The code is clear in that the calculated load on a circuit shall not exceed the circuit's maximum capacity, but does the code allow the actual total connected load to exceed the circuit's maximum capacity? For instance, the calculated load on a 20 amp 120 volt branch circuit with 13 general duplex receptacles in a commercial office is 2340 VA @ 180 VA per receptacle and is within the circuit's maximum capacity of 2400 VA. Plugged into the same circuit is a number of small office appliances such as printers, a copy machine, computers, desk calculators, battery chargers, fax machines, typewriters, task lighting, etc. with a total connected load based on nameplate data exceeding 2400 VA. Yet, the actual recorded averaged peak demand over a one week period is less than 50% of the circuit's capacity. This is owing to the fact that nameplate data represent maximum values and that many office appliances operate in spurts and at different times and at various loadings. Hope this example is clear. Appreciate any response. Thanks.
Can actual total connected load exceed circuit capacity?
- Thread starter gonggd
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Re: Can actual total connected load exceed circuit capacity?
If every connected load equipment, on all services, throughout the US, are turned on at the same time there would be blackouts everwhere.
If the load is all self starting after an outage, there will never be any electricity restored.
Without load/time diversity we would have meltdown.
[ August 28, 2003, 03:58 PM: Message edited by: bennie ]
If every connected load equipment, on all services, throughout the US, are turned on at the same time there would be blackouts everwhere.
If the load is all self starting after an outage, there will never be any electricity restored.
Without load/time diversity we would have meltdown.
[ August 28, 2003, 03:58 PM: Message edited by: bennie ]
websparky
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Re: Can actual total connected load exceed circuit capacity?
gonggd,
I know it seems wrong but, if all of the devices were drawing their full load at the same time, the 20A CB would trip. Like you said, the nameplate rating is "max" load, not typical load.
Take a look at this below;
Table 220.13 Demand Factors for Nondwelling Receptacle Loads
Portion of Receptacle Load to Which Demand Factor Applies (Volt-Amperes)
Demand Factor (Percent)
First 10 kVA or less at 100
Remainder over 10 kVA at 50
gonggd,
I know it seems wrong but, if all of the devices were drawing their full load at the same time, the 20A CB would trip. Like you said, the nameplate rating is "max" load, not typical load.
Take a look at this below;
Table 220.13 Demand Factors for Nondwelling Receptacle Loads
Portion of Receptacle Load to Which Demand Factor Applies (Volt-Amperes)
Demand Factor (Percent)
First 10 kVA or less at 100
Remainder over 10 kVA at 50
Re: Can actual total connected load exceed circuit capacity?
Gonggd:
If a receptacle is installed for a specific appliance, that appliances nameplate data should be used 220.3B1. However, if it is a general purpose receptacle, you use the 180 VA per outlet.
No diversity factors are allowed for a branch circuit (50% over 10kva applies to feeders and services in nondwellings units). If it is a general use outlet, the NEC (nor you) can not dictate or predict what someone will plug in. Adding nameplate ratings and comparing to the acutal use is both interesting and informative, but the 180VA per outlet is the only NEC number I would compare them to.
Steve
Gonggd:
If a receptacle is installed for a specific appliance, that appliances nameplate data should be used 220.3B1. However, if it is a general purpose receptacle, you use the 180 VA per outlet.
No diversity factors are allowed for a branch circuit (50% over 10kva applies to feeders and services in nondwellings units). If it is a general use outlet, the NEC (nor you) can not dictate or predict what someone will plug in. Adding nameplate ratings and comparing to the acutal use is both interesting and informative, but the 180VA per outlet is the only NEC number I would compare them to.
Steve
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Can actual total connected load exceed circuit capacity?
Please note, however, that every single load that is connected is taken into account, one way or another, in the load calculation. The difference between ?calculated load? and ?total connected load? comes from two types of NEC rules. One type of rule lets us calculate a load based on something other than the device?s nameplate. Using 180 VA per receptacle outlet is one example of this type of rule. The other type lets us take credit for the likelihood that not everything will be running at the same time. Applying a 65% demand factor when an apartment complex has 7 dryers is an example of this type of rule.
Once the facility is built, the code no longer keeps track of how many vacuum cleaners, hair dryers, lights, and stereos the homeowner plugs in at a time (i.e., the 180 VA per receptacle does not change). If too many things are plugged in, the circuit breaker will trip. It may annoy the homeowner, but it is safe, and that is all the NEC cares about. On the other hand, if the homeowner wants to add more receptacles or to install a new hot tub, then the relationship between ?calculated load? and ?total connected load? is no longer the same. The load calculation needs to be revised to account for the new load.
Yes.
Please note, however, that every single load that is connected is taken into account, one way or another, in the load calculation. The difference between ?calculated load? and ?total connected load? comes from two types of NEC rules. One type of rule lets us calculate a load based on something other than the device?s nameplate. Using 180 VA per receptacle outlet is one example of this type of rule. The other type lets us take credit for the likelihood that not everything will be running at the same time. Applying a 65% demand factor when an apartment complex has 7 dryers is an example of this type of rule.
Once the facility is built, the code no longer keeps track of how many vacuum cleaners, hair dryers, lights, and stereos the homeowner plugs in at a time (i.e., the 180 VA per receptacle does not change). If too many things are plugged in, the circuit breaker will trip. It may annoy the homeowner, but it is safe, and that is all the NEC cares about. On the other hand, if the homeowner wants to add more receptacles or to install a new hot tub, then the relationship between ?calculated load? and ?total connected load? is no longer the same. The load calculation needs to be revised to account for the new load.
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