Standard Calculation (Fixed appliances four or more)

[wireguy8169]

Hello,

I am using the text "Electrical Systems" for an NEC course (I used this book back in my apprenticship too), anyhow while doing the standard load calculation specifically the part for fixed appliances the text does not deduct the portion of the load for the water heater (240V is specified not a on demand or anything like that). Even on the website that the text gives a video tutorial it shows the same calculation and again does not deduct that portion of the load before doing the demand for the neutral part of the load. Here is the example

Dishwaser =1650 Disposer=950 Compactor=700 Water Heater=7500 Attic Fan=1400 for a total of 12,200VA

there are four or more appliances so I am permitted to apply the demand factor

So I use the demand 12200 x .75 = 9150VA (this is the general load) Phase

I go on to figure the neutral load, I re do the calculation deducting the water heater (this still leaves me four or more so I am permtitted to apply the demand factor)

So now I have (1650 +950+700+1400=4700VA )

Apply the demand factor 4700 x .75 = 3525VA (for my neutral load)

This is the answer I get
Phase = 9150 Neutral = 3525

The text gives an answer of phase 9150 and neutral 9150 (I do realize you do not have to apply a demand factor at all but this is what the chapter is covering)

Thanks

 

[Hv&Lv]

Hello,

I am using the text "Electrical Systems" for an NEC course (I used this book back in my apprenticship too), anyhow while doing the standard load calculation specifically the part for fixed appliances the text does not deduct the portion of the load for the water heater (240V is specified not a on demand or anything like that). Even on the website that the text gives a video tutorial it shows the same calculation and again does not deduct that portion of the load before doing the demand for the neutral part of the load. Here is the example

Dishwaser =1650 Disposer=950 Compactor=700 Water Heater=7500 Attic Fan=1400 for a total of 12,200VA

there are four or more appliances so I am permitted to apply the demand factor

So I use the demand 12200 x .75 = 9150VA (this is the general load) Phase

I go on to figure the neutral load, I re do the calculation deducting the water heater (this still leaves me four or more so I am permtitted to apply the demand factor)

So now I have (1650 +950+700+1400=4700VA )

Apply the demand factor 4700 x .75 = 3525VA (for my neutral load)

This is the answer I get
Phase = 9150 Neutral = 3525

The text gives an answer of phase 9150 and neutral 9150 (I do realize you do not have to apply a demand factor at all but this is what the chapter is covering)

Thanks

Maybe I am lost...
Where did you get that .75% deduction for the neutral from? Are you over 200 amps? I can't see where 220.61 would come into play here. Look at an example in Annex "D"

 

[Dennis Alwon]

Look at 220.53-- If it is a dwelling then you are allowed a 75% demand factor for calculations of a service or a feeder.

Look at your example and let's take out the water heater all together. There would be the calculation of 3525va for the neutral. 3525/120= 30 amps. There is no reason that the neutral would need to be larger than the ungrounded conductors so I think your calculation is correct. You would need to be careful that the grounded conductor is not ever smaller than the equipment grounding conductor. 215.2(2) In this case you have 9150va/ 240 = 38 amps thus a 40 amp circuit would be needed with a 10 egc. The neutral should work in this case.

 

[Hv&Lv]

Look at 220.53-- If it is a dwelling then you are allowed a 75% demand factor for calculations of a service or a feeder.

I was refering to the 75% on the neutral calculation.

 

[Dennis Alwon]

I was refering to the 75% on the neutral calculation.

Why would that not be the case. As I said if all the loads were 120V load the neutral would be reduced 75% also. No need for a neutral to be larger than the ungrounded conductors.

 

[wireguy8169]

Maybe I am lost...
Where did you get that .75% deduction for the neutral from? Are you over 200 amps? I can't see where 220.61 would come into play here. Look at an example in Annex "D"

In the text they are showing the steps of the Standard calculation for a one family dwelling. The fixed appliance calculation (220.53 is for the appliances four or more.) Below is what they are showing in the calculation I have to figure the neutral. If I am figuring a neutral load regardless of the total ampacity of the service at the end, I would only use those loads that utilize a neutral (or of course I can use a full size neutral but that is not what is being asked) So if I do the phase calculation part as is done I come up with 9150VA for the phase.

When I do the neutral part I pull out the Water Heater load due to no neutral (still have four fixed in place appliaces so I can use 250.53 as a demand factor) I then end up with (12200 - 7500= 4200) apply the demand factor from 220.53 (75%) and I end up with 3525.

Hope that makes it clear, I appreciate the input. I feel I am correct it makes sense to me and well not to sound like Mr. Spoke but seems logical given the way I read it and the fact that I do not end up below four appliances and that the water heater has no neutral. On another note if I only had a total of four appliances (one being the water heater) I could then use the demand factor to figure the phase load but when I go to figure the neutral load I would be pulling out the hot water at which point when I figure the neutral part of the load I would have less than four appliances (total appliances 4 -1 water heater) thus I would have to apply the full calculated fixed appliance load to the neutral for those three appliances (minus the water heater of course) Hope I did not muddy things up to much with this last part just wanted to give my full thought on this.

Thanks

2. FIXED APPLIANCES: 220.53
l
Dishwasher = 1,650 VA
Disposer = 950 VA
Compactor = 700 VA
Water Heater = 7,500 VA
Attic Fan = 1,400 VA


Φ Total = 12,200 VA x 75% = 9,150 VA 9,150 VA - VA
N Total = 0 VA x 75% = - VA

 

[wireguy8169]

Some More Clarification to the question

Some More Clarification to the question

I just wanted to clarify a few things that may be lost in my OP.

First this is a full Service Calculation using the Standard Method
Second, the way the text breaks it down they take each section (General Lighting, Small Appliance, Cooking equipment, and so on) seperately and apply the demand factors to either the phase or the phase and neutral if it appllies to that section.

So what I have as I stated in the original post is the Fixed in place appliance loads, all of which are 120V except the Water Heater.

I do the calculation and I end up with my calculated KVA, for which I apply a 75% demand factor to due to the fact that there are four or more appliances.

Now the text going through the steps shows a Phase of 9150VA (after the demand factor was applied) and a Neutral demand of 9150VA.

What got me to thinking was the fact that the Water heater is a line to line not line to neutral load so that part of the deman would not be required to be applied to the neutral (regardless of service final ampacity).

I did look at the load calculator on the MH site and it actually even does it different than that.

What that calculator does is takes all the fixed appliance or special appliances as it is listed does the calculation, if there are four or more it applies the demand factor of 75%, but then uses the calculated load before the demand factor was applied in the final feeder calculation.

So I am just wonder if I have been missing something all these years (I actually rarely have had to use the full residential calculation)


IMHO I think that doing the load calculation in this manner is more confusing, I think after you get done with the general lighting loads, small appliance and laundry, things should be broken down into line to line and line to neutral loads. Doing this would give a better representation of what one has for both loads. My experience has bee that the majority of the time other than in the class room, you calculate the phase load and either go to the counter and order a 100A SE or 200A SE cable or Make the neutral the same size as your ungrounded conductors if using individual condutors.

Hope this makes sense

Thanks

 

[Hv&Lv]

I just wanted to clarify a few things that may be lost in my OP.

First this is a full Service Calculation using the Standard Method
Second, the way the text breaks it down they take each section (General Lighting, Small Appliance, Cooking equipment, and so on) seperately and apply the demand factors to either the phase or the phase and neutral if it appllies to that section.

So what I have as I stated in the original post is the Fixed in place appliance loads, all of which are 120V except the Water Heater.

I do the calculation and I end up with my calculated KVA, for which I apply a 75% demand factor to due to the fact that there are four or more appliances.

Now the text going through the steps shows a Phase of 9150VA (after the demand factor was applied) and a Neutral demand of 9150VA.

What got me to thinking was the fact that the Water heater is a line to line not line to neutral load so that part of the deman would not be required to be applied to the neutral (regardless of service final ampacity).

I did look at the load calculator on the MH site and it actually even does it different than that.

What that calculator does is takes all the fixed appliance or special appliances as it is listed does the calculation, if there are four or more it applies the demand factor of 75%, but then uses the calculated load before the demand factor was applied in the final feeder calculation.

So I am just wonder if I have been missing something all these years (I actually rarely have had to use the full residential calculation)


IMHO I think that doing the load calculation in this manner is more confusing, I think after you get done with the general lighting loads, small appliance and laundry, things should be broken down into line to line and line to neutral loads. Doing this would give a better representation of what one has for both loads. My experience has bee that the majority of the time other than in the class room, you calculate the phase load and either go to the counter and order a 100A SE or 200A SE cable or Make the neutral the same size as your ungrounded conductors if using individual condutors.

Hope this makes sense

Thanks

You were right in your calcs IMO. I was lost in the way the first post was written and immediately jumped on the 70% reduction number referenced in another article. Sorry if it caused any confusion. I need to slow down in my reading. Apologies to Dennis also.

 

[wireguy8169]

You were right in your calcs IMO. I was lost in the way the first post was written and immediately jumped on the 70% reduction number referenced in another article. Sorry if it caused any confusion. I need to slow down in my reading. Apologies to Dennis also.

No problem I appreciate all the input helps to make me look deeper. Another thing I have found by way of municipality sheets for doing this calculation, other load calculators from other text or online is this that none of them seem to take into account that the hot water heater while it does fall under a fixed appliance or special appliance, it unlike most of the other appiances utilizes 240V (no neutral).

In a real world situation the load being added to the neutral load thus increasing the size of the neutral conductor would not make that much of a difference in what really matters (dollars and cents) unless of course its a tight bid or your talking a significant amount of cable with todays copper prices. On the other hand in the class room where students are trying to prepare for a test situation it comes down to how the test maker applies the information (in some cases anyhow)

Thanks again