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KILN: 75 Amp, 1 phase - Residential Basement Install

3K views 8 replies 7 participants last post by  Stubbie 
#1 ·
I have a single-phase 240 volt, 75 amp, 16.5 KW KILN that I would like to hook-up myself.

The vented kiln is placed in the basement 60 feet from the service box. The manufacturer of the Kiln has specified that a shut-off box be placed near the kiln for direct wiring (there is no plug). The three wires from the kiln control box are AWG 4 (BLK WH GR).

The Service Panel is a 200 Amp Siemens Series F Type 1 that has space available for a 2 pole breaker.


I have looked at the following so far :

WIRE: One each (Black, White, Green) Single-Conductor Building Wire
(THHN) Stranded, 4 AWG, .318" OD, 600 VAC 95 AMP (The kiln mfg recommends +20% because of distance).

BREAKER: Siemens Q290 90-Amp 2 Pole 240-Volt Circuit Breaker

SHUT-OFF: SIEMENS Safety Switch, NEMA 1, 3W, 2P, 8x14.5x26 Fusible, Amps 100, Voltage 240V, Phase Single, HP 15

CONDUIT: Sched 40, GRAY, Trade Size 3/4, ID .82, OD 1.05


I have very limited electrical experience...some dimmer switches and one 30amp breaker for a garage heater.

Am I going in the right direction? Any suggestions / corrections?? The wire looks huge.

Thanks!
 
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#6 · (Edited)
#4 THHN can only use the 90C amp rating of 95 Amps if both terminations are rated at 90C. According to the Siemens datasheet the Q290 breaker is only rated for 60/75C. Assuming the safety switch is rated at 75, then you can only use the 85 amp rating for the #4 wire. So to go strictly by the table #3 would be required. However, I've heard of some situations where a slightly larger breaker could be used. So #4 might be allowable, but you'll need an electrician to weigh in for sure.

Of course the OP could just use the Siemens 80 amp breaker (Q280), then #4 would easily suffice (again assuming the safety switch is 75C rated).
 
#8 ·
So I guess we need to know from luxou what the safety switch's lugs are rated for (though I'm assuming lost shadow is correct about the safety switch likely being rated to 75C) and also, I suppose more importantly, why the 90 amp breaker for a 75 amp kiln? Was that breaker size suggested by the kiln's manufacturer?
 
#9 · (Edited)
If you have the manufacturer of the kiln please post it.

First thing to understand is kilns are 'continuous loads' they are also required to be on individual branch circuits. Generally speaking every 50' from the panel requires one upsize in awg wire guage after it has been sized correctly for a typical distance under 50' from the panel. So if that comes out to #4 copper and your kiln is 75 feet from the panel you need #3 awg. You will likely find that to be true once you run the actual calculations.


Other things to know ... you must use 90C insulated copper wire. So your correct in using THHN in conduit. However you cannot use the ampacity associated with 90C THHN you must use the ampacity rating for 75C THHN.

You need a disconnect on the wall close to the kiln ... you have that correct.

The kiln should have a hard wire terminal block as you would not typically cord and plug connect this kiln with out pin and sleeve.

If I had no instructions and only the data you have supplied this is how I would handle the kiln installation as far as the electrical.

Your wires size would be required to be 125% of 75 amps. 1.25 x 75 = 93.75 amps so you need wire to carry 95 amps with 90c insulation. That's going to be #3 awg thhn copper and for ampacity purposes use the 75C column of table 310.16 of 100 amps.

The circuit breaker is also calculated at 125% ,however there isn't a 95 amp circuit breaker so you go next size up to 100 amps.


So you need a 100 amp disconnect, #3 awg thhn in conduit, 100 amp circuit breaker all this takes into account the continuous load of the kiln and the 60 foot distance. IMO your not enough over 50' to warrant an up size in wire gauge.
 
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