I have several cabinets of servers in a datacenter and each of these cabinets is on a dedicated 208V 30A circuit. The load on each of these circuits is 24A and one circuit is slightly higher (~24.6A).

Recently, a datacenter technician (not an electrician) got in touch with me and said that my breakers in the panel were hot and that I should reduce the load on the circuits.

He took measurements using an infrared gun on three circuits. He said that the temperature was 117F for two of them and 110F for the other. He said that the maximum temp should be 104F. The ambient temperature in the room is roughly 72F.

A couple questions:

  • Is it normal that my breaker temps are reaching 117F on a circuit that's at 80% load or slightly higher?

  • Is the max temp for a breaker really only 104F? If humans and sensitive electronics can handle 104F, then surely a breaker could handle much higher temperatures...

I understand that it's impossible to give an exact answer to these questions when I'm unable to provide specifics on the breaker panel -- I'm just trying to get a general idea of what's 'reasonable' before I start pushing back on this request.

  • 1
    \$\begingroup\$ 47 degC (117F) doesn't seem too bad to me. Why don't you ask him to justify his potentially meddlesome position by explaining his concerns more technically. \$\endgroup\$
    – Andy aka
    Jun 2, 2017 at 18:51
  • 1
    \$\begingroup\$ Post a link to a datasheet on the breakers and we can have a technical discussion. \$\endgroup\$ Jun 2, 2017 at 19:31
  • \$\begingroup\$ @SpehroPefhany I've been told that the breakers are 2-pole 30 amp Square D QOB breakers. I haven't been able to get a precise model number, but it seems the breakers would be very close to this schneider-electric.us/en/product/download-pdf/QOB230 \$\endgroup\$
    – Elliot B.
    Jun 2, 2017 at 20:45
  • \$\begingroup\$ @ElliotB. 40°C is the ambient temperature for calibration of the current trip. At higher temperatures it will tend to trip a bit earlier. Circuit breakers are not precision devices, so there is little to worry about unless the breakers are tripping. \$\endgroup\$ Jun 2, 2017 at 21:28
  • \$\begingroup\$ It shouldn't be over 80% load if those are continuous loads, unless the breakers, panel and assembly are rated for 100% continuous operation. NEC 210.20A. I mention this because it seems like you are targeting the 80% number. \$\endgroup\$ Jun 3, 2017 at 3:59

1 Answer 1


The breakers are likely rated for 40C (104F) ambient.That would be the meaning of 40C marked on the breaker. They are likely rated for connection to wire that has a 60/75C insulation temperature rating. That would also be marked on the breaker. The surface temperature of the breaker can be expected to be higher than the ambient temperature. A surface temperature of 47C should not be a problem if the ambient temperature is 40C, but 47C surface temperature in a 20C ambient could be a problem. That would mean the surface temperature has a 27C rise and would reach 67C in a 40C ambient. However the following information seems to indicate that the breaker is allowed to get hotter than the wire.

UL Surface Temperature Rise Limits

Non-metallic 60 deg. C

Metallic 35 deg. C

UL Terminal Temperature Rise

80% Rated CB 50 deg. C

100% Rated CB 60 deg. C

The above information was found in another forum, not directly from UL.

  • \$\begingroup\$ Ambient temperature in the room is about 21C (71F) -- does that mean the 47C surface temperature on the breaker is a significant problem? \$\endgroup\$
    – Elliot B.
    Jun 2, 2017 at 19:26
  • \$\begingroup\$ See my revised answer. Based on other information I found, I don't think that is a significant problem. You should be able to draw up to the breaker limit. There is a lot of safety margin if the ambient is kept at 21C and likely still some safety margin at 40C. \$\endgroup\$
    – user80875
    Jun 2, 2017 at 19:39
  • \$\begingroup\$ I believe that the 80% limit imposed by the datacenter is based on the total load on the distribution panel. I think distribution panels are sized based on a diversity factor that assumes that various circuits have load variations over time. With a datacenter load, that would not be valid. The individual branch wiring and the breakers should be individually sized to carry full load, but probably only in a 30C ambient. For a 40C ambient, the wiring would be derated to 82/88% for 60/75C wire. \$\endgroup\$
    – user80875
    Jun 2, 2017 at 19:57
  • \$\begingroup\$ Thank you for the information, it has been very helpful! \$\endgroup\$
    – Elliot B.
    Jun 2, 2017 at 20:09
  • \$\begingroup\$ @CharlesCowie the 80% may relate to NEC 210.20A: Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the non-continuous load plus 125 percent of the continuous load. Exception: Where the assernbly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, [then it's ok]. \$\endgroup\$ Jun 3, 2017 at 4:03

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