You are right that 422.11(E) applies. But it allows products rated below 13.3A
to be used as long as product markings permit or instruct them to be used on
the larger (30A) circuit. It does not require the rating or the load to be
large. The marking places the burden on the appliance to have adequate
protection built in and to depend on the branch circuit for overload or fault
protection only at the 30A level. In the product discussed, there was a 15A
supplementary protection in the product providing additional downstream
protection.
We sold many empty 19 inch racks equipped for 30A supplies and no more load
than a fan unless the customer added some. Within the cabinet additional
receptacles of assorted sizes with appropriate protection provided power as
needed.
Alternatively the manufacturer could label the product with a 24A rating even
if the measured load was very low or even zero.
Bob Johnson
ITE Safety
On 06/15/2010 02:19 PM, Ted Eckert wrote:
Here is where I can make an argument that you are not allowed to have a
6 A
load with a 30 A plug.
Article 240.5(A) states that “Flexible cord and flexible cable shall be
protected by an overcurrent device in accordance with their ampacity as
specified in Table 400.5(A) and Table 400.5(B).” This means that for
single-phase circuits, a 30 A plug must be connected to 14 AWG or larger.
You may ask why you can’t have the 6 A load with that 14 AWG wire and
30 A
plug. Now things get more complicated. The electrical code has different
minimum and maximum overcurrent limits for different types of equipment.
Motor driven equipment will have a startup surge that requires different
overcurrent protection than a resistive load. The code separates out motors,
lighting, heating, air conditioners and a few other specialized types of
loads. Article 645 for ITE does not include overcurrent protection, so we
can’t use it. The best we have is Article 422 for appliances. 422.11(E)
covers single, non-motor operated appliances. For equipment rated 13.3 A or
less, the overcurrent protection shall not exceed 20 A. Otherwise, the
overcurrent protection shall not exceed 150 percent of the appliance’s rated
current.
This does have applicability to UL 60950-1. Annex NAE section 2.7 does
specifically reference overcurrent protection for appliances and 422.11. Go a
little further to NAE 3.2.1 and you will find more references to articles 210
and 422. It is 210.19(A)(1) that basically sets the limit of 80% of the
rating of the plug as the maximum for the equipment. UL will typically use
this rather than the adjustments of 422.11(E).
Numerous clauses of the electrical code place maximum limits on
overcurrent
protection. Although there isn’t a specific reference for ITE, it is clear
that the spirit and intent of the code is to make sure that equipment is
placed on a circuit with overcurrent protection that is neither too low or too
high. You don’t want somebody plugging a toaster into a 10,000 A circuit.
In that case, even a direct short might not trip the breaker.
For those not familiar with UL 60950-1 Annex NAE, I recommend it when
trying
to find NEC code requirements. Annex NAE gives the specific NEC and CEC code
references for U.S. and Canadian national differences to IEC 60950-1.
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: Robert Johnson [mailto:[email protected]]
Sent: Tuesday, June 15, 2010 10:39 AM
To: [email protected]
Subject: Re: Power Cords: Cordage size vs Plug
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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