The rules for circuits rated 20 A and less are different than those for over 20 A. In commercial installation in the United States, 20 A is used for general branch circuits and can have any load less than 20 A. Devices drawing less than an ampere must still be protected for connection to a 20 A circuit. It is only once you get above 20 A that there are more gradations of circuit ratings. Since 20 A is the lowest common commercial breaker size, the code has to accept any load less than 20 A. (Note that this is different in Canada. The Canadian code requires 15 A breakers for protecting 15 A outlets even in commercial installations.)
Cords for devices rated less than 20 A can be based on the rating of the product. A lamp that can only accept bulbs rated 150 W or less can have an 18 AWG cord. I don’t know of a specific clause that prevents you from connecting a 15 A load with a 30 A cord. However, you will still be required to prove that the product is safe on that 30 A circuit. That means that wires either need to be sized for 30 A or you have to pass the tests in 60950-1 section 2.6.4.3. I am still not sure of the construction of your device. You describe the 15 A breaker as “supplementary protection”. This term has a specific meaning. In terms of the codes there are “circuit breakers” and “supplementary overcurrent protectors” and the two are different. “Circuit breakers” are tested under UL 489, have arc suppression and have a high ampere interrupt capacity (AIC). This means that they can handle high fault currents and still operate correctly. “Supplementary protectors” are tested under UL 1077 and are not allowed for many purposes. They are typically thermal trip devices and do not require arc suppression. They often can’t handle high fault currents. In the event of a fault, a “supplementary protector” may actually weld closed. They are normally used in conjunction with a UL 489 breaker. The UL 1077 device is there to protect from low level overloads. Power strips have UL 1077 devices to protect against going over the rating of the power strip. Even at 135% of the rating of the power strip, the branch breaker can take an hour to trip. A thermal protector can trip faster at that lower overload level. If you device has UL 489 Listed circuit breakers, you need 30 A wiring from the breakers to the plug, and 15 A wiring from the breakers to the load. If you have UL 1077 Recognized protectors, you need 30 A wiring throughout the device. Regards, Ted Eckert Compliance Engineer Microsoft Corporation [email protected] <mailto:[email protected]> The opinions expressed are my own and do not necessarily reflect those of my employer. From: Kunde, Brian [mailto:[email protected]] Sent: Monday, June 14, 2010 9:38 AM To: [email protected] Subject: Re: Power Cords: Cordage size vs Plug This entire subject confuses me because what I’m being told is not supported by what I see in products. A standard PC power cord has 18AWG wire rated 10A, yet it has a NEMA 5-15 plug rated 15A AND it can be plugged into a 20A receptacle. According to the NEC 240-4, a power cord can be protected by either the Branch Circuit breaker (that in the facility panel) OR the Supplementary Overcurrent Protection in the device (downstream). Isn’t this correct? Isn’t that why PC power cables can be rated 10A but be plugged into a 20A circuit because the cord is protected by the overcurrent protection device inside the PC? You can buy a 3A power cord for your razor and plug it into a 15A or 20A circuit. What protects that cord? Isn’t Power Cords and connectors considered high integrity components and are not likely for fault between the supplementary overcurrent device and the plug, so this is why this is allowed? Isn’t my situation the same? Wouldn’t a 15A breaker inside my device be considered Supplementary Overcurrent Protection and protect the power cord upstream regardless of the plug? Article 400 doesn’t say much about plugs; only the current rating of wire and cords. It would seem that Article 240-4 is the section that deals with plugs verses minimum cord size. Has this section been changed in the newer versions of the NEC? Does Canada have different rules in this regard than the NEC? If so, is every PC in Canada shipped with a 12 AWG power cord? I’m really trying to figure this out. Thanks, The Other Brian ________________________________ From: Ted Eckert [mailto:[email protected]] Sent: Monday, June 14, 2010 12:08 PM To: Kunde, Brian; [email protected] Subject: RE: Power Cords: Cordage size vs Plug Hello Brian, My recommendation is not to use a 30 A plug unless the device in question draws a continuous current of more than 16 A. The code is a bit convoluted, and I won’t get into all the details, but you generally are not allowed to use a 30 A plug on a device rated 16 A or less for continuous current or 20 A or less for noncontinuous current. (The code defines “continuous” as 3 hours or more.) The requirements for plug and cord connections are in Article 400 – Flexible Cords and Cables. I recommend referencing the 2008 version of the NEC, but I believe the relevant tables are the same as the 2005 code. Table 400.5 (A) gives the allowable ampacities for flexible cords and cables assuming a maximum ambient of 30°C. If this is a single phase product, we use Column B of Table 400.5 (A) which does show 10 AWG cable required for up to 30 A. 210.23(B) does state that the maximum continuous load for a 30 A circuit is 24 A, so you might be able to argue for 12 AWG, but that would not be standard practice and it would take some work to get approved. There is another possible issue you will need to address. A 30 ampere circuit is for continuous loads greater than 16 A and less than or equal to 24 A. UL 489 governs the branch circuit breaker. The 30 A breaker protecting your device must hold at 110%, trip in one hour at 135% and trip in 2 minutes at 200%. (All of these are at 25°C) Further, the potential fault current of a 30 A circuit will be higher than for a 20 A circuit because of the lower impedance of the larger wires. As a result, UL and CSA will want the grounding of the system to be based on fault currents, not the normal load current. This will push up the size of the ground wire in the cable. Unless you can find a cable with a larger ground than the conductors, you will have to go with the 14 AWG cable. We also must look in 60950-1 itself. Your circuit is a 30 A circuit. If the 15 A devices are UL 1077 Recognized supplementary protectors, they don’t count for much. They are not the circuit protector as far as table 2.6.3.4 is concerned. You still have a 30 A circuit, so you will need to run the test at 60 A for 4 minutes. You can’t rely entirely on Table 3B for conductor sizes. It is based on the assumption that the branch circuit protection is sized based on the load. That goes back to my first paragraph where I indicated that a 30 A plug is normally used for loads over 16 A. Regards, Ted Eckert Compliance Engineer Microsoft Corporation [email protected] The opinions expressed are my own and do not necessarily reflect those of my employer. From: Kunde, Brian [mailto:[email protected]] Sent: Monday, June 14, 2010 6:34 AM To: [email protected] Subject: Power Cords: Cordage size vs Plug Where can I find a design guide for power cords that shows the minimum size of the cordage vs the current rating of the Plug (for US and Canada)? We have a power cord with a 30A Twistlock Plug and 14AWG cordage. It is used on a device that has a double pole 15A circuit breaker (supplementary protection). Canada inspectors are saying that with a 30A plug the cord must also be rated 30A, which would be 10AWG. I didn’t think what was correct. I’m not very versed in the US NEC (let alone Canada’s NEC), but section 240-4 (1999 version) shows that the minimum cordage size for a 30A plug would be 16AWG, so our 14AWG cordage should be ok. I know you all are probably wondering why we would put a 30A plug on a power cord for a 15A device. It is because this device can be used with other devices we sell that also uses the 30A plug. So out of consistency we try to standardize on this plug where we can. More Info. The device we are powering has a Steady State Current Rating of 14A (calculated worst case). Max Current measured is 11A. Max Continuous Current is only 1.5A. _________________________ LECO Corporation Notice: This communication may contain confidential information intended for the named recipient(s) only. If you received this by mistake, please destroy it and notify us of the error. Thank you. - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc <http://www.ieeecommunities.org/emc-pstc> Graphics (in well-used formats), large files, etc. can be posted to that URL. 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