Calculating Receptacle Layout: VA Calculations

[BlaineXD]

To All:

I have a quick calculation check if anyone is willing. I am scheduling a receptacle panel and want to make sure my thought process is correct.

For the plan, I have a total of 100 receptacles. This is a mix of duplex and quad, and when I change the quads to reflect two duplexs each, I come out to have 117 "duplex" receptacles; 50 continuous load, 67 non-continuous load.

50 x 1.25 x 180VA = 11,250VA (Continuous)
67 x 180VA = 12060VA, but since this is non-continuous over 10,000VA, I have applied a 50% demand factor to everything above 10,000VA. I come up with 11,030VA. (Non-continuous)

Total: 22,280VA.

I am using single-phase 120V, so I then divide 22,280VA / 120V = 185.666A. If I am understanding everything, this will be the total expected load going through my panel.

I am using 20A breakers, which are not supposed to have more than 80% load on them (16A), so I take 185.666A / 16A = 11.6.
So I understand this to mean that I can space roughly 11 receptacles on each 20A circuit breaker and still be ok?

Does any of that make sense to anyone? I am still kinda new to all this. Thanks for taking the time to read and respond.
--Nate

 

[ron]

I'm not sure what your end result question is but if I read your thoughts correctly, this might help:

http://www.mikeholt.com/technical.p...240&type=u&title=NEC Articles 215 through 240

[FONT=Arial, Helvetica, sans-serif]Q. What is the maximum number of duplex receptacles on a two-wire 20 ampere circuit?[/FONT]
[FONT=Arial, Helvetica, sans-serif]Section 220-3(c)(6). The NEC does not have a specific rule that says 10 receptacles on a 15 ampere circuit or 13 receptacles on a 20 ampere circuit. For 15 ampere circuits, the calculation is as follows: 120 volts x 15 amperes = 1800 VA/180 VA = 10 receptacles, and for 20 ampere circuits, 120 volts x 20 amperes = 2400 VA/180 VA = 13 receptacles.[/FONT]

• [FONT=Arial, Helvetica, sans-serif]Many people think that when calculating the number of receptacles the load should be reduced by 80%, but this is not the case! Yes, the NEC limits the load on receptacle circuits to 80% for portable appliances [210-23(a)] and, yes, the maximum continuous load on a branch circuit shall not exceed 80% of the circuit rating (protection device) [220-3(a) and 384-16(c)], but these rules don't apply to the number of receptacles per circuit. I wish the NEC were more specific, but it's not.[/FONT]

 

[charlie b]

Without checking your math, I can see what appears to me to be a contradiction in approach. How can a receptacle be a continuous load? If you know what is going to be plugged in to a given receptacle, and if you know that it will be operating at full capacity for periods in excess of 3 hours, then you should be calculating its load on the basis of the nameplate rating, not on the basis of 180 VA per receptacle. I would treat such outlets under 220.14(A), with the understanding that they are not included in the scope of 220.14(I).

 

[david luchini]

To All:

I have a quick calculation check if anyone is willing. I am scheduling a receptacle panel and want to make sure my thought process is correct.

For the plan, I have a total of 100 receptacles. This is a mix of duplex and quad, and when I change the quads to reflect two duplexs each, I come out to have 117 "duplex" receptacles; 50 continuous load, 67 non-continuous load.

50 x 1.25 x 180VA = 11,250VA (Continuous)
67 x 180VA = 12060VA, but since this is non-continuous over 10,000VA, I have applied a 50% demand factor to everything above 10,000VA. I come up with 11,030VA. (Non-continuous)

Total: 22,280VA.

The demand factor applies to feeders and services. You appear to be calculating branch circuit loads...you cannot apply the demand factor to branch circuits.

I am using single-phase 120V, so I then divide 22,280VA / 120V = 185.666A. If I am understanding everything, this will be the total expected load going through my panel.

I am using 20A breakers, which are not supposed to have more than 80% load on them (16A), so I take 185.666A / 16A = 11.6.
So I understand this to mean that I can space roughly 11 receptacles on each 20A circuit breaker and still be ok?

Does any of that make sense to anyone? I am still kinda new to all this. Thanks for taking the time to read and respond.
--Nate

You have applied 80% twice...first for the continuous load then on the breaker. The breaker can be loaded to 20A non-continuous or 16A continuous, or something in between for a combination of continuous and non-continuous loads. (I wouldn't generally consider receptacle loads as continuous, but you may have a reason to do so.)

 

[Smart $]

Without checking your math, I can see what appears to me to be a contradiction in approach. How can a receptacle be a continuous load? If you know what is going to be plugged in to a given receptacle, and if you know that it will be operating at full capacity for periods in excess of 3 hours, then you should be calculating its load on the basis of the nameplate rating, not on the basis of 180 VA per receptacle. I would treat such outlets under 220.14(A), with the understanding that they are not included in the scope of 220.14(I).

I agree.

 

[BlaineXD]

I'm not sure what your end result question is but if I read your thoughts correctly, this might help:

http://www.mikeholt.com/technical.p...240&type=u&title=NEC Articles 215 through 240

Q. What is the maximum number of duplex receptacles on a two-wire 20 ampere circuit?
Section 220-3(c)(6). The NEC does not have a specific rule that says 10 receptacles on a 15 ampere circuit or 13 receptacles on a 20 ampere circuit. For 15 ampere circuits, the calculation is as follows: 120 volts x 15 amperes = 1800 VA/180 VA = 10 receptacles, and for 20 ampere circuits, 120 volts x 20 amperes = 2400 VA/180 VA = 13 receptacles.

• Many people think that when calculating the number of receptacles the load should be reduced by 80%, but this is not the case! Yes, the NEC limits the load on receptacle circuits to 80% for portable appliances [210-23(a)] and, yes, the maximum continuous load on a branch circuit shall not exceed 80% of the circuit rating (protection device) [220-3(a) and 384-16(c)], but these rules don't apply to the number of receptacles per circuit. I wish the NEC were more specific, but it's not.

This is exactly what I was looking for! I appreciate all the comments from others as well. I especially appreciate everyone pointing out the continuous and non-continuous load differences. I'm going to re-assess my calculation thought process, and hit the computation sheets again. Have a great day!
--Nate

 

[suemarkp]

You may also want to think about what really will be connected to these receptacles. Code says use 180VA if you don't know what will be there, but if you think it will be an office cube with a space heater in it, or have a computer, two monitors, a printer, a desk light, ... then you will be over 180VA. You don't want to trip the breaker because some circuits are over burdened and others under utilized.

So you may not want to put as many receptacles on the branch circuit as code allows. I try to limit it to 4 or 5 receptacles per 20A circuit at most, and many times just a box or two (quad or single), unless it is residential in which case it could be a lot more. I tend to have power run to labs where we have quite a bit of differing equipment that could be brought to one location, so it is nice to have multiple 20A circuits in a given room. Conversely, a long hallway with receptacles could probably have a bunch on one circuit, as could a "social cluster" where all you have is 20 phone chargers plugged into every outlet in the room.