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Power Requirements/Code requirements

By tom.doran ·
I'm trying to document with reference to electrical/building code citations the electrical supply requirements for desktops. Essentially how many desktops with monitors are permited by code on a 20 amp 110volt circuit?
We're setting up a "temporary" (12+ months) work location with an ancient existing wiring system. Our construction manager claims he seen "8 or 10 desktops running off a single extension cord."
I want to have no more than two desktops on a single permanently wired circuit, i.e. we need to spend some money on this project. Otherwise the PC's won't work and a fire hazard is created. Looks like I'll need chapter and verse of the code requirements to persuade management.

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by willcomp In reply to Power Requirements/Code r ...

Believe that the NEC is concerned with total loads, voltage drops, and conductor sizing on a circuit basis. Don' think they speak to how many PCs per circuit.

2 PCs on a 20A circuit is overly conservative. Most PCs have a running load of less than 300 watts. I have a client with 3 servers (powerful desktop PCs with SATA RAID arrays, not true servers), a workstation, and a Laserjet 5SE on a single 20A circuit along with a couple of floor lamps. May not be preferable, but has been running for 8 months with no problems.

You may want to have an EE take a look, but I don't see any problems running 6 PCs and a pinter or two on a single circuit. What you need to look out for are coffee pots, electric heaters, and other comfort/convenience items. Those are resistance loads and may pull up to 1500 watts each.

Also conductor size should be considered. Would prefer to have 12 gauge or larger wire.

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by tom.doran In reply to Power Requirements/Code r ...

The closest relevant item I found in the NEC says design should be 125% of expected load. With PC's marked at 6 amp and monitors at 1.5 amp, two of each gives 15 amps. Twenty amps is only 133% of that, so unless I reading things wrong, two PC's and monitors would be the max.
Six amps is probably max capacity if the box ran wide open so conceivably a few more could power up. I really need to get this guy off the idea of a spider web of extension cords.

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by rawright In reply to Power Requirements/Code r ...

To start, you need to determine the current drawn by the devices you want to connect. A 12AWG copper circuit is rated for 20A service, and the permanently connected load should not exceed 80% of that value (16A). On the rear panel of your PCs and monitors there is a plate that identifies the maximum power requirements for each device - a PC is usually about 2.5A, a monitor around 2A. Printers vary wildly, depending upon the type used, but modern inkjets are quite stingy in the power they use - figure on about .5A average.

Two desktop units per circuit is excessively conservative, especially since it is unlikely that all connected PCs will be drawing maximum currents at the same time. Three per circuit would be certain to fulfill your requirements, and four would not be pushing the limit much, even with a printer connected to one or two of them. In most office environments, PCs spend most of their time idle and draw very little current relative to their maximum ratings.

Given that the wiring is "ancient," though, I'd play it a little safe and install no more than three units on a branch circuit. Be sure to budget for UPS boxes for each, as well, since these can not only save your system during a power failure, but can also smooth out any power variations caused by temporary overloading. Pay attention to air conditioning, too. A loaded 20A circuit is equivalent to a heat source of about 7000 BTUs.

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by wlbowers In reply to Power Requirements/Code r ...

Your saving grace is NEC Sec. 110-3(b) This section requires you to install equipment in accordance with the equipment instructions.

Therefore, you must install electrical equipment so it operates within its voltage rating as specified by the manufacturer.

Designing an ac distribution system for a computer rich environment is not rocket science.

Documenting the reasons for some of the requirements with the NEC is not for the faint of heart. Sometimes it requires diligence and some Wild Turkey.

The label ratings you find on the computers and monitors are maximum requirements.

If you are running the supply in the computer at its max rated current requirements the supply needs to the replaced with a larger unit.

For safety reasons most equipment never draws more than 80% of the rated power even during peak demand, and the majority of computer equipment draws much less than 80% of its nameplate rating, some as low a 30%.

When we design a distribution system for a computer rich environment, a maximum of 6 computers are allowed on a circuit.

This would be adjusted if the circuits feed laser printers as well. (The heaters draw a ton.

There are some areas you need to pay attention to.

K-rated transformers. Look for a k-factor of 7

TVSS devices. One for each Circuit panel. (Transient Voltage Surge Suppression.

Dedicated computer circuit panels (no motors.

Larger Neutrals / Run a Neutral per circuit. Switching supplies produce harmonics that can heat up the neutrals. We run a neutral per circuit.

Verify the Grounding System. Eddie currents induced on a substandard ground will drive you nuts with problems.

Ground loops can cause problems. Sometimes isolated ground receptacles are required.

UPS units are a must. They provide surge protection, over and under voltage protection.

Good Luck

Licensed Journeyman Electrician since 1988.

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