service wire size

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george nicholson

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I requested from a landlord that was providing a tenant service an 800A 3p 208V service for a 8,000sqft catering kitchen. I was given a calculated connected load of 587amps. I was told that all motors, receptacles, and HVAC were taken at 80% of running load. Lights were taken at 100%.

The landlords electrician installed 3ea 225A panels. These are being fed with 4ea 150lf of copper 4/0THHN in 2.5" PVC. Assuming the 675A is OK and I don't agree that it is----what is the proper 225A service wire size?

I would have derated the 75c wire size and used 250MCM NOT 4/0THHN for the 225A panels. The feeders are approx 150lf long and will at best have a continuous load of 195Amps.

Who is right is 4/0 THHN OK for this 225A load or should it have been 250MCM? I questioned the electrical inspector and he says that the 4/0 does not need to be derated in this application?
 
#4/0 THHN is rated for 230 amps at 75 degrees C, not a problem feeding a 225 amp panel. Do you feel that voltage drop is an issue?
 
george nicholson said:
I requested from a landlord that was providing a tenant service an 800A 3p 208V service for a 8,000sqft catering kitchen. I was given a calculated connected load of 587amps. I was told that all motors, receptacles, and HVAC were taken at 80% of running load. Lights were taken at 100%.
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Sounds like you did not provide the calculated load? You may want to provide your own calculated load as per 230-23 & Article 220; there are great examples to follow like Annex D3. Your feeders need to be calculated as well and they're the same see 215-2.

Roughly you shouldn't have to worry about voltage drop unless your distance length gets near or past the supply voltage (208V check voltage drop around 200'+ & 240V check voltage drop around 220'+)

The heating and air-conditioning load must be calculated at 100% not 80%. Lighting is considered a continuous load and it must be calculated as per 220-3(A), it must be calculated at 125%. Motors must be calculated at 125% see 220-3(B)(3), 430-22 & 430-24.
 
Until about 5 years ago I used the conductor size straight from the table but since that time have been forced to use the 80% derating for more than 3 conductors in the raceway in most all local municipalicities.

Those other electrical inspectors and plan review people say that the following is why the service conductors must be derated:

1) "On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor."

2) Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are stacked or bundled in lengths exceeding 24 in., the allowable ampacity of each conductor, as listed in Table 310.16, must be adjusted in accordance with the adjustment factors contained in Table 310.15(B)(2)(A).

3) The Article 310 requirement seems specific. What's missing, however, is a definition of what constitutes "major," as well as any guidance as to how much neutral current might be expected. Likewise, in Articles 210 and 220, how high are "high" harmonic currents?

4) Most buildings can withstand nonlinear loads of up to 15% of the total electrical system capacity without concern, but, when the nonlinear loads exceed 15% some non-apparent negative consequences can be expected. One plan reviewer uses the estimates from EPRI to determone that non-linear loads must be considered.

5) The Electric Power Research Institute (EPRI) estimates that in 1992, 15% to 20% of the total load was nonlinear, and by the year 2,000 it is expected that 50% to 70% of all loads will be nonlinear.

These inspectors say that with almost all lighting being electronic ballast, all controls on HVAC and refrigeration being computer derived, many personal PC stations, and etc that the neutral is definately carrying current and must be counted. In this kitchen instance no large loads are linear so there is not any large linear loading to offset the probable nonlinear loads. Therefore the nonlinear loads may exceed 15% which they consider "major". Thus derate the wire to 80% of the table if the other inspectors are correct.

I am not sure who is correct?
 
A major portion is more than 50%. Most buildings would never reach this amount so the neutral would not be considered a CCC and no derating is required. IMO plan reviewers are off the mark when they make this a blanket requirement.
 
george nicholson said:
Until about 5 years ago I used the conductor size straight from the table but since that time have been forced to use the 80% derating for more than 3 conductors in the raceway in most all local municipalicities....

These inspectors say that with almost all lighting being electronic ballast, all controls on HVAC and refrigeration being computer derived, many personal PC stations, and etc that the neutral is definately carrying current and must be counted. In this kitchen instance no large loads are linear so there is not any large linear loading to offset the probable nonlinear loads. Therefore the nonlinear loads may exceed 15% which they consider "major". Thus derate the wire to 80% of the table if the other inspectors are correct. I am not sure who is correct?
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You do have to use the adjustment factors in Table 310-15(B)(2)(a) but you remain at 100% of conductor until over 4 not 3. The neutral has always been a current carrying conductor and must be counted as such.

You must size the service and feeders to the calculated load in (and according to) Article 220.
 
tryinghard said:
You do have to use the adjustment factors in Table 310-15(B)(2)(a) but you remain at 100% of conductor until over 4 not 3. The neutral has always been a current carrying conductor and must be counted as such.

You must size the service and feeders to the calculated load in (and according to) Article 220.
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You only need to use the adjustment factors when the neutral counts as a CCC. Take a look at 310.15(B)(4).
 
tryinghard said:
The neutral has always been a current carrying conductor and must be counted as such.
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That is not the case at all, often a neutral is not a current carrying conductor to the NEC. 310.15(B)(4)
 
Oh yah :) But aren’t the adjustment factors on the 90? column (if using THHN/THWN of course). The adjustment of 80% on the 90? column would make the 4/0 good for 200A circuit, can you change the breaker to 200A?

In your case I still think a thorough calculated load is needed and not guessed, the calculated load tells the feeder and breaker what size to be. Keep in mind to feeder is sized at the non-continuous load plus 125% of the continuous load 215-2(A)(1).

What size breaker are you using for the feeder?
 
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tryinghard said:
Oh yah :) But aren?t the adjustment factors on the 90? column (if using THHN/THWN of course). The adjustment of 80% on the 90? column would make the 4/0 good for 200A circuit, can you change the breaker to 200A?
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You lost me, unless there is high ambient temperature four 4/0s can supply a 225 amp panel, there is no derating needed for the forth conductor as it is not current carrying in most cases.
 
iwire said:
You lost me, unless there is high ambient temperature four 4/0s can supply a 225 amp panel, there is no derating needed for the forth conductor as it is not current carrying in most cases.
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I understand this but if Georges case does happen to qualify under 310-15(B)(4)(b) or (c) then he would count the neutral as a CCC.

Georges neutral is most likely not a CCC unless the majority of the load is electronic type or drives, (nonlinear loads). His is a kitchen application and I wouldn't count the neutral as current carrying.

George if this is a design-build type project I would interpret "the major portion of the load" as the majority of the load.
 
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