From Newsgroup: uk.d-i-y

I have done a bit of web searching but I've not managed to come up
with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or
a DC value? E.g. I have some cable marked 300v, can I use it for
carrying mains voltage? (It fits all the other obvious rules like
being double sheathed etc.)
--
Chris Green
-+
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From Newsgroup: uk.d-i-y

On 23/02/2026 09:56, Chris Green wrote:

I have done a bit of web searching but I've not managed to come up
with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or
a DC value? E.g. I have some cable marked 300v, can I use it for
carrying mains voltage? (It fits all the other obvious rules like
being double sheathed etc.)

It definitely will not be the rms value as the peak voltage is dependant
on the waveform form factor. rms value gives you the DC heating effect
voltage into a resistive load. I suspect it is the peak voltage rating.
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From Newsgroup: uk.d-i-y

On 23/02/2026 11:48, Andy Bennett wrote:

On 23/02/2026 09:56, Chris Green wrote:

I have done a bit of web searching but I've not managed to come up
with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or
a DC value?-a E.g. I have some cable marked 300v, can I use it for
carrying mains voltage? (It fits all the other obvious rules like
being double sheathed etc.)

It definitely will not be the rms value as the peak voltage is dependant
on the waveform form factor. rms value gives you the DC heating effect voltage into a resistive load. I suspect it is the peak voltage rating.

It is almost certainly the RMS voltage! 300V and 600V ac are both
very common cable ratings.

John

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From Newsgroup: uk.d-i-y

On 23/02/2026 13:51, John R Walliker wrote:

On 23/02/2026 11:48, Andy Bennett wrote:

On 23/02/2026 09:56, Chris Green wrote:

I have done a bit of web searching but I've not managed to come up
with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or
a DC value?-a E.g. I have some cable marked 300v, can I use it for
carrying mains voltage? (It fits all the other obvious rules like
being double sheathed etc.)

It definitely will not be the rms value as the peak voltage is
dependant on the waveform form factor. rms value gives you the DC
heating effect voltage into a resistive load. I suspect it is the peak
voltage rating.

It is almost certainly the RMS voltage! 300V and 600V ac are both
very common cable ratings.

John

So a 10kV peak to peak poor form factor a.c. waveform whose rms voltage
is 230V is perfectly ok on 300V rated cable?

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From Newsgroup: uk.d-i-y

On 23/02/2026 13:56, Andy Bennett wrote:

On 23/02/2026 13:51, John R Walliker wrote:

On 23/02/2026 11:48, Andy Bennett wrote:

On 23/02/2026 09:56, Chris Green wrote:

I have done a bit of web searching but I've not managed to come up
with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or
a DC value?-a E.g. I have some cable marked 300v, can I use it for
carrying mains voltage? (It fits all the other obvious rules like
being double sheathed etc.)

It definitely will not be the rms value as the peak voltage is
dependant on the waveform form factor. rms value gives you the DC
heating effect voltage into a resistive load. I suspect it is the
peak voltage rating.

It is almost certainly the RMS voltage! 300V and 600V ac are both
very common cable ratings.

John

So a 10kV peak to peak poor form factor a.c. waveform whose rms voltage
is 230V is perfectly ok on 300V rated cable?

Lol.

As always the problem of specifying complex technical data to the
'average user' rears its ugly head.

It's more usual to rate stuff like "250V AC/450V DC"to allow for the above.
--
rCLit should be clear by now to everyone that activist environmentalism
(or environmental activism) is becoming a general ideology about humans,
about their freedom, about the relationship between the individual and
the state, and about the manipulation of people under the guise of a
'noble' idea. It is not an honest pursuit of 'sustainable development,'
a matter of elementary environmental protection, or a search for
rational mechanisms designed to achieve a healthy environment. Yet
things do occur that make you shake your head and remind yourself that
you live neither in Joseph StalinrCOs Communist era, nor in the Orwellian utopia of 1984.rCY

Vaclav Klaus

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From Newsgroup: uk.d-i-y

On 23/02/2026 14:15, The Natural Philosopher wrote:

On 23/02/2026 13:56, Andy Bennett wrote:

On 23/02/2026 13:51, John R Walliker wrote:

On 23/02/2026 11:48, Andy Bennett wrote:

On 23/02/2026 09:56, Chris Green wrote:

I have done a bit of web searching but I've not managed to come up
with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or >>>>> a DC value?-a E.g. I have some cable marked 300v, can I use it for
carrying mains voltage? (It fits all the other obvious rules like
being double sheathed etc.)

It definitely will not be the rms value as the peak voltage is
dependant on the waveform form factor. rms value gives you the DC
heating effect voltage into a resistive load. I suspect it is the
peak voltage rating.

It is almost certainly the RMS voltage! 300V and 600V ac are both
very common cable ratings.

John

So a 10kV peak to peak poor form factor a.c. waveform whose rms
voltage is 230V is perfectly ok on 300V rated cable?

Lol.

As always the problem of specifying complex technical data to the
'average user' rears its ugly head.

It's more usual to rate stuff like-a "250V AC/450V DC"to allow for the above.

The standard for testing cables, BS EN 50395 deals with such issues
as follows:
"Unless otherwise stated the test voltage shall be ac of approximately
sine wave form and of frequency between 49 and 61 Hz. The ratio of
peak value to r.m.s value shall be equal to sqrt 2 with a tolerance
of +/- 7%.
The values quoted are r.m.s. values."

John

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From Newsgroup: uk.d-i-y

On 23/02/2026 16:00, John R Walliker wrote:

On 23/02/2026 14:15, The Natural Philosopher wrote:

On 23/02/2026 13:56, Andy Bennett wrote:

On 23/02/2026 13:51, John R Walliker wrote:

On 23/02/2026 11:48, Andy Bennett wrote:

On 23/02/2026 09:56, Chris Green wrote:

I have done a bit of web searching but I've not managed to come up >>>>>> with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or >>>>>> a DC value?-a E.g. I have some cable marked 300v, can I use it for >>>>>> carrying mains voltage? (It fits all the other obvious rules like
being double sheathed etc.)

It definitely will not be the rms value as the peak voltage is
dependant on the waveform form factor. rms value gives you the DC
heating effect voltage into a resistive load. I suspect it is the
peak voltage rating.

It is almost certainly the RMS voltage! 300V and 600V ac are both
very common cable ratings.

John

So a 10kV peak to peak poor form factor a.c. waveform whose rms
voltage is 230V is perfectly ok on 300V rated cable?

Lol.

As always the problem of specifying complex technical data to the
'average user' rears its ugly head.

It's more usual to rate stuff like-a "250V AC/450V DC"to allow for the
above.

The standard for testing cables, BS EN 50395 deals with such issues
as follows:
"Unless otherwise stated the test voltage shall be ac of approximately
sine wave form and of frequency between 49 and 61 Hz.-a The ratio of
peak value to r.m.s value shall be equal to sqrt 2 with a tolerance
of +/- 7%.
The values quoted are r.m.s. values."

John

According to BS EN 50525-1 completed cables for general use that are
rated at 300V line to neutral are tested at 2kV rms for at least 15
minutes. Individual cores are tested for at least 5 minutes at 2kV
if the insulation is >0.6mm thick and at 1.5kV if <=0.6mm.
There is a lot of harmonization of standards, so UL is probably
very similar.
John

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From Newsgroup: uk.d-i-y

John R Walliker <jrwalliker@gmail.com> wrote:

On 23/02/2026 16:00, John R Walliker wrote:

On 23/02/2026 14:15, The Natural Philosopher wrote:

On 23/02/2026 13:56, Andy Bennett wrote:

On 23/02/2026 13:51, John R Walliker wrote:

On 23/02/2026 11:48, Andy Bennett wrote:

On 23/02/2026 09:56, Chris Green wrote:

I have done a bit of web searching but I've not managed to come up >>>>>> with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or >>>>>> a DC value?-a E.g. I have some cable marked 300v, can I use it for >>>>>> carrying mains voltage? (It fits all the other obvious rules like >>>>>> being double sheathed etc.)

It definitely will not be the rms value as the peak voltage is
dependant on the waveform form factor. rms value gives you the DC >>>>> heating effect voltage into a resistive load. I suspect it is the >>>>> peak voltage rating.

It is almost certainly the RMS voltage! 300V and 600V ac are both
very common cable ratings.

John

So a 10kV peak to peak poor form factor a.c. waveform whose rms
voltage is 230V is perfectly ok on 300V rated cable?

Lol.

As always the problem of specifying complex technical data to the
'average user' rears its ugly head.

It's more usual to rate stuff like-a "250V AC/450V DC"to allow for the
above.

The standard for testing cables, BS EN 50395 deals with such issues
as follows:
"Unless otherwise stated the test voltage shall be ac of approximately
sine wave form and of frequency between 49 and 61 Hz.-a The ratio of
peak value to r.m.s value shall be equal to sqrt 2 with a tolerance
of +/- 7%.
The values quoted are r.m.s. values."

John

According to BS EN 50525-1 completed cables for general use that are
rated at 300V line to neutral are tested at 2kV rms for at least 15
minutes. Individual cores are tested for at least 5 minutes at 2kV
if the insulation is >0.6mm thick and at 1.5kV if <=0.6mm.
There is a lot of harmonization of standards, so UL is probably
very similar.

Thanks all, the cable certainly looks the part for carrying mains
voltage. As it ahppens the requirement (making a long lead for a
toothbrush charger) has now disappeared but it's still of interest.

Can one reasonably requir/mandate a three amp fuse in the plug on the
end of the cable? If not then how do you make sure it's not
overloaded? It's certainly not a requirement that all cables (on a
UK plug top) can carry 13 amps safely.
--
Chris Green
-+
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From Newsgroup: uk.d-i-y

On 23/02/2026 18:03, Chris Green wrote:

John R Walliker <jrwalliker@gmail.com> wrote:

On 23/02/2026 16:00, John R Walliker wrote:

On 23/02/2026 14:15, The Natural Philosopher wrote:

On 23/02/2026 13:56, Andy Bennett wrote:

On 23/02/2026 13:51, John R Walliker wrote:

On 23/02/2026 11:48, Andy Bennett wrote:

On 23/02/2026 09:56, Chris Green wrote:

I have done a bit of web searching but I've not managed to come up >>>>>>>> with a definitive answer.

Is an Underwriters Laboratory cable voltage rating an AC RMS value or >>>>>>>> a DC value?-a E.g. I have some cable marked 300v, can I use it for >>>>>>>> carrying mains voltage? (It fits all the other obvious rules like >>>>>>>> being double sheathed etc.)

It definitely will not be the rms value as the peak voltage is
dependant on the waveform form factor. rms value gives you the DC >>>>>>> heating effect voltage into a resistive load. I suspect it is the >>>>>>> peak voltage rating.

It is almost certainly the RMS voltage! 300V and 600V ac are both
very common cable ratings.

John

So a 10kV peak to peak poor form factor a.c. waveform whose rms
voltage is 230V is perfectly ok on 300V rated cable?

Lol.

As always the problem of specifying complex technical data to the
'average user' rears its ugly head.

It's more usual to rate stuff like-a "250V AC/450V DC"to allow for the >>>> above.

The standard for testing cables, BS EN 50395 deals with such issues
as follows:
"Unless otherwise stated the test voltage shall be ac of approximately
sine wave form and of frequency between 49 and 61 Hz.-a The ratio of
peak value to r.m.s value shall be equal to sqrt 2 with a tolerance
of +/- 7%.
The values quoted are r.m.s. values."

John

According to BS EN 50525-1 completed cables for general use that are
rated at 300V line to neutral are tested at 2kV rms for at least 15
minutes. Individual cores are tested for at least 5 minutes at 2kV
if the insulation is >0.6mm thick and at 1.5kV if <=0.6mm.
There is a lot of harmonization of standards, so UL is probably
very similar.

Thanks all, the cable certainly looks the part for carrying mains
voltage. As it ahppens the requirement (making a long lead for a
toothbrush charger) has now disappeared but it's still of interest.

Can one reasonably requir/mandate a three amp fuse in the plug on the
end of the cable?

Mandate, probably not... you could obviously fix a label etc.

If not then how do you make sure it's not
overloaded?

In most cases that is something you control at the appliance end. i.e Is
there any plausible non fault[1] scenario where the appliance that
normally draws (say) 150mA could suddenly start to sustain a draw of 13A?

In the vast majority of cases the answer will be "no".

[1] Fault in this case having a specific meaning i.e. the (typically
very large) current that will flow as the result of a fault like a short
from phase to earth or phase to neutral.

So an overload is only one of the over current situations that you
design for, the more important one (typically) is dealing with fault
currents. Also fault current protection *must* be at the source end of
the supply, whereas overloads protection can be controlled by design and selection of equipment at the destination end of the circuit.

It's certainly not a requirement that all cables (on a
UK plug top) can carry 13 amps safely.

For an appliance protected by a fuse[2] you are not protecting the cable
for 13A, but more likely from >100A - typically for a very short
duration. Possibly counter intuitively, a 13A fuse will often protect a
"3A" flex in these cases. (you can check this using the Adiabatic check
as described here:

https://wiki.diyfaq.org.uk/index.php?title=Calculating_A_Cable_Size#Adiabatic_Check

[2] for the exceptions see:

https://wiki.diyfaq.org.uk/index.php?title=Choosing_a_plug_fuse#Special_Cases --
Cheers,

John.

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