wire sizing

[Ivan Young]

Gentlemen, I am finally taking my Journeyman exam in July and have a few questions.
Lets you have the following circuits: 2) 20 amp 120v receptacle circuits, 2) 20 amp 120v lighting circuits, 1) 20 amp 120 v washer circuit, 1) 40 amp 240v dryer circuit. All circuits are going be piped from the panel to a JB 50 feet away. From the JB the various circuits will be piped to their final destinations. You have to figure the different wire sizes. By my count I have 13 current carrying wires, so I would have to derate by 50%. Since I already know the load and the correction factor I would take each 20 amp circuit and calculate: 20a/ .50=40a. So under the 60 degree column I would pick a #8 wire. For the 40a 240v circuit: 80a. I would pick a #3 wire.
Then based on the different wire sizes I would size my conduit.
How did I do?
Thanks,
Ivan

 

[infinity]

If your conductors have 90? C insulation you should be using their 90? C ampacity for derating.

 

[Gac66610]

also
I believe you only have 12 CCC as the neutral, for the 40a 240v dryer, is a non current carrying conductor

 

[augie47]

Dryer won't matter as either way you are in the 10-20 ccc with an ampacity reduction of 50%, but as Rob pointed out, for derrating purposes you can use the 90? rating (if your insulation allows), so for your 20 amp circuits, a #10 tHHN would suffice

 

[Ivan Young]

Thanks for all the replies guys. I take it the way the calculations were done correctly.
The reason a counted the neutral for the dryer is because the light inside the drum is 120v, so is it ok to count it? I asked a forman on the job and he said the neutral should always be counted. I thought it is counted for receptacles, lighting, and when your load is unbalanced.
Thanks,
Ivan

 

[Smart $]

Dryer won't matter as either way you are in the 10-20 ccc with an ampacity reduction of 50%, but as Rob pointed out, for derrating purposes you can use the 90? rating (if your insulation allows), so for your 20 amp circuits, a #10 tHHN would suffice

Thanks for all the replies guys. I take it the way the calculations were done correctly.
The reason a counted the neutral for the dryer is because the light inside the drum is 120v, so is it ok to count it? I asked a forman on the job and he said the neutral should always be counted. I thought it is counted for receptacles, lighting, and when your load is unbalanced.
Thanks,
Ivan

You didn't specify whether the 120V circuits are two-wire or 3-wire (multiwire branch circuits). Assuming 120/24V supply, you could have 2 mwbc's, 1 non-mwbc for the 20A circuits, and the neutral need not be counted as a ccc for the dryer circuit. So it is entirely possible to have only 7 ccc's, requiring only 70% ampacity reduction.

 

[Ivan Young]

Smart $,
I forgot to mention but all 20 amp circuits are two wire and the dryer is two hots and a neutral

 

[Smart $]

Smart $,
I forgot to mention but all 20 amp circuits are two wire and the dryer is two hots and a neutral

Okay.

 

[Ivan Young]

Smart $,
As to the the calculations, did I do them right? And the second question is: is my foreman right when he said that the neutral should always be counted? I was told the neutral should be counted when the circuit supplies receptacles, lights, or when your loads are not balanced.

 

[garbo]

Dryer neutral.

Dryer neutral.

also
I believe you only have 12 CCC as the neutral, for the 40a 240v dryer, is a non current carrying conductor

I know that the dryer timers and 40 to 60 watt drum lamp are 120 volts. Years ago I ran into a 120 volt motor on a 240 volt dryer.It would be a lot cheaper to run 2 conduits with the price of copper.you should the AHU for a ruling on the dryer neutral. garbo

 

[Ivan Young]

The motor, timer and light on the dryer is 120v, the heating elements are 240v

 

[infinity]

And the second question is: is my foreman right when he said that the neutral should always be counted? I was told the neutral should be counted when the circuit supplies receptacles, lights, or when your loads are not balanced.

Your foreman is wrong. Read 310.15(B)(4):

310.15(B)(4) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).
(b) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).
(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.

 

[Smart $]

Smart $,
As to the the calculations, did I do them right?

Well, your calculation method is fine and perhaps the better way to determine wire size, though as noted you can determine the wire size based on its temperature rating. Many do it the other way... guess at a wire size then multiply by the derating factor to see if that wire size (and type) meets the ampacity requirement.

And the second question is: is my foreman right when he said that the neutral should always be counted? I was told the neutral should be counted when the circuit supplies receptacles, lights, or when your loads are not balanced.

For your case, the 120V circuits' neutrals always count... but that's because they are two-wire circuits, not because they are neutral conductors. The same current flows on both circuit conductors. There is no balanced or unbalanced condition regarding a two-wire circuit. Some prefer the term "grounded conductor" to neutral because of such. Note that at least a three-wire circuit is necessary to not count the neutral conductor, but not true for all three-wire circuits with a neutral. See the code section infinity posted.

 

[Ivan Young]

Thanks Rob and Smart $ for your help, I now have a better understanding. I have one final question that has been bothering me for a long time and no one seem able to give me a straight correct answer, so here goes. Lets say for the same circuits you are told to use THHN wires, how would you handle that?
I picked a #8 wire for the 20 amp circuits because it is less than 100 amps and lets say the terminals are rated at 60c. I asked my foreman and he told me a #10 THHN wire is good. Another guy told me if you use a #10 THHN which is rated for 40 amps, you cannot load that circuit for more than 30 amps which is what the 60c column is rated for. He said you would have to use a #8 THHN wire which is rated at 55 amps in order to meet the 40 amp requirement in the 60c column.
Please help me to clear up this mystery so I can leave you guys in peace
Thanks

 

[Smart $]

Thanks Rob and Smart $ for your help, I now have a better understanding. I have one final question that has been bothering me for a long time and no one seem able to give me a straight correct answer, so here goes. Lets say for the same circuits you are told to use THHN wires, how would you handle that?
I picked a #8 wire for the 20 amp circuits because it is less than 100 amps and lets say the terminals are rated at 60c. I asked my foreman and he told me a #10 THHN wire is good. Another guy told me if you use a #10 THHN which is rated for 40 amps, you cannot load that circuit for more than 30 amps which is what the 60c column is rated for. He said you would have to use a #8 THHN wire which is rated at 55 amps in order to meet the 40 amp requirement in the 60c column.
Please help me to clear up this mystery so I can leave you guys in peace
Thanks

First, let me say that terminal temperature limitations determinations are based on load current, not the breaker rating. I only bring this up because in can make a difference with lightly loaded circuits.

Getting to your question, let's say the 20A circuits have a non-continuous load of 20A. #14 is the minimum wire size for 60? terminals. Article 240 forces the minimum size to #12, but this ends up having no bearing on the determination. Derating for CCCs to 50% pushes us up to wire having an allowable ampacity before adjustment to 40A (20A ? 50%). That puts us at #10 THHN rated 40A, minimum.

For the 40A circuit (similar conditions), #8 is the minimum for terminal rating. Derating for CCCs to 50% pushes us up to wire having an allowable ampacity before adjustment to 80A (40A ? 50%). That puts us at #4 THHN rated 95A, minimum.

 

[Ivan Young]

Thanks Smart $,
I thought than circuits rated under 100 amps were supposed to use the 60C column and over 100 amp the 75C column
I think the point that I have to remember is that even though the wires are rated for a higher amperage because of derating it is still a 20 amp circuit and can only be loaded as such. So basically what you are saying is after your derating you would pick the wire the new apmerage based on the wire type.

 

[Smart $]

I thought than circuits rated under 100 amps were supposed to use the 60C column and over 100 amp the 75C column

In general, yes. However, if the 100A or less equipment is listed for higher temperature terminations, you can use the column which matches that rating. Many <=100A panelboards are rated for 75? terminations.

I think the point that I have to remember is that even though the wires are rated for a higher amperage because of derating it is still a 20 amp circuit and can only be loaded as such.

Correct, essentially. However, there are cases where the circuit is rated less than the ocpd protecting it, such as when the final ampacity of the conductor does not correspond to a standard ocpd rating and you are permitted to use the next higher standard ocpd rating. For example, after derating a conductor has an adjusted ampacity of 28A. You may be permitted to protect this circuit with a 30A ocpd, yet you can only load it to 28A.

So basically what you are saying is after your derating you would pick the wire the new apmerage based on the wire type.

If I understand your statement correctly, yes you can use the tabled ampacity of the wire type for adjustments and corrections, but the wire size cannot be smaller than determined for terminal rating.

 

[Ivan Young]

Thanks again Smart $, you have cleared most of the questions I had. The last part of the question I messed up expressing it clearly.
Lets you were using TW wire, you would have actually use a #8, but if you are going to use THHN you would use a smaller wire #10.
Thanks for clearing up the mystery. No wonder you are Smart $
Thanks again

 

[Smart $]

Lets you were using TW wire, you would have actually use a #8, but if you are going to use THHN you would use a smaller wire #10.

That's correct.

 

[Ivan Young]

Gentlemen,
I have another question. When installing multi-wire cables in conduit, you are supposed to treat it as one conductor. My question is, where would you find the dimensions of the cable so would could figure out conduit fill?