Overfilled 1-1/2 conduit or derating?

[Flex]

Came across an 1-1/2 emt that feed out to the 4 rooftop units. It had 4 3pole 30A ckts in it. In all 13 #10 conductors counting the ground. None of the conduit is on the roof so temperature really isnt an issue. I was wondering if this was a legit install though. Doesnt some sort of derating come into play? Also do the 3 phases per fan count as one circuit or is each phase its own circuit?

 

[rcwilson]

3-phase fan circuit = 3 current carrying wires. Four circuits = 12 wires, ground doesn't count. (Doesn't matter anyway).

10-20 conductors in a conduit = 50% derate per Table 310.15 (B)(3) (a).

#10 awg XHHW-2, THHN and other 90C rated conductors' ampacity = 40 amps.

50% of 40 amps = 20 A, which is breaker size.

So it's OK if conductors are rated 90C.

If they are only 75C (RHW, THWN THW, XHHW, etc) ampacity is only 35 Amps, 50% = 17.5 A. Next larger breaker = 20 amps. All OK.

 

[augie47]

I'm afraid I miss something with Bob saying "o.k." I would agree if your breakers were 20 amp, but I understood you to say 30 amp.
IMHO, you might still be o.k., but only if the units MCA is 20 amps or less. You need to check your nameplate data,

 

[infinity]

IMHO, you might still be o.k., but only if the units MCA is 20 amps or less. You need to check your nameplate data,

I agree the MCA is the key. The derated conductor ampacity would have to be greater than or equal to the MCA of each unit. The size of the OCPD doesn't really matter as long as it doesn't exceed the MaxOCPD listed on the units.

 

[Flex]

So if the MCA on each unit is less than 20A then the install is ok? Also arent the lugs on the breaker only good for 75degree, so therefore the wire must be calculated the same way? Or am I just thinkin into this to much.

 

[infinity]

So if the MCA on each unit is less than 20A then the install is ok? Also arent the lugs on the breaker only good for 75degree, so therefore the wire must be calculated the same way? Or am I just thinkin into this to much.

The 90? C ampacity can be used for derating. The final useable ampacity cannot exceed the 75? C rating if the terminals are 75? C. I'm undecided if 240.4(B) for conductor sizing will be applicable to this type of installation. I'm thinking no.

 

[augie47]

The 90? C ampacity can be used for derating. The final useable ampacity cannot exceed the 75? C rating if the terminals are 75? C. I'm undecided if 240.4(B) for conductor sizing will be applicable to this type of installation. I'm thinking no.

Rob,
As long as the MCA is 20 or less and the MOCP is 30, do you see a problem ?

 

[infinity]

Rob,
As long as the MCA is 20 or less and the MOCP is 30, do you see a problem ?

No problem there, but I'm wondering if your adjusted ampacity is less than the MCA could you use the next size up rule. IMO you cannot.

 

[texie]

I also agree that the MCA is required to answer this question. If the MCA is 20 amps or less it's good to go and breaker size is not an issue as long as it does not exceed the max OCPD on the data plate. It would seem likely that the MCA is not above 20 given the 30 amp breaker that is already installed.

 

[augie47]

No problem there, but I'm wondering if your adjusted ampacity is less than the MCA could you use the next size up rule. IMO you cannot.

I agree. IMHO, your MCA cannot exceed the adjusted ampacity.

 

[texie]

I agree. IMHO, your MCA cannot exceed the adjusted ampacity.

I agree.

 

[ramsy]

If either MCA or amp clamp read over 14A, a thermal scope may show burnt terminations.

Adjusted feeder ampacity for inductive loads (higher amps) is a separate 80% factor frequently ignored.

Twelve inductive #10's in 30?c conduit exceed 75?c at 15 amps, without continuous loads.

Unless the 4 AHU's don't run simultaneously, where B.310.15(B)(1) engineering supervision could apply a load diversity avoiding Tble 310.15(B)(3)(a); substituting Annex B, Table B.310.15(B)(2)(11), per 310.15(b)(3) informational note.

Conduit fill should always be suspect, failure and liability from electrical installs support cottage industries for thermal-camera imaging, expert-fire investigators, and litigators for bad-faith insurers denying claims.

 

[don_resqcapt19]

... Adjusted feeder ampacity for inductive loads (higher amps) is a separate 80% factor frequently ignored. ...

Code section please.

 

[ramsy]

Code section please.

Code sections requiring inductive load adjustment, before 310.15 derating:
1) Section II of 430. With some exceptions, conductors shall not be less than 125 percent of motor full-load current
2) 220.18(A) combination loads. (ie) non HVAC or fans, "total calculated load shall be based on 125 percent of the largest motor"

Motor loads need adjustment, but no such factor exists in derating tables of ? 310.15, where power factor frequently gets forgotten.

Code sections with inductive-load nameplates prepared for 310.15 derating:
1) 220.18(A) Air conditioning only, equipment with nameplate load current. ? 440.6
2) 220.18(B) inductive lighting, with "total-ampere ratings"

If the Op's rooftop units are all HVAC nameplates, then no separate motor adjustment applies, and 20A FLA should read below 75?c.

 

[rcwilson]

It had 4 3pole 30A ckts in it.

Disregard my late Friday post. I did read 20A CB.

Mea culpa.