Continuous loads

[DavidFahey]

THe code states the continiuos loads are those loads use for 3 hrs or more. Would that include Computer and Monitiors? Silly question but if you have an office building with 60 computers at 600 watts then at noncontinuous that would be 36000 watts but a continuious it would be 45000 watts there is a big difference on a 120 load thats 96 amps, if I did the math right. So what is the soultion?

 

[stevee]

First, remember the "continuous load" rule is used for sizing of conductors and OCPD's not the actual amount of load being used. If you have 60 computers at 600W each then you have 36kW of load at 120V but your conductors and OCPD's would have to be sized for 45kW of load for temperature rise due to the continuous load. So let's look at an example...

Each computer is rated 600W. This means you can put up to 3.2 computers on a single 20A circuit using #12 copper conductors. Since you can't connect 0.2 of a computer you have to round down to 3 computers per 20A 120V circuits. This means you will need 20 - 20A 120V circuits for 60 computers.

Ok, now lets assume all the computers will be fed from the same panelboard and no other load is connected to this panelboard. If you are in a large commercial building I would have to assume you are utilizing a 3p-4w, 208/120v system. If you balance your loads that would mean you have 7 circuits on phase A, 7 circuits on phase B and 6 circuits on phase C. Again, each branch circuit is 120V, 20A using #12 copper conductors.

To determine the panelboard rating is quite simple using Ohms Law for Power.
If P = E x I x 1.73(for 3Ph), you can determine ampacity by transposing the formula to solve for current and plugging in the known variables.
I = P / E x 1.73
if
P= 60 x 600W = 36000W
and
E= 208V
than
I = 36,000 / 359.84 = 100A

Since the load is continuous you need to multiply the current by 125% to determine conductor ampacity and OCPD size.

100A x 125% = 125A

So the rating for the feeder OCPD supplying the panelboard would be 125A, and you would need to use #1 copper conductors (at 75'C). Again, all of the branch circuits (20 of them) would be 20A, single pole (120V) circuits using #12 copper conductors.

Keep in mind, computer loads are non-linear so oversizing the neutral conductor may need to be considered.

In addition since you typically don't install a panelboard that does not have extra capacity I would recommend that you increase the capacity to 200A, it wouldn't be much more expensive and you would have the flexibility to connect more load if necessary.

Just my thought.............

 

[DavidFahey]

All that makes since but when your doing a service and feeder calculation continuous loads are considered like lighting and any other load that is on for 3 hrs or more and that is my question are computers considered to be continuous or not does 3 hrs mean 3 hrs not matter what the equipment but thanks for your imput

 

[infinity]

A computer is not a continuous load. By definition it would have to draw it's maximum current for 3 hours or more. That means that the hard drive CD-ROM, DVD and every other running aspect of the PC would have to be running at maximum current for three continuous hours. This will never happen.

Here's the actual Article 100 definition:

Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

 

[Doug S.]

A computer is not a continuous load. By definition it would have to draw it's maximum current for 3 hours or more. That means that the hard drive CD-ROM, DVD and every other running aspect of the PC would have to be running at maximum current for three continuous hours. This will never happen.

+1 for that.