You are faced with compliance with two standards in the US. The National
Electrical Code (NEC) NFPA 70, and UL 60950, which has modified IEC 60950 to
be compatible with the NEC.
The code sections relevant to your needs are article 240.5 wherein it mentions
"...it shall be considered to be protected when applied within the appliance
or luminaire listing requirements." That means in accordance with UL 60950.
I'll cover that further down.
You can also fall back to the table 400.5(A) which requires 10 AWG (min) for a
30A circuit. The intent is to assure that conductors cannot be damaged by
fault conditions without being protected by overcurrent devices. The use of
downstream overcurrent is permitted by the code (a situation called tap
conductors - see 240.2 and 240.21), but not applicable to this situation
mostly because of the requirement for raceways. I therefore don't believe the
supplementary overcurrent protection can be used for upstream protection
except within the product.
Contrary to other comments, I don't believe there is any requirement the load
rating or current be above some figure (like 16A) when using a 30A connector.
I see no safety reason to worry about underload conditions. If there is some
requirement, I would appreciate a code citation.
15-20 amp branch circuits, which are the smallest in standard use in the US,
have special exemptions for lighter gauge cords like 18 or 20 AWG and tinsel
cords, but lately there has been concern about protection of these and even
heavier cords, which resulted in the introduction of arc fault circuit
interrupter (AFCI) requirements in recent codes.
Now we get into an interesting situation. The only clause of UL 60950 dealing
with the wire gauge or ampacity of the power supply cord is article 3.2.5.1.
This specifies the power cord gauge based on the rated current of the
equipment, not on the plug. This would seem to imply that you could put a 30
amp (or greater) plug on a 6 amp rated product and use an 18 AWG cord.
There is a modification in Annex NAE clause 3.2.5: "...Power supply cords
shall have conductors with cross-sectional areas sufficient for the rated
current of the equipment. Conductors shall be sized based on the requirements
in the National Electrical Code, ANSI/NFPA 70..." This is certainly a circular
reference and conceivably be misinterpreted, since the code points right back
to the listing agency. I think you will find that agencies will interpret this
to mean that a 30A plug will need a cord with 30A ampacity.
There is also a reference in Annex P1 to UL 817, Cord Sets and Power Supply
Cords, and to UL 82, Flexible Cord and Fixture Wire. I have not checked these
standards for how they interact with the plug rating.
This point would be worthwhile to address with changes to UL 60950-1.
Bob Johnson
ITE Safety
On 06/14/2010 12:38 PM, Kunde, Brian wrote:
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.
_________________________
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mistake, please destroy it and notify us of the error. Thank you.
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