Brian,
In order to get 100% continuous current through a circuit breaker in North America, the breaker has to be rated for 100% use. I have heard and suspected that these are rare, somewhat of a special-order item, and naturally, more expensive than the common circuit breaker that cannot handle 100% current continuously. Actually, when selecting the components of branch circuits, you have to consider more than the ampacity of the conductor and the circuit breaker. Temperature, particularly at the terminations, is the first item that has to be checked in sizing conductors in a wiring system. Per the National Electrical Code, you size the conductors based on NEC 110.14(C)(1). This means that unless both terminations of a conductor are Listed and marked otherwise, the ampacity of a conductor under 100 A is based on the 60°C column of Table 310.15(B)(16) (formerly Table 310.16), and for over 100 A or over 1 AWG, the ampacity of the conductor is based on the 75°C column of Table 310.15(B)(16). Afterwards, you can use higher temperature conductors for temperature adjustments. Working in telecommunication infrastructure, I have caught lots of people over the years choosing wire sizes for tray-cable systems using the free-air tables (former Table 310.17), or using the 90°C column because they were using THHN wire. So, given the above, you either size a standard branch circuit with receptacles and luminaires as prescribed in Article 210. But, if you have dedicated loads, you need to consider whether your loads are continuous or not, size the branch circuit for the sum of the non-continuous loads plus 125% of the continuous loads, then consider the temperature ratings of the terminations, because they are the weak link. Finally, as Pete mentioned, and following the do-as-I-say-not-as-I-do rule in the lab, circuit breakers can get very hot to touch at 100% rating, and they sometimes trip early because of the heat. Best regards, DON GIES ALCATEL-LUCENT SENIOR PRODUCT COMPLIANCE ENGINEER BELL LABS - GLOBAL PRODUCT COMPLIANCE LABORATORY 600-700 Mountain Avenue Room 5B-104 Murray Hill, NJ 07974-0636 USA Phone: +1 908 582 5978 Fax: +1 908 582 0582 [email protected] MEMBER, ALCATEL-LUCENT TECHNICAL ACADEMY From: Kunde, Brian [mailto:[email protected]] Sent: Thursday, May 17, 2012 11:17 AM To: [email protected] Subject: Re: Circuit breakers in Europe (Continuous vs. Non-Continuous Loads) Question, in North America are common branch circuit overcurrent protection devices (circuit breakers) listed for operation at 100 percent of its rating? If not, I assume you can purchase circuit breakers that are rated for 100% operation. Like you said, NEC 210-19 specifies the current rating of the conductor, or wire. Isnt the wire used in building wiring normally sized to handle 100% the continuous current rating of the branch circuit? For instance, wont 14AWG handle 15 amps continuously? 12AWG = 20 amps, 10AWG = 30 amps, and so on? NEC 210.23 Permissible Loads, states An individual branch circuit shall be permitted to supply any load for which it is rated. The rest of that section refers to branch circuits supplying more than one outlet, receptacle or load. So I assume for individual branch circuits or a dedicated circuit seeing a load from a single device, the device can draw 100% continuous current of the branch circuit rating as long as the conductors and overcurrent device is so rated. I think this is what Pete was referring to in his email. Every time this topic comes up I learn something new. Thanks to all. The Other Brian From: Don Gies [mailto:[email protected]] Sent: Wednesday, May 16, 2012 12:11 PM To: Kunde, Brian; [email protected] Subject: Re: Circuit breakers in Europe (Continuous vs. Non-Continuous Loads) Other Brian, In the US and Canada, branch circuits and other circuits (e.g. services) are rated for the sum of the continuous loads plus 125 percent of the non-continuous loads. This is stated, for instance, in NEC 210.19 for minimum ampacity of branch circuit conductor. Continuous Load is in the definitions (Art. 100) as: Continuous Load. A load where the maximum current is expected to continue for 3 hours or more. Likewise, in the Canadian Electrical Code, Part I, Rule 8-104, maximum circuit loading is 80% of the rating of the circuit where a fused switch or circuit breaker is marked for continuous operation at 80% of the ampere rating of its overcurrent devices (or is unmarked per Appendix B). A continuous load is defined in that same rule as: (3) The calculated load in a consumers service, feeder, or branch circuit shall be considered a continuous load unless it can be shown that in normal operation it will not persist for (a) a total of more than 1 h in any two-hour period if the load does not exceed 225 A; or (b) a total of more than 3 h in any six-hour period if the load exceeds 225 A. Except for the definitions of continuous loads, the requirements are essentially the same in the US and Canada (though they are stated as reciprocals in one country versus the other). An easy example to demonstrate continuous and non-continuous loads would be to compare a standard electric heater to a blow dryer. The standard electrical heater plugs into a 120V ac, 15 A receptacle, and is considered a continuous load. It is never rated more than 1500W. Roughly this equates to 12 A current at 120 V ac, or 80% of the 15 A nominal circuit rating (12.5 actually, but you are normally allowed to exceed the current rating by no more than 10%). Blow dryers that plug into 120 V ac, 15 A outlets, on the other hand, are often rated for up to 1800 W. which equates to a current of 15 A. Why is this possible? Because a blow dryers use is short-duty or non-continuous, and will not be used for more than 3 hours in the US or for more than 1 hour in a 2 hour period in Canada. Best regards, DON GIES ALCATEL-LUCENT SENIOR PRODUCT COMPLIANCE ENGINEER BELL LABS - GLOBAL PRODUCT COMPLIANCE LABORATORY 600-700 Mountain Avenue Room 5B-104 Murray Hill, NJ 07974-0636 USA Phone: +1 908 582 5978 Fax: +1 908 582 0582 <mailto:[email protected]> [email protected] MEMBER, ALCATEL-LUCENT TECHNICAL ACADEMY -----Original Message----- From: Kunde, Brian [mailto:[email protected]] Sent: Wednesday, May 16, 2012 10:45 AM To: [email protected] 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: <mailto:[email protected]> [email protected] <mailto:[mailto:[email protected]]> [mailto:[email protected]] On Behalf Of Pete Perkins Sent: Tuesday, May 15, 2012 6:07 PM To: <mailto:[email protected]> [email protected] 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 <mailto:[email protected]> [email protected] - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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