<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=utf-8"><meta name=Generator content="Microsoft Word 15 (filtered medium)"><style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
p.msonormal0, li.msonormal0, div.msonormal0
{mso-style-name:msonormal;
mso-margin-top-alt:auto;
margin-right:0in;
mso-margin-bottom-alt:auto;
margin-left:0in;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
span.EmailStyle18
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal>Mark,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>This is due to remanent flux in the transformer core. If the first half cycle of the line tries to magnetize the core in the same polarity, it will saturate the core and draw high current. One possible approach is to put a resistor in series with the line and cut out the resistor after the transformer core resets. An incandescent bulb, say 75W if you can find one, would be a good resistor.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>That said, try an SL22 10008 NTC.<o:p></o:p></p><p class=MsoNormal><a href="https://www.ametherm.com/inrush-current/transformer-inrush-current.html?gclid=CjwKCAjw_47YBRBxEiwAYuKdw6_IzT-R5wYlOv0eMMWHP5owpuxpxYzNUo4YC3SoXzx-6Xj7CdVNiBoCnDsQAvD_BwE">https://www.ametherm.com/inrush-current/transformer-inrush-current.html?gclid=CjwKCAjw_47YBRBxEiwAYuKdw6_IzT-R5wYlOv0eMMWHP5owpuxpxYzNUo4YC3SoXzx-6Xj7CdVNiBoCnDsQAvD_BwE</a><o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Karl W4KRL<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><b>From:</b> Tacos <tacos-bounces+karl.w4krl=gmail.com@amrad.org> <b>On Behalf Of </b>Mark Whittington<br><b>Sent:</b> Tuesday, May 22, 2018 6:57 AM<br><b>To:</b> Tacos <tacos@amrad.org><br><b>Subject:</b> Advice sought - isolation transformer inrush current limiting<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><div><p class=MsoNormal>I've got a 1KW isolation transformer that usually trips the 15A breaker at the breaker box when I turn it on, load or no. I'm considering installing a pair of NTC thermistors on the input side of the transformer, but I've no idea how to size them or if there might be a better option.<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Any advice would be appreciated.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Cheers.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>-Mark<o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div></div></div></div></body></html>