Commenting on the second part of your post first...
Bjenks said:
I ask this because Mike Holt's Electrical NEC exam prep example 11.20 keeps taking the full VA rating of the receptacle for each individual circuit (which I am assuming means dedicated). So for 30 circuits he has a load of 72kW which is 300A on a 240 service. If they are all PCs at 400W it would seem very excessive.
For dedicated outlets with known loads you use the actual load, period. If the load type is known but the actual load it puts on the system is not, use your best disgression to determine what the load will likely be. If you cannot, for whatever reason, use the full capacity of the dedicated circuit. In doing so, also take into consideration whether the load is continuous or non-continuous, as the former is only permitted to load the circuit to 80% of its ampacity rating (e.g. 16A max on a 20A circuit). These circuits fall under 220.14(A).
I am not familiar with the example you are referring to. Not knowing all the particular parameters I cannot comment on its validity. In your case, if you have 30 dedicated-use circuits at 400W each, the calculated load will include 12kVA, and nothing more, for these circuits.
Bjenks said:
When I do load calculations for a commercial building I first calculation the general receptacle loads. I do this by taking the larger of 1VAxSq ft of the building versus the number of 5-15/20 receptacles x 180VA. I have several questions on this: If I know any of the loads that are going to be on a yoke do I take it out of the 180VA calculation and put in the actual load?
This is a somewhat loaded (pun intended) question. I'll approach the answer assuming the load or loads are on a semi-dedicated or general-use circuit, as I discussed dedicated circuits above.
The first question to ask is how many receptacles are on the circuit. If it is maxed out, then you cannot use actual loads over 180VA and still use 180VA for the non-specific loads on this circuit, as that would put you over the ampacity of the circuit. For example, you have a 20A circuit with max no. of receptacles. Putting a 400VA load on one of the receptacles will diminish the available power to the remaining receptacles to 2000VA. That would be 2000VA ? 12 recepts. = ~167VA per receptacle. In this case, use 180VA per receptacle.
However, if this is a semi-dedicated circuit, i.e. the number of receptacles is less than max and certain loads are known, then by all means use specific loads over 180 VA and 180VA for each unused recept., as long as the total VA on the circuit, using 125% of continuous loads, does not exceed its ampacity times its voltage. (BTW, 125% of load is the inverse calculation of the 80% of ampacity mentioned earlier for continuous loads.)
Bjenks said:
What if one of the receptacles gets changed to a dedicated 20A receptacle with an unknown load do I now change that load to 120x20A? What if the customer says I want 30 dedicated receptacles for each of his PCs that are rated at 400W do I use 400W or 20A x 120V or 180VA?
Answered above.
I make no warranty on the credibility of the answers above :grin:
[minor editing for readability]
