Auxiliary electric heat is grouped in banks of 5 kw. The conductor size is 6 awg. The circuit ampacity/breaker size is 60 amps. If you have 15 kw, you either have an extra "single point service kit" added, or two 6 guage/ 60 amp circuits. One would have two 5 kw elements and the other would have one 5 kw and the load of the blower motor. Of course there are plenty of exceptions where manufacturers use unequal sized elements to handle staging better. If you have a Square D HOM panel, I have had to replace at least 100 of them on electric heat. If the breaker overheats, it may fry that lug on the bus in the panel. If you have a single point service kit, the wire is larger as someone previously stated and the breaker larger. If you have a #6 AWG wire coming to the breaker, it can't have a breaker greater than 60 amps. Going to a larger breaker will help nothing; and that night when your breaker tripped from a problem, you would have had something similar to an electrical fire instead. If the bus is bad, sometimes we can change it out if it is available. Another circuit may have to be run and you might have to get a new or different panel. Square D makes an excellent product called QO and it doesn't have the same problems in rural areas with electric heat. Now sometimes, in the winter we get "brown power". When everyone on a small residential line, usually in the country, uses a lot of power during a cold spell the voltage goes down. Low voltage causes motors and Electric furnaces to use more amperage. So, yes, inspect the bus, replace the contact grease/antioxidant on the bus then replace the breaker if it is the right size for the wire. If not, an electrician needs to get eyes and hands on it. Electric heat is only safe if we deliberately keep it that way. Electric furnaces are not to be played with because other lives depend on its safety systems and electrical codes. I typed this delayed response for people who will still read it forever.
At the start of the OP's problem details, he stated he had (apparently a single) ~61/62a (measured) heating load tripping a 60a cb. He was going to replace it with a
125A cb for his
temp heating need with 0* temps eminent. He was talked off the death-by-brute ledge and used an 80a cb as a temporary.
I told him that based on his reporting of "a 15KW load", he needed 80A cb
and wire properly sized for the continuous load. This was for a 15kw load branch circuit, the wiring
between sub-panel and furnace point of connection. How that breaks out in the furnace became details later. In post 19, OP stated he had #8's on the 60a cb! He was told he needed #3 copper/#4Al.
Post #22 brought up NEC 424.22(B) with the 48A load limit requiring a 60A circuit max. So the OP could run one feeder to the furnace and break out into 2 circuits with a mini-sub also functioning as a local disconnect OR run 2 circuits to the furnace from sub panel. Either method can easily meet the requirement of 48A max circuit loads.
Conductors and cb's used with continuous loads such as heating elements are (supposed to be) sized at 125% to reduce the heating affect at the various connection points. The OP's problem was he had 15KW load wired on a branch circuit at 105% of rating where it should have only been running at 80%. The cold snap caused this problem to get worse b/c the aux heating was running longer = more heating damage to the undersized circuit.
I get what you are trying to state above, but the 10KW circuit would need the #6 copper/60A cb circuit and the other 5KW circuit only needs #10 copper/30A cb circuit. See the nameplate in post #33 as a clear example. Always refer to the installation manual. AKA: RTFM
When a voltage lower than rated voltage is applied to a heating element, it draws less current. Motors may draw more depending on load.
(Please use carriage returns. Walls of text are a turnoff.)
