looked but I haven't found that exception. It may be hidden somewhere tho.

Maybe 200.7(B)

Just IMHO, as the question specifically states the word "grounded", I would say ...TRUE.

I have not heard of any inspectors that would have any types of issue with a 'white' conductor on the secondary side of a XFR.
.

Yup 200.7(B) I guess I didn't look hard enough

Here is the given answer:
"Nope  it does  250.70.A   200.7.B"

I got that wrong too. I said True based on 250.20.

So much to learn, and when I die, it's all gone <:

Thanks,
Phil

Some follow up notes as I dive deeper into this question and answer.

250.70(A) doesn't exist in NEC2017. Does it exist in 2014? (I don't have that codebook).
250.70(1) through (4) do exist but it has to do with grounding/bonding connections to electrodes.
I don't see any relation to this question.

200.7(B) relates to this question and points to 250.20(A).

250.20(A) is AC systems to be grounded with (A) being less than 50V.
If this transformer is fed from one 240V leg, wouldn't 250.20(A)(1) & (2) be True and therefore a Neutral required?
Which would make the question False, as is the answer given, "Nope it does 250.70.A 200.7.B"

What if the transformer is fed from two 120V ungrounded with an existing Neutral? Answer is still False.

Kind of a weird question,...and answer.
What do you think?
Am I over-thinking this one? I've been know to do that <;

Cheers,
Phil