Wire Overheating

sssss

Member
Location
VA
I'm re-using existing feeders, remove from the old load at the disconnect switch, extend it to feed a boiler (480V) with new feeders. The feeder is (2) set of #350 MCM, 3 phase with ground wire. The upstream breaker is set to 620A, and meter is installed at the boiler to monitor the power usage. The load result came back never go above 600A, but the feeders runs extremely hot. The temperature goes about 190'F at the disconnect switch. Is there any reason it happens?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I'm re-using existing feeders, remove from the old load at the disconnect switch, extend it to feed a boiler (480V) with new feeders. The feeder is (2) set of #350 MCM, 3 phase with ground wire. The upstream breaker is set to 620A, and meter is installed at the boiler to monitor the power usage. The load result came back never go above 600A, but the feeders runs extremely hot. The temperature goes about 190'F at the disconnect switch. Is there any reason it happens?
Poor connections.
Overload.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Are the three phase sets properly balanced in the conduit?
Are there 1 or 2 conduits holding the 2 sets of 350 MCM?
Are both sets the same temperature?
What is the ambient temperature?

-Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
These conductors have a 75C rating of 310A.

This means that under the expectations used to create table 310.16, you would _expect_ a temperature of 75C(167F) at 620A for the set... Any deviation from the design conditions of 310.16 and the temperature will be higher or lower.

-Jon
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
This means that under the expectations used to create table 310.16, you would _expect_ a temperature of 75C(167F) at 620A for the set... Any deviation from the design conditions of 310.16 and the temperature will be higher or lower.
My expectation would be that when the design conditions of 310.16 are met, all other unspecified conditions have to be pessimal in order to actually get close to 75C. E.g. the 2 conduits in question are run alongside 10 other conduits, and the whole group is embedded in thermal insulation.

But 190F = 88C, so if the ambient at the disconnect is 30C, that should only happen if there's a high resistance connection in the disconnect, yes?

Cheers, Wayne
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
My expectation would be that when the design conditions of 310.16 are met, all other unspecified conditions have to be pessimal in order to actually get close to 75C. E.g. the 2 conduits in question are run alongside 10 other conduits, and the whole group is embedded in thermal insulation.

But 190F = 88C, so if the ambient at the disconnect is 30C, that should only happen if there's a high resistance connection in the disconnect, yes?

Cheers, Wayne

I agree that the design conditions of 310.16 are very very conservative, and that in actual practice actual operating temperatures will be lower than the values in 310.16.

My point was that these high temperatures are not that high relative to the 'rated' value, and it is likely that only small installation errors could cause the issue.

IMHO a high resistance connection is very likely but only one possibility. It could be at the terminations or it could be poor contact in the disconnect itself.

Another possibility is that there is some error in the parallel sets, causing circulating currents or unbalanced current. In this case the connections might be fine but subject to excessive current.

-Jon
 

Greg_SSE

New User
Location
So Cal
Occupation
Control Engineer
I've had issues with 'unbalanced' conduit fills - meaning that the net current in the conduit is not 0. If there is a net current then there is a magnetic field around the conduit and that leads to eddy currents in the conduit. At low currents this is pretty small, but at larger currents this can lead to heating. If the 3 phase motor leads are split (so a pair of smaller wires can be used for each leg), put one of these splits in one conduit and the other 3 wires in a second.

L1A, L2A, L3A in one conduit, and L1B, L2B, L3B in a second.

If it is L1A, L1B, L3A in one and L2A, L2B, and L3B in the second there will be 'net conduit current' that is not 0, which will create the magnetic field

Good news is that for your application maybe you just need to move connections around on both ends of the run to fix this
G
 
Last edited:

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
If the cables run -all 6 lives- in a same conduit -according to NEC Table 310.15(B)(3)(a) an adjustment of 0.8 is required.

Then, for 350 mcm copper conductor 90oC insulated, NEC Table 310.15(B)(16) I=350 A. Then for 2 sets 2*350*0.8=560 A.

However, if the permissible conductor temperature it is 90oC [194 oF] then you are at limit on the conductor.

I don't know how the cables run, indeed, and where the temperature is measured or how accurate the measurements are. In my opinion, 180oF on the outside jacket it could be o.k.
 

zooby

Member
Location
Indiana
Occupation
maint. electrician
how were temps obtained? a lil thermal imaging should rule out termination issues and/or disconnect blade issues.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2017 NEC
If the cables run -all 6 lives- in a same conduit -according to NEC Table 310.15(B)(3)(a) an adjustment of 0.8 is required.
Solve 310.15(B)(2) for Temp. requires balanced 3-Phase load is 460kW to match 88 deg.C (190 deg.F) from six 350cu in same pipe.

Same 88 deg.C with 498kW load, requires Pf = 0.87 with three 350cu per pipe. Boiler load would need to have 0.87 power factor?
 
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