I have a single phase circuit with 120VAC L and N. This is going through a step up transformer to up the Voltage to circa 1500VAC. question is how and where to connect the surge suppression devices? ie on the 120V side of the XFMR or do they have to be on the 1500VAC side?

Also I have a distribution system like a PDU upstream of the 120VAC supply to the XMFR, could the surge suppression be in there?

  • \$\begingroup\$ Can you please give us some more context here? If you're only concerned about mains power input surges, then a 120VAC suppressor pretty much anywhere upstream of your xfmr would be sufficient. However, if you're asking a question about something you're building where a surge suppression device is required by an applicable elec. code (or other legal/liability concern), then we'd need to see that code in order to answer correctly. \$\endgroup\$ Commented Jan 28, 2016 at 2:44
  • \$\begingroup\$ I don't believe there are any specific code requirements its purely to protect the downstream equipment from mains surges. Codes are NFPA70, API 17F or ISO 13628-6 \$\endgroup\$
    – Douglas
    Commented Jan 28, 2016 at 2:59

1 Answer 1


Purely for protecting downstream equipment from mains surges, a 120VAC suppressor pretty much anywhere upstream of the xfmr should be sufficient (so long as it's rated for the full load the xfmr draws, of course). Most surge suppressors work by shorting surge current from L to N, or from L to GND, so the further upstream you place them, the fewer components between surge origin & surge suppressor can be damaged (such as the windings in your xfmr when the surge suppressor effectively shorts the secondary windings, allowing momentary huge currents in both primary & secondary windings).

  • \$\begingroup\$ i'm not sure I fully understand what you mean about it being rated for the full load the xfmr draws? xfmr is rated at 2.5kVA at a 300V tap so max current is about 8.33A but the breaker protecting this branch is set at 20A because of the other stuff involved also but i'm looking at using phoenix contact possibly something like 280067 or 2804429. I don't really understand the operation or how to size these etc. \$\endgroup\$
    – Douglas
    Commented Jan 29, 2016 at 1:54
  • \$\begingroup\$ Well, one digit is missing from the first model you listed (280067_), but the 2804429 is for 240/415 volt (full single-phase, or 3-phase) usage, rather than for (split-phase) 120 volt circuits. It could still work, but would be "overkill" at the least, and at most require installing a new "secondary" breaker box to split down the 240V circuit into 2 120V circuits. As for the "full load the xfmr draws" part, most transformers operate at ~80% efficiency @ rated capacity, so a 2.5KVA rated xfmr should draw around ~3.2KVA, so any surge protector rated for >=4KVA should suffice (with safety margin) \$\endgroup\$ Commented Jan 29, 2016 at 2:13
  • \$\begingroup\$ apologies, 2800675 \$\endgroup\$
    – Douglas
    Commented Jan 29, 2016 at 3:00
  • \$\begingroup\$ Ok, that one's rated to protect up to an 80A load on a 120V circuit (or ~9.6KVA), but I just noticed a discrepancy between your 2.5KVA transformer rating & your OP statement about using a 120V primary. To get 2.5KVA from a 120V source requires >20A current, even before adding any losses, so either your primary windings are on a 240V circuit, of your xfmr is capable of overloading your 20A circuit before it reaches its rated load. \$\endgroup\$ Commented Jan 29, 2016 at 4:20
  • \$\begingroup\$ Where / how did you get the 80A and the 9.6KVA from? I don't see those numbers anywhere on the data sheet? in ref to the discrepancy, Are you are referring to the P=IV where P=2500 and V=120 I=20.83A? or something different? \$\endgroup\$
    – Douglas
    Commented Jan 29, 2016 at 4:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.