30 amp connection - too hot baby!

MarkC · May 2021

I have a 30 amp brkr circuit with 28 amp max current limited (Do I need to reduce the max current to ~ 24 amps to be code compliant?). The L6-30P/R connection point gets very hot (170 oF) in a matter of hours. The plug is a "factory installed" part of a 10 AWG cable. The receptical is a Hubble brand that I installed myself - and have double checked good/tight wire installation. Both "plugs" get very hot - much hotter than the cable (that gets somewhat hot ~130 oF)
Is this normal? Do I need to worry about such temperatures? I've reduced the max current to 22 amps and will "test" again today, however, why would 30 amp rated connectors get so hot?

BB. · May 2021

The NEC has one rating for "standard" circuits, and another rating for "continuous use" circuits.

Circuit breakers are rated to trip at 100%+ rated load, and rated to not trip at 80% or less current. In between, the breaker may never trip, trip after hours, or whatever.

I was looking any outlet and cord temperature ratings--And I did not find anything specific. However, for NEC wiring, there are temperature ratings:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Size	Copper Conductors				Aluminum Conductors Copper-Clad Conductors
Size		60° C (140° F)	75° C (167° F)	90° C (194° F)		60° C (140° F)	75° C (167° F)	90° C (194° F)
AWG Kcmil		Types	Types	Types		Types	Types	Types
AWG Kcmil
		TW	RHW	TBS		TW	RHW	TBS

		UF	THW	SA		UF	THHW	SA, SIS

			THWN	SIS			THW	THHN, THHW

			XHHW	FEP			THWN	THW-2

			USE	FEPB			XHHW	THWN-2

			ZW	RHH			USE	RHH,
				THHN				RHW-2
				THWN				USE-2
				XHHW				XHH, XHHW
								XHHW-2, ZW-2

So--I would start to worry if the insulation was in the 140-194F range... And of course, Ambient temperature affects wiring temperature (cool 70F room vs 95F room in summer sunny conditions)... And cord ratings:

http://www.chem.cmu.edu/groups/bier/procedure_files/general/powercords.pdf

If you wanted to send 30 amps through the plug/cord continuously--Then I would do the 1.25 / 0.80 derating (1/0.80=1.25)

30 amps * 1.25 NEC continuous derating = 37.5 ~ 40 amp rated branch wiring/circuit breaker suggested.

Or, as you propose, run at 80% of circuit rated capacity:

30 amps * 0.80 NEC continuous current derating = 24 amps max continuous load

Remember of of the power equations is Power = I^2 * R (current squared * resistance)... So as current is increased with fixed resistance, the heating effect goes up even more.

Say you have a 0.1 Ohm resistance and 30 amps vs 24 amp load:

P = 30a^2 * 0.1 Ohms = 90 Watts heating
P = 24a^2 * 0.1 Ohms = 57.6 Watts heating
24a/30a = 0.80 = 80% (or 20% reduction in current)
57.6w/90w = 0.64 = 64% (or 36% reduction in heating)

Hubble makes good connectors (in general)--So I don't think you have a "cheap" connector issue.

I have seen a spool of extension cord get pretty hot. The coiled cord/poor air circulation heats the cord vs just laid out straight point to point.

-Bill

MarkC · May 2021

Bill - I set the current on my EVSE at 22 amps and in an hour, the connection temp went to 104 oF - probably about what your calc method would predict. Next try is 24 amps and check - likely where I'll stay. That is plenty of "juice" to charge my Leaf overnight. 240*24= 5760 VA. The on board charger is rated for 6.6kw, so all should be good. Added benifit is that I can probably stay with my 30 amp breaker set.
Thanks for your reply,
Mark

30 amp connection - too hot baby!

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