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Fuse for 24V Transformer
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Serious Machining
electronics forum beginner


Joined: 20 Sep 2005
Posts: 10

PostPosted: Wed Jul 12, 2006 10:26 pm    Post subject: Fuse for 24V Transformer Reply with quote

I'm wiring a remote ON/OFF switch for a large room (window mount) air
conditioner which is otherwise accessible only via a 8 foot step
ladder. Historically, everyone has been going to the fuse panel to do
this. I need to use a low-voltage 24VAC control circuit to keep
everyone happy (don't bother to ask me why). The A/C is on a 240 VAC,
20 amp circuit.
..
The 240 VAC to 24 VAC transformer I have is a White-Rogers (Steveco)
90-T40M2 and it will be controlling a Supco DP30242 (W-R 90-244)
contactor. The same fellow who insists on having the 24 VAC control
circuit, also wants the transformer's 240 VAC input to be fused.
..
As the transformer's output is 40 volt-amps (I know, almost but not the
same as Watts), my simple math tells me that if the transformer was
100% efficient, it should only draw a max of 167 mA. In looking at
fuses, I see that Bussman makes all sorts of 5mm x 20mm fuses under 1
amp. I'm considering using 300 mA fast acting fuses as that's what's
readily available from my local Grainger store.
..
Am I on the right track here ? I just hope my 'friend' doesn't insist
that the 24 VAC wiring also be fused. Or should it, just to be
absolutely safe ? The transformer will be installed in a tight
location where there will not really be any air flow to draw off heat.
And it will likely be energized (even with the A/C OFF) all the time,
all year long.
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Chris
electronics forum Guru


Joined: 24 Mar 2005
Posts: 723

PostPosted: Wed Jul 12, 2006 11:26 pm    Post subject: Re: Fuse for 24V Transformer Reply with quote

Serious Machining wrote:
Quote:
I'm wiring a remote ON/OFF switch for a large room (window mount) air
conditioner which is otherwise accessible only via a 8 foot step
ladder. Historically, everyone has been going to the fuse panel to do
this. I need to use a low-voltage 24VAC control circuit to keep
everyone happy (don't bother to ask me why). The A/C is on a 240 VAC,
20 amp circuit.
.
The 240 VAC to 24 VAC transformer I have is a White-Rogers (Steveco)
90-T40M2 and it will be controlling a Supco DP30242 (W-R 90-244)
contactor. The same fellow who insists on having the 24 VAC control
circuit, also wants the transformer's 240 VAC input to be fused.
.
As the transformer's output is 40 volt-amps (I know, almost but not the
same as Watts), my simple math tells me that if the transformer was
100% efficient, it should only draw a max of 167 mA. In looking at
fuses, I see that Bussman makes all sorts of 5mm x 20mm fuses under 1
amp. I'm considering using 300 mA fast acting fuses as that's what's
readily available from my local Grainger store.
.
Am I on the right track here ? I just hope my 'friend' doesn't insist
that the 24 VAC wiring also be fused. Or should it, just to be
absolutely safe ? The transformer will be installed in a tight
location where there will not really be any air flow to draw off heat.
And it will likely be energized (even with the A/C OFF) all the time,
all year long.

Try 1/4A slo-blow for the input side of your transformer. Grainger has
those, too. A fast blow fuse will pop on you. That's because the 11
ohm DC resistance of your contactor coil might cause momentary inrush
current..

Don't worry -- you don't need to fuse the secondary of the transformer.

And the 90-T40M2 is made to be mounted in an electrical box, I believe.
Since your contactor will only require 6VA when on, you shouldn't have
any problem with heating.

Buy your friend a cold six of beer if he's giving you free advice -- he
seems to be leading you in the right direction.

Good luck
Chris
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jasen
electronics forum Guru Wannabe


Joined: 18 Jun 2006
Posts: 204

PostPosted: Thu Jul 13, 2006 11:31 am    Post subject: Re: Fuse for 24V Transformer Reply with quote

On 2006-07-12, Serious Machining <whatelse@qwest.net> wrote:
Quote:
I'm wiring a remote ON/OFF switch for a large room (window mount) air
conditioner which is otherwise accessible only via a 8 foot step
ladder. Historically, everyone has been going to the fuse panel to do
this. I need to use a low-voltage 24VAC control circuit to keep
everyone happy (don't bother to ask me why). The A/C is on a 240 VAC,
20 amp circuit.
.
The 240 VAC to 24 VAC transformer I have is a White-Rogers (Steveco)
90-T40M2 and it will be controlling a Supco DP30242 (W-R 90-244)
contactor. The same fellow who insists on having the 24 VAC control
circuit, also wants the transformer's 240 VAC input to be fused.

As the transformer's output is 40 volt-amps (I know, almost but not the
same as Watts), my simple math tells me that if the transformer was
100% efficient, it should only draw a max of 167 mA. In looking at
fuses, I see that Bussman makes all sorts of 5mm x 20mm fuses under 1
amp. I'm considering using 300 mA fast acting fuses as that's what's
readily available from my local Grainger store.

I think you want a slow fuse on that transformer fast fuses before
transformers are succeptible to surges when the power comes on.
(something I only learned about recently)

Quote:
Am I on the right track here ? I just hope my 'friend' doesn't insist
that the 24 VAC wiring also be fused.

you could use an in-line fuseholder for that... no big deal.

Quote:
Or should it, just to be absolutely safe ?

if something shorts out your wiring it could save the transformer.

Quote:
The transformer will be installed in a tight
location where there will not really be any air flow to draw off heat.
And it will likely be energized (even with the A/C OFF) all the time,
all year long.

how much current will it be suppllying most of the time?

--

Bye.
Jasen
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ehsjr
electronics forum Guru


Joined: 03 May 2005
Posts: 863

PostPosted: Sat Jul 15, 2006 9:43 pm    Post subject: Re: Fuse for 24V Transformer Reply with quote

Serious Machining wrote:
Quote:
I'm wiring a remote ON/OFF switch for a large room (window mount) air
conditioner which is otherwise accessible only via a 8 foot step
ladder. Historically, everyone has been going to the fuse panel to do
this. I need to use a low-voltage 24VAC control circuit to keep
everyone happy (don't bother to ask me why). The A/C is on a 240 VAC,
20 amp circuit.
.
The 240 VAC to 24 VAC transformer I have is a White-Rogers (Steveco)
90-T40M2 and it will be controlling a Supco DP30242 (W-R 90-244)
contactor. The same fellow who insists on having the 24 VAC control
circuit, also wants the transformer's 240 VAC input to be fused.
.
As the transformer's output is 40 volt-amps (I know, almost but not the
same as Watts), my simple math tells me that if the transformer was
100% efficient, it should only draw a max of 167 mA. In looking at
fuses, I see that Bussman makes all sorts of 5mm x 20mm fuses under 1
amp. I'm considering using 300 mA fast acting fuses as that's what's
readily available from my local Grainger store.
.
Am I on the right track here ? I just hope my 'friend' doesn't insist
that the 24 VAC wiring also be fused. Or should it, just to be
absolutely safe ? The transformer will be installed in a tight
location where there will not really be any air flow to draw off heat.
And it will likely be energized (even with the A/C OFF) all the time,
all year long.



The fuse on the input side of the transformer is
NOT to protect the transformer. It is to protect
the wiring on the input (240V) side, in case the
transformer draws too much current. So you don't
need a fuse close to some computed value based on
40 va. A slow-blow 1 amp fuse would be fine in
an assembled piece of equipment.

In fact, if your transformer is UL listed for
installation in a j-box, the mains branch circuit
breaker provides the correct protection.
Bastardizing things by adding a fuse not called
for by the manufacturer's instructions will make
things *less* safe, not safer.

Ed
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cornytheclown@hotmail.com
electronics forum beginner


Joined: 23 Oct 2005
Posts: 16

PostPosted: Mon Jul 17, 2006 3:39 am    Post subject: Re: Fuse for 24V Transformer Reply with quote

Serious Machining wrote:
Quote:
I'm wiring a remote ON/OFF switch for a large room (window mount) air
conditioner which is otherwise accessible only via a 8 foot step
ladder. Historically, everyone has been going to the fuse panel to do
this. I need to use a low-voltage 24VAC control circuit to keep
everyone happy (don't bother to ask me why). The A/C is on a 240 VAC,
20 amp circuit.
.
The 240 VAC to 24 VAC transformer I have is a White-Rogers (Steveco)
90-T40M2 and it will be controlling a Supco DP30242 (W-R 90-244)
contactor. The same fellow who insists on having the 24 VAC control
circuit, also wants the transformer's 240 VAC input to be fused.
.
As the transformer's output is 40 volt-amps (I know, almost but not the
same as Watts), my simple math tells me that if the transformer was
100% efficient, it should only draw a max of 167 mA. In looking at
fuses, I see that Bussman makes all sorts of 5mm x 20mm fuses under 1
amp. I'm considering using 300 mA fast acting fuses as that's what's
readily available from my local Grainger store.
.
Am I on the right track here ? I just hope my 'friend' doesn't insist
that the 24 VAC wiring also be fused. Or should it, just to be
absolutely safe ? The transformer will be installed in a tight
location where there will not really be any air flow to draw off heat.
And it will likely be energized (even with the A/C OFF) all the time,
all year long.

I do quite a bit of work with 24 volt control circuits and I've never
felt the need to fuse the input side of the transformer..... I always
fuse the low voltage side...and usually use transformers with built in
circuit breakers. Transformers for the most part dont just fail....Ive
seen them 50 years old still kicking it....problems usually arise on
the low voltage side in my experience.
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jasen
electronics forum Guru Wannabe


Joined: 18 Jun 2006
Posts: 204

PostPosted: Mon Jul 17, 2006 10:52 am    Post subject: Re: Fuse for 24V Transformer Reply with quote

On 2006-07-17, cornytheclown@hotmail.com <cornytheclown@hotmail.com> wrote:
Quote:

Serious Machining wrote:

I do quite a bit of work with 24 volt control circuits and I've never
felt the need to fuse the input side of the transformer..... I always
fuse the low voltage side...and usually use transformers with built in
circuit breakers. Transformers for the most part dont just fail....Ive
seen them 50 years old still kicking it....problems usually arise on
the low voltage side in my experience.

a fuse on the primary will stop a fire if the transformer fails.
(many transformers have it built in)

a thermal switch on the transfromer and or a small enough fuse on the
secondary will save the transformer if a fault develops in ther low
voltage stuff.

Bye.
Jasen
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