Panel Loading Calculation

Status
Not open for further replies.

jpalme07

Member
Hey guys...I'm also a Noob and loooking for some help.

I'm working on a project where we are installing a new 60A branch circuit in an existing panelboard and the jurisdiction wants us to verify the existing feeders are adequate for our loading. Our actual new design load is in the 7.24 KVA range.

The panel board is 225A with #2 wires feeding it. It is 120/208V, 3phase, 4W. The C/B's are as follows:
(11) 20A, 1p, receptacle breakers
(1) 30A, 3p, heater (for pool) breaker
(4) 30A, 1p dedicated receptacle for treadmills breakers


Is there a quick way for me to estimate loading? The examiner tried to tell me us 80% of the breaker size...does that seem right? Can I estimate a 20A, 1p at 1.92 KVA (20A * 80% * 120)? How do I deal with the three phase load estimate?

I'm guessing using this conservative estimation will cause us to have to upgrade wire size, but without tracing all 16 circuits, how else can this be approached?

Any help appreciated!
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Keep in mind that you don?t have to prove that you know exactly what the load is. All you have to prove is that adding the new load will not overload the panel or its feeder conductors. There are several ways of doing this. One way, as you suggested, is to trace out all circuits, find out exactly what is powered by each, write out the list, add up the VA, and convert to amps. Your facility is relatively small, so this is actually a viable alternative. But it is not necessary.

Another way is to install an ammeter on the feeder, and measure the load for 30 days, at 15 minute increments. Take the maximum load that you measure, add 25%, then add the proposed new load to that value. As long as you are still below 225 amps (the panel rating), you have successfully demonstrated that your new load is acceptable. This process is described in the Exception to NEC 220.87.

In your case, there is an easy way out. Please note that this method does not establish the design basis for the facility, but it does prove that your new load will not overload the panel. What you do is to perform a ?worst case? calculation that assumes each breaker is loaded to 80% of its rating.
? (11 circuits) times (20 amps) times (80%) times (120 volts) = 21,100 VA
? (1 circuit) times (30 amps) times (80%) times (208 volts) times (1.732) = 8,646 VA
? (4 circuits) times (30 amps) times (80%) times (120 volts) = 11,520 VA
? Total load VA before addition = 41,286 VA.
? Additional load = 7,240 VA.
? Total load after addition = 48,526 VA
? Divide 48,526 by 208 volts, and divide again by 1.732, and you get 135 amps.
? 135 is well below the panel rating of 225.
? This proves that no matter what the actual load is, the addition cannot cause an overload of the existing panel.
? Q.E.D.

Welcometo the forum.
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
How are you claiming 225 amps with #2 Even if you were using 2? it would be 175 amps.
 

barone

Member
Follow your conductors

Follow your conductors

You should be more concerned with how much load you have on your feeder conductors rather than the panel that is connected at the end of them (panel size is also important, but less so in your case). If you have #2's feeding a 225Amp panel board then you are limited to the neighborhood of 115amps, not taking into account any derating factors.

Another method of determining existing load is to request that the owner provide you with 1 year of electrical bills with the recorded demand, in lieu of using the recording meter. But keep in mind this will only do for the service lateral or a single feeder fed from a single meter. If you have distribution upstream from your panel this will not work for you.
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
It is not basic to an engineering curriculum. They don?t cover this information in a BS-EE degree program. We have to learn it in the field, and we don?t learn it all on day one.here.
I am completely agree with charlie. This is not subject of any engineering in university. we should not care of easy or difficult question, we must just help and solve the problem.
 

jpalme07

Member
Wow...lots of great stuff! Thanks Charlie and everyone for the input. If I know the (11) 20A breakers are all receptacles...can I use the demand factors to further reduce the loading per Table 220.42 in the NEC? If so, we could probably prove in the 115A feeders as adequate. This panel is way downstream in a large hotel (so electric bills won't help).
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Wow...lots of great stuff! Thanks Charlie and everyone for the input. If I know the (11) 20A breakers are all receptacles...can I use the demand factors to further reduce the loading per Table 220.42 in the NEC? If so, we could probably prove in the 115A feeders as adequate. This panel is way downstream in a large hotel (so electric bills won't help).
Not table 220.42, but table 220.44. Also, don't forget Charlie's calculation is a quick "worst case" calc. If you can find out how many receptacles on each circuit and the actual load of the heater and treadmills, you can do a more accurate calculation.

What size breaker is feeding the #2s to the panel?
 

Smart $

Esteemed Member
Location
Ohio
... If I know the (11) 20A breakers are all receptacles...can I use the demand factors to further reduce the loading per Table 220.42 in the NEC? If so, we could probably prove in the 115A feeders as adequate. This panel is way downstream in a large hotel (so electric bills won't help).
Not table 220.42, but table 220.44. Also, don't forget Charlie's calculation is a quick "worst case" calc. If you can find out how many receptacles on each circuit and the actual load of the heater and treadmills, you can do a more accurate calculation.

What size breaker is feeding the #2s to the panel?
Actually, charlie's calculation is quick, but it's not the worst case scenario calc. Worst case receptacle loads would be calculated at 100% capacity, or 20A (2400VA) per circuit, as these loads could very well be non-continuous loads.

I agree with david... with the situation as presented, you are best off counting your receptacles at 180VA each, applying the demand factor if warranted, and determining the exact dedicated loads on the subsystem... essentially doing a 100% accurate NEC load calculation per Article 220.
 

Hameedulla-Ekhlas

Senior Member
Location
AFG
Wow...lots of great stuff! Thanks Charlie and everyone for the input. If I know the (11) 20A breakers are all receptacles...can I use the demand factors to further reduce the loading per Table 220.42 in the NEC? If so, we could probably prove in the 115A feeders as adequate. This panel is way downstream in a large hotel (so electric bills won't help).
I think (11) 20A breaker for receptacles are the existing. If they are existing, the demand factor has already been applied. You dont need to apply or change the existing load because calculation has been done already.

So, for your new load you can apply demand factors .
 
Status
Not open for further replies.
Top