Existing Loads and NEC 220.87

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hbendillo

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Is there any room for engineering judgment for determining existing loads? My example is this: Panelboard A is to be refed from Panelboard B. The connected loads for both panelboards are known and verified. The total connected load for Panelboard B when the load is transferred exceeds the rating of the Panelboard by 3%. That is if you consider the connected loads at 100% demand.

Panel A feeds VAV boxes with electric heat throughout the facility. Panelboard B mainly feeds air handling units that are strictly motors with no electric heat.

I am confident that the diversity in the load will allow the addition of the load to Panelboard B. However, the NEC gives no recourse other than NEC 220.87.

Is this strictly an AHJ call or can I use my own engineering judgment?
 
I am not sure what you mean here
The total connected load for Panelboard B when the load is transferred exceeds the rating of the Panelboard by 3%. That is if you consider the connected loads at 100% demand.
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Are you talking about the calculated load? Art. 408.36 requires the overcurrent protective device to not have a rating higher than the rating of the panelboard.

Feeders are based on 215.2(A). Are both of these sections compliant. I don't believe you can be 3% over on the load and you are only considering non continuous loads...

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts
III, IV, and V of Article 220. Conductors shall be sized to
carry not less than the larger of 215.2(A)(1)(a) or (b).
(a) Where a feeder supplies continuous loads or any
combination of continuous and noncontinuous loads, the
minimum feeder conductor size shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.
(b) The minimum feeder conductor size shall have an
allowable ampacity not less than the maximum load to be
served after the application of any adjustment or correction
factors.
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Annex B is allowed with engineering supervision but I am not sure that allows you to over rule the other sections of the code. That being said many areas will allow exceptions with a signed doc from the engineer
 
Dennis Alwon said:
I am not sure what you mean here


Are you talking about the calculated load? Art. 408.36 requires the overcurrent protective device to not have a rating higher than the rating of the panelboard.

Feeders are based on 215.2(A). Are both of these sections compliant. I don't believe you can be 3% over on the load and you are only considering non continuous loads...
Click to expand...

Yes, OCPD is compliant with 408.6. Yes, we are considering non-continuous loads in both panelboards. The loads are all motors and heat strips which leaves me with no NEC demand factors to use to reduce the calculated demand below the level of the connected loads. My calculated demand is 1.0 x connected load unless I am missing something.
 
One of my engineering managers was an advocate of assigning diversity factors based on engineering judgement. His argument was basically that if you're putting your stamp on it, and you have a good basis of argument for the adjustment, you can calculate the loads however you deem accurate, as long as you can clearly communicate your reasoning to the building officials.

An example was an HVAC project where we were looking at replacing all the existing HVAC (Z-box system with perimeter radiant heat) in a high-rise with new VAV and electric reheat. I don't remember exactly the numbers or diversity we used, but we applied something like a 65% demand on all the reheat based on conversations with the mechanical engineer and some common sense about the position of the sun in the sky. We added notation to the plans communicating our reasoning, and the officials agreed with it.

Lots of times the code calculations yield a much oversized system. This is evident when you see a commercial building with something like a 3000A service and the utility is running it off of a 500kVA transformer for years with no problems.
 
I'd put our Fluke recording meter on it for a month with a 15 minute sample rate and see what I get. That's the simplest way we do it.

We have one facility with several sets of MCC's and it's the only way I can keep track for the customer when they add and remove loads. If I just added the HP of all the motors, I'd be way over.
 
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