De Rating

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monkey

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For de rating purposes, is the neutral in a dryer circuit counted as a CCC?

And if so, and after de rating, the ampacity is 28 amps, must the wire be up sized or would a 30 amp breaker qualify as the "next size up"?

Additionally, since it is a dedicated load, could the actual load be used for the wire size calculation instead of going to #8?

This is a commercial application.

Thanks.
 
monkey said:
For de rating purposes, is the neutral in a dryer circuit counted as a CCC?

And if so, and after de rating, the ampacity is 28 amps, must the wire be up sized or would a 30 amp breaker qualify as the "next size up"?

Additionally, since it is a dedicated load, could the actual load be used for the wire size calculation instead of going to #8?

This is a commercial application.

Thanks.
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Check 310.15(B)(4) (a) and (b).

If (a) applies, then neutral is not a CCC.
If (b) applies, then neutral is a CCC.

Conductors must be sized for the load.

Overcurrent protection is sized for the conductors.

240.4(B)(1) permits you to use the 30 amp breaker to protect 28 amp rated wire since it is not a multioutlet branch circuit.
 
e57 said:
Turn it on with a clamp-on meter and find out - odds are it is not...
Click to expand...

That will not tell us if it is a current carrying conductor as far as the NEC is concerned.

A conductor can carry current and not be a current carrying conductor as far as the NEC is concerned.

If the service is 120/240 the neutral will not be a current carrying conductor.

If the service is 208Y/120 as in many apartment buildings the neutral is a current carrying conductor.
 
True Bob, My thinking was the dryer itself - I recently installed a code required 4-wire cord on one to an empty neutral terminal. i.e. it was unused...
 
Everything in a dryer (drum and timer motors, light, etc.) except the heating element is line-to-neutral. Check out the schematic next time you have the chance.

I have had quite a few customers who bought a new dryer to replace one that ran but did not heat, only to have me discover that they had one phase out.
 
LarryFine said:
Everything in a dryer (drum and timer motors, light, etc.) except the heating element is line-to-neutral. Check out the schematic next time you have the chance.
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Not the one I was just mentioning... After removing the neutral/ground terminal bond it had - the neurtal terminal was of no use. All of the electronic controls were powered by a transfomer.
 
LarryFine said:
That's a new one on me. Does the transformer primary have 240- and 208-volt taps?
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New one on me too, I wonder how they do it?

They would need a motor rated 208 through 240 and still handle 10% plus or minus of those voltages for a total range of 187 to 264 volts.
 
That transformer setup would have to be similar to these self contained Heating and A/C units we've been doing lately. These have 208V and 240V taps that must be changed upon installation to match voltage available.

Tom

I might have to pull my dryer apart.LOL
 
iwire said:
That will not tell us if it is a current carrying conductor as far as the NEC is concerned.

A conductor can carry current and not be a current carrying conductor as far as the NEC is concerned.

If the service is 120/240 the neutral will not be a current carrying conductor.

If the service is 208Y/120 as in many apartment buildings the neutral is a current carrying conductor.
Click to expand...

Interesting.

Let's assume the 240 (or 208)-volt heating element pulls 18-amps and the 120-volt motor and controls pulls 4-amps.

On a 120/240 service, the neutral load is 4-amps.

What is the neutral current on a 208/120 service?
 
hardworkingstiff said:
Interesting.

Let's assume the 240 (or 208)-volt heating element pulls 18-amps and the 120-volt motor and controls pulls 4-amps.

On a 120/240 service, the neutral load is 4-amps.

What is the neutral current on a 208/120 service?
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It depends, do all the line to neutral loads come from the same phase.

If so the answer is 4 amps for either voltage system.

If they tried to balance the 120 motor against the lamp and controls by using both ungrounded phases the neutral current will be less than 4 amps.
 
iwire said:
It depends, do all the line to neutral loads come from the same phase.

If so the answer is 4 amps for either voltage system.
Click to expand...

So why would the neutral need to be counted as a CCC with the 208Y/120 but not on the 120/240? Is there a different heat generated with the 4-amp neutral load on the 208Y/120 than there is on the 120/240?
 
hardworkingstiff said:
So why would the neutral need to be counted as a CCC with the 208Y/120 but not on the 120/240? Is there a different heat generated with the 4-amp neutral load on the 208Y/120 than there is on the 120/240?
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Yes.

If you think about it this way, it may be easier to understand: If you have a fully-balanced 4-wire Y load, there is no neutral current. Now, for each amp you reduce one phase's load by, the neutral increases by the same amount.

Let's say we have a 15-amp load on each phase. If you reduce one phase's load to zero, the neutral current will increase to that same 15 amps. If you only have two phases and a neutral, you get the same results: 15 amps on each of two phases, and a full 15 amps on the neutral as well.

For a 240/120 circuit, and a 15-amp load on each hot, the neutral load is zero. For every amp one phase's load is reduced, the neutral increases the same. But, you must have that phase reduction to have the neutral current increase.
 
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Larry,

I believe the circuit you are discussing is different than the dryer we've been discussing. They are wired differently.

It appears you are talking about a MWBC of a 208Y/120 single-phase panel and the dryer is more like a 208-volt heater load sharing the "hot" conductor with a 120-volt motor and control load.
 
hardworkingstiff said:
So why would the neutral need to be counted as a CCC with the 208Y/120 but not on the 120/240? Is there a different heat generated with the 4-amp neutral load on the 208Y/120 than there is on the 120/240?
Click to expand...

Because the code requires it with 208Y/120 {310.15(B)(4)(b)} and does not with 120/240. {310.15(B)(4)(a)}

In this particular example (the dryer) it does not matter but the code is written for every situation not just a dryer.


hardworkingstiff said:
Larry,

I believe the circuit you are discussing is different than the dryer we've been discussing. They are wired differently.

It appears you are talking about a MWBC of a 208Y/120 single-phase panel and the dryer is more like a 208-volt heater load sharing the "hot" conductor with a 120-volt motor and control load.
Click to expand...

By NEC definition both circuits (Larry's and the Dryers) are multiwire branch circuits.
 
iwire said:
.....
By NEC definition both circuits (Larry's and the Dryers) are multiwire branch circuits.
Click to expand...

OK, but that still doens't affect the question. How is the heat different so that the Neutral is a CCC for a 208Y/120 vs. 120/240. This question is only for the dryer circuit.

As for Larrry's example of reducing the current in one phase casuing the neutral current to rise is understood (but I believe does not apply in this case). Larry, look at it this way. According to your example, if we were able to turn on dryer heating element and not any of the 120-volt loads, there would be equal current on the two ungrounded phases feeding the heater. The neutral would be the same current? Of course not.
 
hardworkingstiff said:
How is the heat different so that the Neutral is a CCC for a 208Y/120 vs. 120/240. This question is only for the dryer circuit.
Click to expand...

If all the line to neutral loads come from one phase conductor the current will not be different.

If the line to neutral loads are split between the phases the current on the neutral will be less in the 120/240 circuit than the neutral current in a 208Y/120 supplied circuit.

It strikes me that the 'heat' will be close to the same in either circuit.
 
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