The 80% rule is not specific to plug connections. The excerpt below shows the rule applying to all branch circuits. However, if you search through the code, you will find similar clauses for feeders and services.
NFPA 70 210.19 Conductors - Minimum Ampacity and Size. (A) Branch Circuits Not More Than 600 Volts (1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor sized, before the application of any adjustment or correction factors, shall have an allowabvle ampacity not less than the noncontinuous load plus 125 percent of the continuous load. Exception No. 1: Where the assembly, including overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the branch circuit conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load. If you ask 10 code experts about the origin of this requirement, you will likely get 11 answers. Exception 1 seems to indicate that ratings for circuit breakers are part of the issue. However, my NEC Code Handbook indicates that the rule is designed to prevent overheating of conductors. One explanation of the difference between North America and European requirements goes back to voltage, construction methods and history. The higher voltages of Europe lead to lower currents for a given power consumption. Brick and stone construction is more common in Europe, particularly in the cities where electrification started. As a result, there was a lower fire risk due to high currents heating conductors. Electric shock remained the focus of codes and fire risk was a lesser concern. The United States and Canada run off of a lower voltage and there is a greater prevalence of wood-frame construction. The quick adoption of high-current appliances such as air conditioners and kitchen appliances resulted in a much higher demand for power and higher currents. The resulting fire risk from overheated conductors would have been higher making it a stronger focus of North American codes. The amount of power available in even a small American home is fairly large. It's rare to see even small homes built with less than a 150 A service with large homes sometimes having a 400 A service. That gives plenty of power to overheat wires with. Ted Eckert Compliance Engineer Microsoft Corporation ted.eck...@microsoft.com<mailto:ted.eck...@microsoft.com> The opinions expressed are my own and do not necessarily reflect those of my employer. -----Original Message----- From: Kunde, Brian [mailto:brian_ku...@lecotc.com] Sent: Wednesday, May 16, 2012 7:45 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: Circuit breakers in Europe Since this topic has been brought up, I'm hoping you experts could clarify the 80% rule for me for North America (don't forget Canada). I was given the impression that the 80% rule (as we call it) applies to the maximum continuous current of the plug rating regardless of whether there is one receptacle or multiple receptacles on that circuit. A product can draw 100% the current rating of the plug for periods of time but not more than 80% average current or continuous current. The reason for the rule is to avoid overheating in the plug/receptacle connection. Is this right or wrong? So say I have a 30 amp plug on my product and it is wired into a dedicated receptacle and circuit. My product can be rated and draw up to 30 amps but it cannot draw more than 24 amps continuous. Correct? If the product is wired direct without the plug/receptacle it can draw up to 100% of the circuit rating continuously. Correct? Please confirm or clarify. We gathered this information from CSA who inspects our products in Canada and if we rate a product over 80% of the plug rating they require we also provide an average current and/or continuous current rating for that product. Thank you. The Other Brian -----Original Message----- From: emc-p...@ieee.org<mailto:emc-p...@ieee.org> [mailto:emc-p...@ieee.org]<mailto:[mailto:emc-p...@ieee.org]> On Behalf Of Pete Perkins Sent: Tuesday, May 15, 2012 6:07 PM To: EMC-PSTC@LISTSERV.IEEE.ORG<mailto:EMC-PSTC@LISTSERV.IEEE.ORG> Subject: RE: [PSES] Circuit breakers in Europe PSNet, One issue that I'm not sure has been reflected in this thread is that the NA 80% restriction for utilization circuits is that it is assumed that there will be a number of duplex plugs wired into each circuit, as allowed by the NA code. Therefore, any single product may not use the entire rated current from the circuit (20A, for instance) but is limited (80% = 16A) so that other products may be plugged into the same circuit with out overloading that circuit. If a product uses the full, rated current from a circuit breaker then that product must be wired to a 'dedicated ccircuit' meaning that an electrician must be called to wire in a circuit with only one outlet (not a duplex outlet) on that breaker. (After getting the required permit, it might cost as much to wire a new circuit - if it is even possible - as many commercial electronic products cost.) It's pretty clear that customers do not want to hire an electrician to wire a new circuit for a new product thus the pressure to meet the 80% limit with the products. Industrial customers deal with rewiring to code better than commercial or household folks do. Been there, done that. :>) br, Pete Peter E Perkins, PE Principal Product Safety Engineer PO Box 23427 Tigard, ORe 97281-3427 503/452-1201 fone/fax p.perk...@ieee.org<mailto:p.perk...@ieee.org> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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